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LASA 

Lidar Atmospheric Sounder and Altimeter 

EARTH OBSERVING SYSTEM

Volume lid 

INSTRUMENT PANEL REPORT 

NASA 

National Aeronautics and 

Space Administration 

1987
EARTH OBSERVING SYSTEM REPORTS 

Volume 1 Science and Mission Requirements Working Group Report 

Volume II From Pattern to Process: The Strategy of the Earth Observing System 

Science Steering Committee Report 

Volume ha Data and Information System 

Data Panel Report 

Volume lib MODIS 

Moderate-Resolution Imaging Spectrometer 

I " Instrument Panel Report 

Volume lic HIRIS & SISEX 

High-Resolution Imaging Spectrometry: Science Opportunities for the 1990s 

Instrument Panel Report 

Volume lid LASA 

Lidar Atmospheric Sounder and Altimeter 

Instrument Panel Report 

Volume lie HMMR 

High-Resolution Multifrequency Microwave Radiometer 

Instrument Panel Report 

Volume hf SAR 

Synthetic Aperture Radar 

Instrument Panel Report 

Volume hg LAWS 

Laser Atmospheric Wind Sounder 

Instrument Panel Report 

Volume IIh Altimetric System 

Panel Report

11
LIDAR ATMOSPHERIC SOUNDER AND ALTIMETER

PANEL FOR THE EARTH OBSERVING SYSTEM 

Robert J. Curran, Chairman 

Richard R. Nelms, Executive Secretary 

Frank Allario 

Robert A. Bindschadler 

Edward V. Browell 

Jack L. Bufton 

Robert L. Byer 

Steven Cohen 

John J. Degnan 

William B. Grant 

Reynold Greenstone 

William S. Heaps 

Benjamin M. Herman 

Antony Jalink 

Dennis K. Killinger 

Laurence Korb 

James B. Laudenslager 

M. Patrick McCormick 

S. Harvey Melfi 

Robert T. Menzies 

Volker Mohnen 

James Spinhirne 

H. J. Zwally

Ill
PREFACE 

The Lidar Atmospheric Sounder and Altimeter (LASA) Panel was organized to develop 

the scientific objectives and corresponding engineering requirements for a lidar instrument as 

part of NASA's Earth Observing System (Eos) study. The initial concept of LASA was 

described in Volume I of the Earth Observing System Reports (Butler et al., 1984). The Panel 

members were selected by the Earth Science and Applications Division of NASA 

Headquarters. 

The primary task of the LASA Panel was to develop the specific objectives for this lidar 

instrument, give a rationale for the observational requirements, and identify the envelope of 

instrument parameters needed to satisfy the observational requirements. This task was accomplished through a series of panel meetings held over a one and one-half year period. The 

necessary simulation calculations and report writing were performed by the Panel members 

at their respective home organizations. Meeting logistics as well as the difficult job of document preparation were most ably handled by Ms. Lori Nelson, Ms. Terry Edgar, and Ms. 

Dianna Gemma of Birch & Davis Associates, Inc. It is a pleasure to acknowledge the support 

provided by the staff of Birch & Davis Associates, Inc. Recording of the proceedings of the 

Panel meetings and the arduous task of final editing of this report was performed by Mr. 

Reynold Greenstone of ORI, Inc. The Panel wishes to acknowledge the strongly professional 

efforts of Mr. Greenstone, without which this report would be considerably less well 

organized. 

A number of individuals aided in discussions with the Panel to develop its relationship 

to other instruments in Eos. Drs. Robert S. Fraser and Howard Gordon discussed the contribution LASA could make to correcting passive observations to be made in the visible and 

infrared. Dr. Ramesh Kakar described the anticipated accuracy and resolution that microwave 

instruments will provide in remotely sensing atmospheric water vapor. Drs. Wayne Esaias and 

William L. Barnes helped in developing a synergism between LASA and the ModerateResolution Imaging Spectrometer (MODIS). Dr. Robert Thomas helped in defining the scientific rationale for LASA's use in determining ice and snow mass balance. 

Robert J. Curran 

Chairman, LASA Instrument Panel 

Washington, D.C. 

lv
EXECUTIVE SUMMARY 

One of the major opportunities facing Earth 

scientists in the next decade is the deployment of 

new satellite instrument technologies. These new 

devices can provide both unique observational 

measurements not previously achievable and measurements with greater accuracy, precision, and 

resolution than passive and active remote sensors 

currently in use. This report provides recommendations and background for an active laser remote-sensing system designed to be a significant 

contributor to Eos, the Earth Observing System of 

the 1990s. 

The new active laser remote-sensing system has 

been designated the Lidar Atmospheric Sounder 

and Altimeter (LASA). LASA will be part of a 

complement of new instruments to fly in polar orbit. 

Working together, these instruments will form an 

Earth observing system to provide the information 

needed to further understanding of the fundamental, global-scale processes which govern the Earth's 

environment. The outline of Eos is set forth in Volume I of the Earth Observing System Reports (Butler et al., 1984). The Science and Mission Requirements Working Group identified the role of active 

laser observations in Eos. 

The LASA Panel was created to consider the 

role that the rapidly evolving light detection and 

ranging (lidar) technology could play in furthering 

the interdisciplinary objectives of Eos. The conclusions and recommendations of the LASA Panel discussed in this report are directed both toward a 

series of atmospheric composition and structure 

measurements, and toward a set of surface ranging 

measurements using laser techniques. 

Lidars operate in the ultraviolet, visible, and 

infrared regions of the electromagnetic spectrum. 

Their use permits derivation of remotely sensed atmospheric profiles with unprecedented resolution. 

Lidar can be used to measure distances to centimeter-level precision over a wide range of terrestrial 

baselines. In a retroranging mode, distance measurements can be used to determine relative motion 

and deformation of the lithospheric plates, and they 

can also be used to study the Earth's crustal movements. In an altimetric mode, lidars can be used to 

measure the topography of the Earth's surface. In 

performing altimetry and retroranging, lidar's 

shorter wavelengths offer surface footprints in tens 

of meters leading to improved measurement and discrimination of land features. Lidar's improved resolution has also enabled the accurate detection of 

small crustal displacements for accurate measurements of tectonic plate motions. 

The remote measurement of atmospheric, land, 

and oceanic properties with active optical techniques has progressed rapidly since the invention of 

the laser in 1960. Lidar can be used to measure

remotely from space atmospheric parameters such 

as the height of the planetary boundary layer (PBL), 

the vertical and horizontal distribution of atmospheric aerosols, cloud-top heights, the vertical distributions of atmospheric trace gases such as water 

vapor and ozone, and the vertical distributions of 

atmospheric pressure and temperature. In making 

composition measurements of the atmosphere, lidar 

provides enhanced vertical resolution (50 to 100 m 

for aerosols and clouds and 0.5 to 1 km for gases). 

Such high resolution is consistent with many atmospheric science requirements. For many of these parameters lidar provides a new measurement capability; for others, lidar provides measurements with 

improved vertical resolution and decreased size of 

the ground footprint. 

As an optical device, lidar has the advantage of 

high-sensitivity detection, with capability in some 

cases to detect single photons. (Photomultiplier 

tubes and solid-state detectors are well developed at 

lidar wavelengths.) A further lidar advantage is its 

high level of monochromaticity. With these advantages, lidar may be used to uniquely measure molecular concentrations against interfering background spectra. With the development of tunable 

laser sources, molecular species such as water vapor 

and ozone as well as the atmospheric structure parameters, pressure and temperature, can be measured through the Differential Absorption Lidar 

(DIAL) technique. Thus, lidar has a unique capability to make many measurements from space that 

cover an altitude range from the stratosphere 

through the troposphere to the surface of the Earth. 

At most of the available wavelengths, strong 

return signals from the ground and clouds are obtained with lidar. Additionally, laser sources with 

pulse lengths in the picosecond range and high repetition rates (>1 kHz) have been achieved. These 

pulse characteristics, coupled to modern signal processing and electronic filtering techniques, permit 

measurement of physical heights to an unprecedented centimeter resolution and accuracy. Ice and 

terrain roughness at centimeter scales can be measured via laser-pulse waveform analysis for footprints 

an order of magnitude smaller (with correspondingly higher resolution) than can be measured with 

radar. 

The LASA concept incorporates two separable 

remote-sensing techniques. Sensing of atmospheric 

properties is the primary role of the LASA instrument proper whereas measurement of Earth surface 

features is the role of a separately designated instrument identified as the Geodynamics Laser-Ranging 

System (GLRS). The LASA instrument can function as an altimeter with somewhat limited vertical 

resolution as compared to GLRS, which has the 

potential of achieving 10 cm resolution. 

V
SCIENCE OBJECTIVES 

The LASA Panel concentrated on five major 

scientific objectives derived from the major thrusts 

of Eos:

1. Improve understanding of the global hydrological cycle through observations of atmospheric water vapor. 

2. Improve understanding of the role of aerosols and clouds in the physical climate 

system. 

3. Improve understanding of the large-scale 

structure and dynamics of the Earth's atmosphere through globally distributed highresolution observations of atmospheric composition and structure. 

4. Improve understanding, through altimetry, 

of the processes that influence ice and snow 

amount. 

5. Improve understanding of geodynamic processes through retroranging to locally and 

globally distributed reflectors and through 

altimetric mapping of land topography. 

In order to arrive at a recommended set of experiments responsive to these broad scientific objectives, the LASA Panel reviewed the current status 

and future potential of lidar techniques in the Eos 

era. These reviews included the technology readiness of laser sources, lidar detectors, telescopes, 

and optical receivers. The status of calibration techniques and associated subsystems such as laser 

wavemeters and spectrometers was also reviewed. 

The LASA Panel recommends that the LASA 

instrument package be incorporated within Eos in 

a time-phased manner consistent with the technological readiness of the lidar components. The measurement objectives in the areas of the global hydrological cycle, aerosols and clouds, and 

atmospheric state parameters and ozone are to be 

met by the LASA instrument proper. The further 

measurement objectives, ice-sheet mass balance and 

land topography, and crustal movements and tectonic processes are to be addressed by the GLRS 

instrument. Depending upon the state of technology, the LASA instrument and the GLRS instrument can be used to meet the following measurement objectives that are responsive to the major 

thrusts of Eos. 

LASA INSTRUMENT MEASUREMENT

OBJECTIVES 

Global Hydrological Cycle 

• Measure the total column content of water 

vapor with a horizontal spatial scale on the

order of 1 km and an accuracy of ± 10 

percent. 

• Measure the vertical profiles of atmospheric 

water vapor from the stratosphere, through 

the troposphere, to the ground with a vertical 

resolution of 1 km and with an accuracy better 

than 10 percent at night and 15 percent during 

the day. 

Aerosols and Clouds 

• Measure cloud-top heights to a resolution of 

±50 m and also measure cloud-top parameters. Measure the height of the planetary 

boundary layer with a resolution of 50 m and 

the heights of volcanic aerosol layers with a 

resolution of 1 km. Measure the vertical profiles of atmospheric aerosols over an altitude 

range from the ground to 30 km, with a resolution from 50 m to 2 km, and an accuracy 

of 15 percent. 

Atmospheric-State Parameters and 

Ozone 

• Measure the surface pressure with an accuracy of ±2mb. 

• Measure the vertical profiles of temperature 

and pressure from the stratosphere, through 

the troposphere, to the ground. Measure temperatures with an accuracy better than 1 K 

and with a vertical resolution of 1.25 km in 

the lower troposphere, 2.5 km or better in the 

mid-troposphere and upper troposphere, and 

3 km in the lower stratosphere. Measure vertical profiles of pressure in the troposphere 

with an accuracy better than 0.4 percent (i.e., 

approximately 2 mb at 5 km) with a vertical 

resolution of 1 km. 

• Measure the total column content of ozone 

with an accuracy of 1 to 2 percent. 

• Measure the vertical profiles of ozone, especially in the region below the stratospheric 

peak to the ground, with a vertical resolution 

better than 2 km, and an accuracy better than 

1 percent nighttime and 10 percent daytime. 

GLRS INSTRUMENT MEASUREMENT

OBJECTIVES 

Ice-Sheet Mass Balance and Land 

Topography 

• Measure ice and terrain topography to better 

than 10 cm vertical resolution and 300 m horizontal separation with a 70 m footprint. 

vi
• Measure the surface roughness and surface 

height distribution, at centimeter scales, of 

bare terrain, sea ice, and surfaces covered by 

vegetation, by analysis of the range distribution within a laser footprint. 

• Measure topographical profiles of selected 

terrain to characterize volcanic, erosional, 

catastrophic geomorphic and subsurface 

processes. 

Crustal Movements and Tectonic 

Processes 

• Measure three-dimensional regional scale (25 

to 500 km) crustal movements to a precision 

of 1 cm in seismically active zones using dense 

networks of retroreflectors. 

• Measure regional scale (100 to 1,000 km) 

tectonic plate movements and intraplate 

rigidity using more broadly distributed 

retroreflectors. 

INSTRUMENT CAPABILITIES 

LASA sounding experiments from space will 

provide direct measurements of atmospheric properties. Profiles of tropospheric and lower-stratospheric water vapor and profiles of ozone, pressure, 

and temperature can be obtained with 1 to 2 km 

vertical resolution in keeping with the science 

objectives. 

In altimetry, the unique collimation ability of 

laser pulsed radiation provides a small footprint (approximately 100 m) on the Earth's surface. The resulting high-resolution time-of-flight data are 

unique, and can resolve the important scales of icesheet elevation profiles, sea ice structure and distribution, and micro-roughness of terrain. 

The retroranging experiments can provide a 

unique set of high-density, quasi-simultaneous crustal movement measurements. In a seismically active 

zone, ranging measurements will determine the intersite distances along a strike of a major fault to 

accuracies on the order of 1 cm. A more globally 

distributed network of cube-corner reflectors will be 

used to develop a set of tectonic plate movement 

and plate internal deformation measurements. Retroreflectors placed on the surfaces of ice sheets and 

smaller glaciers will provide similar data for studies 

of ice dynamics.

HERITAGE 

Significant lidar investigations have been performed from both aircraft and space platforms. Lidar measurements have already been made from the 

NASA ER-2 aircraft, and an experiment is planned

to measure the vertical distribution of lower-tropospheric water vapor from the NASA ER-2 aircraft in 

1988. In addition, a technology development flight 

with lidar to measure vertical profiles of aerosols 

and cloud-top heights with a multi-wavelength lidar 

system is planned for a 1990-91 Shuttle flight. Spacebased altimetry measurements with a solid-state 

laser were demonstrated on several Apollo missions 

in the 1970s. The Apollo Lunar Laser Altimeter 

returned very useful data sets on lunar topography 

and demonstrated the viability of space-based lidar 

systems. 

TECHNOLOGY REQUIREMENTS 

The LASA Panel recognizes that laser technology is rapidly evolving in response to NASA and 

other national needs. In particular, the development 

of tunable solid-state laser materials is progressing 

rapidly in that portion of the optical spectrum that 

covers experiments proposed in this report. Additionally, semiconductor lasers arranged in arrays are 

rapidly developing as replacements for conventional 

flashlamp pumping, with potential improvements of 

an order of magnitude in laser power, efficiency, and 

operating lifetimes. Flashlamp lifetimes are also 

constantly being improved. Gas laser technology in 

both excimers and carbon dioxide is being developed for long-lifetime space applications. Improvements in any one or all of these technologies before 

1990 will substantially improve the Eos lidar capability to perform both daytime and nighttime measurements with full duty cycles. 

Although there are limitations on lidar performance in space based on present technology, the 

Panel has assumed, for the most part, that the required laser technology will be available in the Eos 

time frame. The Panel did recognize that some experiments may be constrained to limited duty cycles 

and to nighttime operation for the earlier Eos experiments. These constraints are dictated by limitations in average power available and the relatively 

long instrument servicing intervals. 

SYNERGISTIC SCIENCE 

Spaceborne lidar instruments will provide observations whose characteristics include high spatial 

resolution with limited coverage. The coverage limitations follow primarily from the limitations in laser 

efficiency and available power, and secondarily from 

the limitations in laser system lifetime. These characteristics of lidar systems make their observations 

ideal for studies which synergistically use other Eos 

observations. An example of the synergistic use of 

the LASA data would be the combination of LASAdetermined vertical profiles of water vapor together 

with two-dimensional depictions of water vapor 

vii
distributions from an Eos scanning microwave radiometer. In this example, the LASA instrument 

would give high vertical resolution profiles of water 

vapor at nadir with a horizontal spacing of approximately 1 km along the sub-satellite track. The microwave instrument would provide observations 

from which can be derived water vapor distributions 

with low vertical and horizontal resolution but broad 

spatial coverage. 

The lidar altimetry data sets are highly synergistic with data products from radar altimeters and 

imaging radar instruments. This is true for ice, terrain, and ocean surface targets where the small footprint of the laser can be used to calibrate radar data 

and provide a high-resolution (vertical and horizontal) nadir track.

CONCLUSION 

Eos will provide an ideal forum in which the 

strongly synergistic characteristics of the lidar systems can be used in concert with the characteristics 

of a number of other sensors to better understand 

the Earth as a system. Progress in the development 

of more efficient and long-lasting laser systems will 

insure their availability in the Eos timeframe. The 

necessary remote-sensing techniques are being developed to convert the LASA observations into the 

proper scientific parameters. Each of these activities 

reinforces the promise that LASA and GLRS will 

be a reality in the Eos era. 

viii
CONTENTS

Page 

PREFACE ..................................................................................................iv 

EXECUTIVESUMMARY ................................................................................v 

Science Objectives ............................... ...............................................vi 

LASA Instrument Measurement Objecti .. ves.....................................................vi 

GLRS Instrument Measurement Objectives .....................................................vi 

Instrument Capabilities...........................................................................vii 

Heritage...........................................................................................vii 

TechnologyRequirements ........................................................................vii 

Synergistic Science ................................................................................vii 

Conclusion........................................................................................viii 

ACRONYMS ..............................................................................................x 

I. INTRODUCTION...................................................................................... 1 

Earth Observing System: Concept and Overview ............................................... 1 

LASA Science Issues ............................................................................. 1 

Atmospheric Structure and Composition ........................................................ 1 

SurfaceRanging .................................................................................. 4 

II. LASA SCIENCE: ATMOSPHERIC SOUNDING .................................................... 8 

LidarTechniques.................................................................................. 8 

Cloud Observations............................................................................... 9 

Planetary Boundary Layer Observations......................................................... 11 

Tropospheric Aerosol Observations .............................................................. 13 

Stratospheric Aerosol Observations.............................................................. 15 

Water Vapor Observations........................................................................ 19 

Atmospheric Pressure Observations ............................................................. 28 

Atmospheric Temperature Profile Observations ................................................. 31 

Ozone Observations .............................................................................. 33 

LidarScanning.................................................................................... 37 

Synergistic Science ................................................................................ 39 

III. LASA SCIENCE: ICE-SHEET DYNAMICS AND SURFACE ALTIMETRY ...................... 41 

Lidar Altimetry Applications .................................................................... 41 

Measurement Objectives ......................................................................... 41 

Synergistic Science ................................................................................ 42 

Altimeter Instrument Description................................................................ 42 

Measurement Simulations ........................................................................ 42 

Examples of Laser-Altimeter Data .............................................................. 49 

IV. LASA SCIENCE: GEODYNAMICS AND LASER-RANGING APPLICATIONS ................. 54 

Introduction....................................................................................... 54 

Measurement Objectives and Strategy........................................................... 56 

Demonstration Experiments and Performance Simulations ..................................... 62 

SynergisticScience ................................................................................ 67 

V. HERITAGE, CONCEPTS, AND LASER TECHNOLOGY ASSESSMENT ........................ 70 

LidarHeritage .................................................................................... 70 

Instrument Concepts ............................................................................. 71 

Laser Technology Assessment for Atmospheric Measurements ................................. 73 

Laser Technology Assessment for Surface-Ranging Measurements ............................. 78 

Comparison of LASA Objectives with Laser Capabilities ...................................... 79 

VI. Fos COMMAND AND DATA MANAGEMENT SYSTEMS FOR LASA ......................... 83 

DataRates........................................................................................ 83 

Command Uplinking and Control................................................................ 83 

System Redundancy .............................................................................. 84 

Ground Computer Facilities...................................................................... 84 

VII. CONCLUSIONS AND RECOMMENDATIONS ................................................... 85

APPENDIX A: EYE SAFETY ISSUES ....................................................................87 

REFERENCES .............................................................................................88 

lx 
ACRONYMS 

AFGL Air Force Geophysics Laboratory 

ANSI American National Standards Institute 

AOL Airborne Oceanographic Lidar 

APD Avalanche Photodiode 

BRDF Bidirectional Reflectance Distribution Function 

BUV Backscattered Ultraviolet 

CLS Cloud Lidar System 

cw Continuous Wave 

DARPA Defense Advanced Research Projects Agency 

DC Direct Current 

DIAL Differential Absorption Lidar 

DIALEX Differential Absorption Lidar Experiment 

FHST Fixed-Head Star Trackers 

FOV Field-of-View 

FWHM Full-Width Half-Maximum 

GCM General Circulation Model 

GEM Goddard Earth Model (of Gravity) 

GEOS II Geodynamic Satellite II 

GLRS Geodynamics Laser-Ranging System 

GMT Greenwich Mean Time 

GPS Global Positioning System 

GRM Geopotential Research Mission 

GSFC Goddard Space Flight Center 

HMMR High-Resolution Multifrequency Microwave Radiometer 

IOC Initial Orbiting Configuration 

IRU Inertial Reference Unit 

LAGEOS Laser Geodynamics Satellite 

LASA Lidar Atmospheric Sounder and Altimeter 

LASE Lidar Atmospheric Sensing Experiment 

Lidar Light Detection and Ranging 

LITE Lidar In-Space Technology Experiment 

MACS Modular Attitude Control System 

MCP/PMT Microchannel Plate/Photomultiplier Tube 

MMS Modular Mission Spacecraft 

MODIS Moderate-Resolution Imaging Spectrometer 

MPE Maximum Permitted Exposure 

NEE Noise Equivalent Energy 

NEP Noise Equivalent Power 

OAST Office of Aeronautics and Space Technology 

OMC Observed Minus Calculated 

OPO Optical Parametric Oscillator

x 
ACRONYMS (continued) 

PBL Planetary Boundary Layer 

PSC Polar Stratospheric Cloud 

RF Radio Frequency 

rms Root Mean Square 

SAGE Stratospheric Aerosol and Gas Experiment 

SAM II Stratospheric Aerosol Measurement II 

SBUV Solar Backscatter Ultraviolet 

SLR Satellite Laser Ranging 

SMRWG Science and Mission Requirement Working Group 

SNR Signal-to-Noise Ratio 

SPE Signal Photoelectron 

STP Standard Temperature and Pressure 

TFE Tropopause Fold Event 

TOMS Total Ozone Mapping Spectrometer 

UV Ultraviolet 

VLBI Very Long Baseline Interferometry

xi 
I. INTRODUCTION 

EARTH OBSERVING SYSTEM:

CONCEPT AND OVERVIEW 

Through the centuries, vast amounts of data 

and much phenomenological knowledge about the 

Earth have been accumulated through ground, airborne, and space observations. However, to acquire 

systematic, synoptic, and temporal knowledge of 

our planet, and an understanding of the mechanisms 

underlying the global processes that affect it, requires a multidisciplinary approach with long-term 

observations from space. The key to progress in 

Earth science for the next decade will be to address 

those multidisciplinary questions that concern the 

integrated functioning of the Earth as a system. The 

fundamental processes that govern and integrate 

this system include the global hydrological cycle, the 

biogeochemical cycles, climate processes, and geologic activities. A plan to address the integration of 

these cycles and processes has been developed and 

reported in Earth Observing System (Eos) (Butler 

et al., 1984). 

In order to address the multidisciplinary challenges confronting Earth science, observational capabilities must be employed that range in scale from 

detailed in situ and laboratory measurements to the 

global perspective offered by satellite-based remote 

sensing. Eos will use polar-orbiting space platforms 

to provide this global perspective. It is, of course, 

recognized that information obtained from space 

must be used in concert with information obtained 

from conventional observations. 

The understanding of the global integrated 

functioning of Earth will require, in addition to integrated observation systems, a data system to ensure that information from all systems will be extensively exploited. The integration of ground-based, 

airborne, and spaceborne measurements to obtain 

quasi-instantaneous coverage (on the order of 2 

days) with broad areal, almost global extent, is the 

motivation for creating Eos. This new entity, Eos, 

will enable a comprehensive multidisciplinary approach to understanding Earth as a system. 

Eos is envisioned as a user-friendly informationgathering system with near-global coverage, augmenting to the maximum extent, existing operational systems. The implementation strategy for Eos 

is to maintain the ongoing time series of Earth science data, to broaden the measurement basis commensurate with technology achievements, and to 

demonstrate new and more powerful instrument capabilities. Through this evolutionary process, Eos 

will eventually serve all Earth science communities.

LASA SCIENCE ISSUES 

The recommendations of the Lidar Atmospheric Sounder and Altimeter (LASA) Panel discussed in this report are directed toward a series of 

atmospheric composition and structure measurements, as well as a set of surface-ranging measurements. The LASA instrument concept incorporates 

two separable remote-sensing techniques. Sensing 

of atmospheric properties is the primary role of the 

LASA instrument proper whereas measurement of 

Earth surface features is the role of a separately 

designated instrument identified as the Geodynamics Laser-Ranging System (GLRS). The LASA instrument can function as an altimeter with somewhat limited vertical resolution as compared to 

GLRS, which has the potential of achieving 10 cm 

resolution. Accordingly, the material that follows 

separates into two parts: atmospheric composition 

and structure, and surface-ranging measurements. 

The LASA instrument may operate in the ultraviolet, visible, and infrared range of the electromagnetic spectrum, and provides vertical profiles of 

water vapor, aerosols, clouds, ozone, and other atmospheric gases. Temperature and pressure profiles 

may also be determined as can locations of atmospheric discontinuities such as the top of the planetary boundary layer (PBL) and the tropopause. In 

addition, aerosol distributions can be used as tracers 

of atmospheric motions, and, using the Doppler 

technique, a direct measurement of winds is possible (see the report of the Panel on Lidar Atmospheric Wind Sounder). Laser altimetry will be used 

to observe the accumulation of ice and snow on the 

Greenland and Antarctic ice sheets and to measure 

the topography of the Earth's surface. GLRS will 

perform high-accuracy distance measurements over 

a wide range of terrestrial baselines and, by ranging 

off retrorefiectors, observe both relative motion and 

internal deformation of the Earth's lithospheric 

plates. Similarly, GLRS will also be used to study 

local scale crustal movements. 

ATMOSPHERIC STRUCTURE AND

COMPOSITION 

There are many features of the atmosphere, 

both structure and composition, that need elucidation. These features include the distribution of moisture as water vapor and as cloud droplets, characteristics of the PBL, variations in temperature and 

pressure, the distribution and nature of atmospheric 

aerosols, and the distribution of atmospheric ozone. 

All of these features can be studied by remote sensing using spaceborne lidars.
Water Vapor 

The transfer of water between its three phases, 

solid, liquid, and vapor, among the land, oceanic, 

and atmospheric reservoirs is important in energy 

transport around the Earth. The movement of water 

between the oceans, the marine and terrestrial atmosphere, the land, the subsurface reservoirs, etc., 

constitutes the global hydrological cycle. The mechanisms of the atmospheric circulation are strongly 

coupled to the components of this hydrological 

cycle. Because of the distinct features of the global 

circulation, the spatial distribution of water is highly 

variable over the globe. The hydrological cycle plays 

an important role in atmospheric chemistry, in 

weather and climate, and in the Earth radiation 

budget. 

Water vapor is an important mechanism for the 

transport of energy in the form of latent heat, primarily from tropical regions poleward. This latent 

heat is converted into sensible heat upon recondensation, primarily in the regions of mid-latitude cyclones. Latent heat release is the primary energy 

source for tropical cyclones. Thus, the vapor phase 

latent heat represents a large source of potential 

energy, and therefore, the water vapor distribution 

is key to global dynamics and circulation. 

Water vapor is of prime importance in the transfer of infrared radiation. Since water vapor is a 

highly variable atmospheric constituent that is 

highly infrared-active, its effect on radiation transfer is significant, both spatially and temporally. A 

long-term increase of water vapor, with its accompanying effects on infrared radiation transfer, could 

result in greatly enhancing the predicted long-term 

effects on climate from increasing carbon dioxide in 

the atmosphere. 

To date, measurements of water vapor have 

been primarily made from ground-based radiosonde 

stations and have generally been limited to populated land areas. Most of the precipitable water vapor is contained in the first one kilometer above the 

Earth's surface. To reasonably characterize this portion of the hydrological cycle, observations of the 

vertical profile of water vapor must be made with a 

resolution of 1 km or better for this part of the 

atmosphere. Huge gaps exist in data bases over 

oceanic and other inaccessible areas. Further, standard measurement techniques have been limited to 

the lower and mid-troposphere due to difficulties in 

determining the small amounts of water vapor in the 

upper troposphere and the stratosphere. In the 

stratosphere, the data base is extremely sparse. 

Planetary Boundary Layer 

The planetary boundary layer (PBL) determines near-surface fluxes of heat, moisture, particles, and trace gases. The PBL is that portion of the 

lower troposphere that marks the top of the convective activity near the Earth's surface. It is normally 

about 1 km thick and capped by a temperature inversion. As a result, exchange of atmospheric properties, such as heat, moisture, particles, and trace 

gases between the region above the PBL and the 

free troposphere, is greatly inhibited. The flux of 

heat and moisture from the Earth's surface to the 

lower atmosphere is largely governed by the intensity of convection within the PBL, as well as the 

thickness of this layer. Under conditions of deep 

convection, the PBL may extend through the entire 

troposphere up to the tropopause. For the most 

part, detection of the PBL is accomplished from 

analysis of upper-air radiosonde soundings, and has 

been confined to data points at standard radiosonde 

stations. This leaves large gaps in the data base and 

an incomplete description of the global PBL. To 

significantly improve our understanding of the PBL 

and its interaction with the surface and the free 

troposphere, measurements of PBL height are 

needed with a resolution of approximately 0.1 km. 

Clouds 

It is well known that many cumulus-type clouds 

under highly unstable atmospheric conditions actually penetrate the tropopause and reach into the 

lower stratosphere. The role of these clouds in the 

budget of stratospheric water vapor is presently unknown. It may be that these clouds represent a 

source of stratospheric water vapor from the troposphere through the tropopause. The existence of 

subvisible cirrus clouds has been known for some 

time. Their presence has been detected by aircraft 

and satellite observations and by inference from certain atmospheric optical effects. Little is known of 

their frequency of occurrence, their spatial distribution, and the physical and optical properties from 

which one could assess their importance on the radiation budget. 

Pressure and Temperature 

Pressure and temperature are basic to the inference of atmospheric properties from many passive remote sounders, and are crucial for interpretation of all atmospheric processes. While 

reasonably good data coverage exists over land 

masses in the northern hemisphere, the data bases 

over oceanic areas, and much of the land areas of 

the southern hemisphere, are very poor. Increased 

data coverage for these two variables is needed at a 

higher vertical resolution than the 2 to 4 km presently available. 

Aerosols 

Aerosols affect atmospheric radiative transfer 

processes, clouds, and chemistry. The recent El Chichon eruption inspired several studies on the subject 

2
of volcanic aerosols in the atmosphere. These studies indicated that, in the lower troposphere, temperature decreases on the order of 2 K could result 

from intense volcanic activity. Considerably larger 

stratospheric changes in temperature were predicted, and evidence of these effects in tropical regions was observed. Determination of the distribution and optical densities of volcanic aerosol layers 

is important. With the exception of data from lidar 

and from solar photometers, our global knowledge 

of these layers depends on existing satellite observations such as those from the Stratospheric Aerosol 

Measurement II (SAM II) and Stratospheric Aerosol and Gas Experiment (SAGE) missions. Some 

data are available on tropospheric aerosols from 

conventional balloonsonde data, but the data base 

is inadequate. 

Ozone 

The global ozone distribution needs further 

study, particularly to establish possible trends in total ozone concentrations. There is continuing concern for the possibility of ozone destruction in the 

stratosphere due to the release of chlorofluorocarbons and a satellite- and ground-based data base on 

ozone distribution is being developed. In the troposphere, ozone is thought to have increased in recent years because of increases in methane and 

other gases. In order to detect early indications of 

a trend in either the stratosphere or troposphere, a 

more detailed measurement base is required. 

Lidar Contributions Complementary to 

Passive Remote Sensing 

The experiments proposed for the LASA observations of atmospheric structure and composition 

provide many avenues of interaction with remotesensing experiments, with either currently operational or proposed Eos instruments. These interactions may be one or more of the following types: 

1. The LASA-measured data will in many instances provide a completely independent 

data set from that determined by other experimental techniques, thus allowing for a 

cross-check of results. 

2. The LASA-measured data can be used as 

input data for other experiments to assist in 

the data analysis. 

3. The LASA-measured data will provide initial solution estimates for other experiments 

requiring mathematical inversion solutions. 

Considering (1) above, the LASA experiments 

provide direct sounding information on atmospheric 

parameters, in contrast to passive techniques used 

in other experimental packages. Furthermore, the

data analysis techniques employed in the LASA experiments involve direct data reduction, in contrast 

to inversion techniques required by passive experiments. Examples include proposed experiments to 

determine vertical profiles of water vapor, temperature, and ozone. The current techniques for inferring these profiles use passive instruments, requiring inversions of the data to recover the profiles. 

The active technique proposed in the LASA package will provide independent data and direct profile 

recovery. Lidar and Differential Absorption Lidar 

(DIAL) will present independent profiles for comparison with passive sounders. In addition, LASA 

will provide, in most cases, considerably higher vertical resolution than those techniques requiring inversions. For instance, features such as cloud-top 

altitudes and PBL heights can be resolved to approximately 0.1 km or better. Water vapor profiles 

can be determined using the DIAL approach to a 

vertical resolution of approximately 1 km. 

Considering (2) above, the LASA data will be 

of considerable use in improving the data analysis 

from other experiments. For example, the tropospheric and stratospheric aerosol data obtained by 

lidar will be invaluable in removing aerosol effects 

from surface imagery and albedo measurements. As 

was the case after the eruption of El Chichon, artifacts due to the stratospheric aerosols produced by 

this eruption caused significant errors in the retrieval of sea-surface temperature and the concentrations of nitrogen dioxide, water vapor, and other 

gases obtained by existing satellite measurements. 

LASA measurements of the PBL, when combined 

with passive microwave measurements, will provide 

better information on the vertical distribution of 

water vapor. LASA measurements of cloud properties and heights, particularly for optically thin 

clouds, will assist in most passive measurements of 

atmospheric parameters, and will be a substantial 

aid in refining cloud-top heights determined by infrared passive sounders. LASA tropopause heights 

will be a great aid in infrared and microwave inversion techniques for determining temperature 

profiles. 

Most profiles determined by lidar and DIAL 

could be used as initialization information ("initial 

estimates") for experiments that fall into category 

(3). Examples would be the water vapor, temperature, and ozone profiles obtained through lidar, and 

also obtained by inversion of passive measurements. 

The lidar profiles could be used to provide "initial 

guess" profiles for the inversion algorithms used by 

the passive techniques. Aerosol profiles from LASA 

will also be used to complement the passive aerosol 

measurements from the SAM II and SAGE 

instruments.
Lidar Technology Considerations 

The LASA capabilities can be divided into two 

broad categories that are distinguished by the degree of technological difficulty anticipated in developing the instrumentation. These categories are: (1) 

existing technology with engineering improvements, 

and (2) future technology requiring continued research efforts. 

Lidar measurement capabilities using existing 

technology (only engineering improvements required) include: 

• water vapor column content 

• boundary layer height 

• relative aerosol profiles 

• cloud-top heights 

• optical depth of clouds 

With anticipated technology developments, the 

following measurements may become operational 

within the Eos era: 

• water vapor profile (lower troposphere) 

• surface pressure and pressure-altitude 

profiles 

• surface temperature and temperaturealtitude profiles 

• total aerosol optical thickness 

• total ozone column content 

• ozone profiles 

SURFACE RANGING 

Lidar is an ideal tool for very accurate measurement of distance. Laser ranging, either directly to 

the Earth's surface or to retroreflectors on the surface, can be used to accurately determine geometric 

positions. A particularly interesting application of 

satellite-borne laser surface altimetry is in its use in 

understanding ice-sheet dynamics and variability. 

The most notable application of satellite laser retroranging is in improved understanding of geodynamical processes. These applications are described 

in the two sections which follow. 

Ice-Sheet Dynamics and Variability 

At present, it is not known whether the existing 

ice sheets of Greenland and Antarctica are growing 

or shrinking. There are mounting concerns that future changes of climate may disturb the equilibrium 

of polar ice masses and hence global sea levels (Polar 

Research Board, 1984a, 1984b). Present estimates 

of mass balance, which are based on estimates of 

the mass input (accumulation rate) and mass output 

(ablation, bottom melting, and iceberg discharge), 

are probably only accurate to ± 50 percent. An uncertainty in ice-sheet mass balance of ± 50 percent 

corresponds to a sea-level change of 3 cm/decade or 

an average ice elevation change of 100 cm/decade. 

Results from the few areas that have been studied 

in detail by surface measurements indicate that 

either thickening or thinning of the local ice is the 

rule rather than the exception. Steady state appears 

to be rare. It is also not known whether the ice flow 

is inherently stable or subject to accelerated 'flow. 

Further, knowledge of how the ice sheets respond 

to climate change is rudimentary (Thomas et al., 

1979; Polar Research Board, 1984b). 

The boundaries of ice sheets and adjacent floating ice shelves are influenced by the seasonal cycle 

of temperature, radiation, precipitation, and ablation, as well as longer-term variations of atmosphere, ocean, and solar insolation. Two important 

parameters that can be uniquely observed by satellite for the study of the long-term behavior of ice 

sheets and ice shelves are changes in surface elevation and changes in the position of ice-shelf fronts. 

Other key parameters observable by satellite include 

iceberg discharge, ice velocities, extent and duration 

of summer melting, and possibly accumulation 

rates. Ice thickness has been surveyed by airborne 

radar sounders, and ongoing field projects will continue to provide detailed regional measurements of 

ice flow, internal characteristics, and ice bedrock 

interactions. Most of the ice parameters of interest 

for the study of ice dynamics should be averaged 

over at least 1 year, consistent with the slow response of glaciers to changes in boundary conditions. One exception is the extent and duration of 

surface melting in summer, which requires daily observations by passive microwave sensing for studies 

of interannual variations. 

Repetitive, precise measurement of ice-surface 

elevation by satellite altimetry (Zwally et al., 1981) 

has been recognized in recent years to be the only 

means of determining changes in ice volume, which 

is equivalent to determining the net mass balance. 

In addition, detailed repetitive profiling of the surface slope, which determines the driving force for 

the ice flow toward the sea, will provide information 

on the time-dependent behavior of the ice flow. This 

has been exceptionally difficult to obtain by conventional measurements. Therefore, repeated elevation 

surveys by satellite altimetry will provide a direct 

measure of the volume change and overall mass balance, as well as the redistribution of mass within 

the ice sheets. 

Several recent reports support, in principle, the 

approach described in this report for studying icesheet dynamics and variability. They include: The 

Polar Regions and Climatic Change (Polar Research 

Board, 1984b), Research Emphases for the U.S. 

Antarctic Program (Polar Research Board, 1983), 

and Environment of West Antarctica: Potential Carbon Dioxide-Induced Changes (Polar Research 

Board, 1984a). 

4
The required accuracy for satellite measurement of elevation changes is estimated by looking at 

the probable magnitude of thickening or thinning 

rates that might be observed and the significance of 

these changes in terms of changes in ice volume and 

sea level. The mass input per year averaged over the 

whole of Greenland and Antarctica is about 20 cm 

of ice per year, which is about one part in 1W of the 

total ice volume (average thickness is about 2,000 

m). The ratio of average elevation change to sealevel change is about 30:1 (2,000 in thickness divided by sea level equivalent of 70 m). Therefore, a 

50 percent mass imbalance would produce an average ice elevation change of 10 cm/yçar and a sealevel change of about 0.3 cm/year, a.figure somewhat 

larger than estimates of current rise in sea level. 

The accumulation rate is not uniform over the 

ice sheets; in central regions, accumulation rates are 

about 2 to 5 cm/year. Therefore, in those regions, a 

50 percent mass imbalance represents only a 1 to 2 

cm/year elevation change. To permit detection of 

this change over a 5- to 10-year period requires a 

relative measurement accuracy of about ±10 cm 

and averaging of multiple measurements over approximately 10 km2 areas. The spacing of profiles 

should be less than 5 km and the spacing of height 

measurements along each profile track should be 

about 300 in to provide the density of coverage for 

detection of elevation changes without biases or aliasing due to ice-surface undulations (predominant 

wavelengths are about 5 to 10 km). The satellite 

orbit must also reach at least 81° latitude, with 87° 

most desirable, to cover most of the Antarctic ice 

sheet and provide overlapping measurements for 

calibration, referencing of successive orbits, and validation of precision. The anticipated Eos orbit of 

98.2° will be ideally suited for this application. In 

operation, the altimeter will measure the elevation 

of the snow surface, beneath which there is a gradual transition to solid ice over a depth of 50 to 

100 m. To translate changes in surface elevation to 

changes in ice-column mass, it is assumed that the 

depth of the snow-to-ice transition and the elevation 

of the bedrock topography are invariant during the 

period of measurement. 

Ice-sheet topography has already been successfully mapped using a satellite radar altimeter 

(Brooks et al., 1978; Thomas et al., 1983). Because 

the accuracy of radar altimetry is limited by the 

rather large footprint of the radar beam (20 km 

diameter), changes, within the resolving power of 

the radar would take decades to be detected. A 

satellite laser altimeter system with a range precision of ±10 cm and footprint diameter of 70 in has 

been designed (Bufton et al., 1982). Accurate repetitive mapping of the seaward boundary of the 

floating ice shelves has also been accomplished with 

radar altimetry (Thomas etal., 1983). Other studies 

have shown the feasibility of measuring accumulation rates and summer melt zones with passive microwave images (Zwally, 1984). Ice-surface velocities can be measured by laser retroranging to 

reflectors placed on the ice sheets for studies of glacier dynamics. Measurements by altimetry, laser 

ranging, and passive and active microwave imaging 

on Eos are all complementary and will provide 

unique data for studying ice-sheet dynamics and 

variability. 

The immediate science goals and objectives for 

ice-sheet dynamics and variability studies can be 

summarized as follows: 

• determination of ice-sheet mass balance 

• studies of ice dynamics 

• investigation of stability/surging/flow changes 

Geodynamics 

Over the past 2 decades remarkable progress 

has been made in understanding geodynamic processes through the use of space techniques. Highaccuracy determinations of satellite orbits by lasertracking systems have been used to develop detailed 

representations of the Earth's gravity field. Variations in the height of the gravity equipotential surface (geoid) have been used to determine the mechanical strength of the crust and provide insight 

into the dynamics of tectonic plate motion. Insight 

into the processes responsible for mountain building 

and convection within the mantle has been developed by the synergistic merger of space and ground 

gravity and geodetic data with seismic, geological, 

and thermal information. Determination of wholeEarth tides, of polar motion, and variations in the 

length of day have all benefited from centimeteraccuracy laser tracking of satellites. Most importantly, laser tracking of satellites has permitted the 

determination of distances between sites to centimeter accuracies. Centimeter-level changes in baseline lengths for distances from a few tens of kilometers to several thousand kilometers are being 

used to monitor the motion of tectonic plates and to 

study crustal deformations. In addition, measurements of regional-scale crustal movements are providing important data for modeling strain accumulation in regions of past and future large 

earthquakes. Indeed, within the last few decades, 

the study of crustal dynamic processes has occupied 

one of the most prominent places within solid-Earth 

geophysics research. The theory of plate tectonics; 

in which the mechanically strong layers of the crust 

and uppermost mantle are fractured into a limited 

number of moving, relatively rigid units (lithospheric or tectonic plates), has provided a unifying 

geodynamic theory. Plate tectonics explains the geography of continents and oceans, the occurrence 

of earthquakes, volcanoes, and mountains, and the 

distribution and evolution of various rock types and 

geologic features. Tectonic plates move relative to 

one another at velocities of several centimeters per
year. The resistance to this motion at the contact 

points between plates causes stress to accumulate, 

which may subsequently be relieved by earthquakes. 

Although tectonic plates are regarded as being 

relatively rigid within the context of first-order tectonic plate theory, significant internal deformations 

may occur. Strain related to stress accumulation on 

faults is important both at the boundaries between 

major plates and within certain plate interiors. Over 

geologically significant time spans, new tectonic 

plate boundaries are created and destroyed by plate 

rifting and suturing. The tectonic processes produce 

vertical as well as horizontal motions. Compressional folding and/or fault-related block uplift are 

two phenomena involved in mountain development 

within most major chains. 

Tectonic plate motions, regional crustal deformations, fault-related strain accumulation, and 

other tectonic processes produce crustal movements 

of up to several centimeters per year. The fundamental scientific issues are to determine the contemporary rates for these motions, determine their spatial and temporal variations, and to develop models 

that explain these phenomena in terms of basic physical processes occurring in the solid Earth. The data 

required to carry out this task are the high-accuracy 

determinations of the relative motion between various points on the Earth's surface separated by tens 

to thousands of kilometers. The data must be obtained with improved temporal and spatial resolution unobtainable from ground-based observations. 

A unique geodetic surveying capability is provided 

by laser ranging from an Eos platform to cube-corner reflectors on the ground via GLRS. 

The basic experiment involves a time-of-flight 

measurement of the distance between Eos and the 

ground targets. These data are used to determine 

the positions of the survey sites. The motions are 

deduced by resurveying the network at a later date 

to observe changes in the target site locations. As 

discussed in later sections of this report, laser ranging from ground-based observatories to satellite reflectors has already provided significant information 

on crustal dynamic properties. In the Eos concept, 

the sophisticated laser-ranging and optical-tracking 

equipment would be located on the satellite. This 

would permit orders-of-magnitude-greater density 

of sites than is possible with current techniques at 

selected locales throughout the world. In combination with the ground-based systems, this system 

would provide a truly global, centimeter-accuracy, 

geodetic network. 

Thus, the operation of a laser-ranging instrument from an Eos platform to ground-based retroreflectors permits dramatic improvements in the 

temporal and spatial resolution with which geodynamic processes can be studied. The measurements 

would expand current capabilities to measure tectonic plate motion and provide unique opportunities 

to search for time variations in the plate velocities.

Such a system could also provide a capability to 

observe the precursory geodetic motions believed to 

occur before the onset of large earthquakes and, in 

a broad sense, provide significant information on 

the evolution of the Earth and other planets. 

The retroreflectors can also be attached to ice 

sheets to monitor their horizontal and vertical movements and complement measurements relating to 

the ice-sheet dynamics discussed earlier in this 

report. 

Height determinations may also be made by operating the laser-ranging system in an altimetric 

mode without cube-corner retroreflector targets. 

Terrain topographic data can be correlated with 

gravity observations to determine how the lithosphere flexes under mountain belts. Flexure properties are useful in understanding the mechanical 

state and evolution of the lithosphere. Additionally, 

mapping of surface slopes can provide important 

input to hydrologic studies of rain runoff, and detailed mapping of height distributions within selected physiographic provinces is useful in both geological and geophysical studies. Retroranging data 

can be used to refine the orbital ephemeris of the 

Eos platform to the centimeter level. Since a highly 

accurate prediction of the Eos platform position can 

be made with gravity field models for some period 

of time following retroranging, few centimeteraccuracy measurements of the marine geoid or 

height of the underlying terrain may be possible if 

the LASA retroreflector instrument (GLRS) is operated in the altimetry mode or in conjunction with 

the higher-power, large-aperture LASA altimeter. 

This would lead to a further improvement in our 

knowledge of the Earth's gravitational field and of 

the mechanisms of support and gravity compensation for topographic features. 

Specifically, the spaceborne laser-ranging system would build up a data set for studying the following fundamental geodynamic questions: 

What are the spatial distributions in crustal 

movement near various seismic zones, and 

how do the patterns change with distance into 

the tectonic plate interiors? 

In this regard, detailed information on the pattern of strain provides insight into the forces responsible for crustal movement and, in turn, insight 

into the mechanism for transfer of stress between 

plate boundaries and interiors. This information is 

useful in modeling the mechanical structural properties of the Earth as they vary from one region to 

another. 

Simultaneous surveying of a grid of cube-corner 

retroreflectors separated by roughly 50 km in a network with dimensions of several hundred kilometers 

would provide insight into earthquake mechanisms. 

In combination with ground observations within a 

few tens of kilometers about a fault zone the data 

would provide insight into the failure mechanisms 

6
associated with earthquake occurrences and how aftershocks occur, and would provide more detailed 

understanding of how an earthquake relieves and 

redistributes tectonic stress. The detection of precursory crustal movements offer the potential for 

aiding earthquake prediction. 

• What are the patterns of internal crustal deformations within tectonic plate interiors? 

In discussing this point, we note that although 

first-order tectonic plate theory regards the plates 

as rigid units, significant internal deformations exist 

in many locations. Studies of the patterns of crustal 

movements over baselines of several hundred kilometers in North America, places in Eurasia, and 

elsewhere would provide information on the contemporary rates of these internal deformations. 

These would be used in studies of the mechanisms 

responsible for intraplate deformation. 

Altimetric measurements are important for tectonophysics studies over both continental and 

oceanic regions. Terrain profile data, for example, 

can be used in studies of the mechanisms of support 

for topographic features. Studies correlating topography and gravity are basic to understanding how 

the Earth's lithosphere bends or supports crustal 

loads associated with mountain belts, islands, etc. 

Such studies provide information on the thickness 

of the lithosphere and its strength, on the distribution of density of rock within the crust and upper 

mantle, and insight into the evolution of the crust. 

Over continental regions with significant topographic relief, measurements with a spatial resolution of a few hundred meters would provide a valuable data set for these studies. Measurement 

accuracy of a few meters is sufficient for geodynamic 

applications over land. Over oceans, an accuracy of 

about 1 cm is required for accurately mapping the 

ocean geoid and determining ocean circulation 

parameters. 

The measurements with the GLRS instrument 

are complementary to those made with other techniques. The crustal movement measurements provide a densification of the existing network of 

ground-based laser and very long baseline interferometry (VLBI) sites. They would extend well beyond the capabilities of conventional local surveys, 

which are very labor-intensive and of limited utility 

in mountainous regions. The networks should be 

designed to complement rather than compete with 

those that will be surveyed with highly mobile receivers operating on microwave signals from the 

Global Positioning System (GPS) satellites. The altimetric measurements would provide high-resolution topographic mapping to complement the 

coarser but more global coverage available from microwave altimeters. Gravity and magnetic data 

available from current sources and the planned 

Geopotential Research Mission (GRM) satellite can 

be used in combination with these data to provide 

a more complete picture of the geodynamics of the 

Earth than could be inferred from either data set 

taken by itself.
II. LASA SCIENCE: ATMOSPHERIC SOUNDING 

LIDAR TECHNIQUES 

Lidar sounding is a remote-sensing technique to 

measure profiles of atmospheric backscatter from 

aerosols and molecules and extinction properties of 

the atmosphere with an active laser transmitter. The 

lidar signal return (power) from a range Z is given 

by:

- EA3cT

2Z 

where E is the energy per pulse of the laser source, 

A is the collection area of the receiving telescope, 

1 is the overall efficiency of the receiving system, 3 

is the atmospheric volume backscattering coefficient, c is the speed of light, and T is the two-way 

atmospheric transmission from the transmitter to 

the range Z. 

In the simplest form, a lidar system consists of 

three basic components: (1) the laser transmitter as 

a source of energy, (2) a suitable telescopic receiver 

to collect backscattered light, and (3) a detector 

system to produce an electrical signal proportional 

to the received power. In general, the transmitter 

and detector systems are collocated. The wavelength range of lidar operation is normally from 0.2 

to 10 I1.m, with either pulsed or continuous operation of the laser transmitter. 

Active laser sensors, such as lidar, offer the possibility of significantly enhanced vertical resolution 

compared with that typically achieved with passive 

sounders. Time gating of the detector signal permits 

a vertical resolution limited ultimately by the laser 

pulse length, detector sensitivity, and background 

sources of light. Since lidar provides its own light 

source, it can perform measurements under both 

daytime and nighttime conditions. 

Lidar operates by detecting laser radiation 

transmitted through the atmosphere and backscattered from the atmosphere itself, or from surfaces 

such as clouds or the ground. Developments in laser 

atmospheric sounding have reached the point where 

there have been ground-based measurements (for 

the past 20 years) and aircraft-based measurements 

(for the past 10 years) of many important atmospheric properties, including clouds, aerosols, temperature, ozone, and water vapor distributions. 

Because of the global coverage provided by the 

Eos platform, LASA can be utilized to investigate 

long-range transport of haze layers, aerosols, and 

several atmospheric species, tropospheric/stratospheric exchange processes, boundary layer properties, and free tropospheric conditions. 

In the following sections, the science objectives 

and the experimental approaches associated with 

each lidar measurement technique are described.

The techniques include those associated with backscatter lidar for measuring cloud-top heights, PBL, 

tropospheric and stratospheric aerosols, the DIAL 

techniques associated with the measurement of 

water vapor and ozone concentrations, and atmospheric pressure and temperature measurements. It 

is emphasized that each measurement category complements the other. For example, with the backscatter lidar technique, cloud-top heights, PBL, and 

tropospheric and stratospheric aerosol profiles can 

be measured simultaneously, and, with a pulsed 

DIAL system, these measurements can still be obtained but also profiles of gaseous species, temperature, and pressure can be obtained. In addition, 

the total column content of gaseous species can be 

determined. 

Differential Absorption Lidar Technique 

In general, the Differential Absorption Lidar 

(DIAL) technique depends on the existence of a 

spectral feature (absorption line) in the atmosphere 

that is specific to a gas molecule, and whose spectroscopic characteristics are well known (i.e., line 

intensity, line position, and line broadening as a 

function of temperature and pressure). The absorption line must be free of spectroscopic interference 

from other molecules. In order to detect the molecular feature, the wavelength of the laser is tuned to 

overlap the absorption feature, preferably at the 

center of the line. A second laser wavelength is required, whose wavelength is shifted away from the 

molecular feature far enough to minimize molecular 

absorption, but sufficiently close so that the scattering properties of the atmosphere are virtually identical at the two wavelengths. The two laser wavelengths are pulsed and can be fired within a time 

spacing that is short compared to the time scale of 

atmospheric dynamics, i.e., <100 ps. The two laser 

pulses are backscattered to the receiver telescope 

by atmospheric aerosols and molecules, so that the 

return signals represent a time history of the scattering and absorption properties of the atmosphere 

for each laser-pulse pair. This time history is related 

to a spatial profile of these properties through the 

equation (czt)/2 = R, where At represents a time 

interval that is selected to correspond to a specific 

range interval, AR. The ratio of the return signal 

P0,,, at the 'on' wavelength to the return signal, off, 

at the 'off , is measured. Through the 

lidar equation, this ratio can be related to the molecular concentration of the gas in the atmosphere 

as a function of distance from the transmitter. Another useful mode for a DIAL experiment is to measure column content by employing a pulsed or continuous wave (cw) laser and using reflection from 

the ground (or from layers such as clouds) to return 

8
the two laser wavelengths to the receiving telescope. 

In the infrared, pressure broadening of atmospheric 

spectral lines as a function of altitude is large relative to the ultraviolet (UV) and visible portion of 

the spectrum. In this wavelength region one can 

selectively probe various segments of the absorption 

line by tuning the wavelength of the continuously 

transmitting laser from the central peak into the 

wings of the absorption line to determine the vertical layering of the selected molecule. Conceptually, 

this is similar to passive solar-reflection techniques. 

In the case of DIAL, however, the source of radiation is monochromatic, allowing a single absorption 

line to be probed. 

Generically, lidar (including DIAL) measurements from space have three distinct advantages: 

(1) the capability to probe the troposphere between 

the clouds with minimal signal loss from low-level 

clouds and haze layers, (2) increasing signal strength 

with range from the top of the atmosphere down, 

due to the exponentially increasing atmospheric 

scattering, and (3) high horizontal spatial resolution 

with very good vertical resolution when used in the 

pulsed mode. 

CLOUD OBSERVATIONS 

Clouds are critical to the flow of energy and the 

cycling of water and trace species through the atmosphere. The climatic effects of changes in cloud 

cover are not well understood. 

Cloud cover reduces solar input to the surface 

of the Earth through reflection of solar radiation, 

but cloud cover also reduces the infrared output 

from the surface, which is an opposing process. The 

net radiant flux change is a function of the density 

and temperature, or height, of the cloud. For cirrus, 

frontal clouds, or high cumulonimbus, the infrared 

process may dominate. Only direct observations, 

which include lidar height measurements, are likely 

to adequately resolve the actual radiative effects of 

these clouds. 

A principal limitation to current understanding 

and modeling of the physical climate system is the 

inability to correctly account for the role of clouds. 

Better predictive cloud models are required for 

studies such as the influence of anthropogenic carbon dioxide on climate. Improved cloud climatologies are possible from a thorough study of the current satellite data base. However, existing 

observations from passive sensors do not adequately 

describe all cloud characteristics. The important 

cloud-height measurement is only a roughly inferred 

quantity, and optically thin clouds are poorly resolved. Future improvements can come from active 

sensors such as lidar. The fundamental measurement objective for LASA in cloud studies is to unambiguously describe the vertical structure of the 

cloud field. Passive techniques such as infrared imagery are inadequate for cirrus clouds where the 

emissivity is very much less than one. Even more 

advanced methods, such as carbon dioxide slicing, 

can obtain height resolutions to no better than several kilometers. 

In addition to the cirrus problem discussed 

above, there are other direct applications that may 

be mentioned. The persistence and height of marine 

stratus are readily obtained from lidar measurements. Additional applications involving advanced 

analysis of lidar returns for cloud microphysics are 

possible. These would be important for cloud radiation parameterization. 

Measurement Approach 

For clouds, the basic lidar measurement approach will be to obtain vertical lidar backscattering 

profiles at a single wavelength. In addition, the polarization characteristics of the lidar pulse can be 

used to determine the thermodynamic phase of hydrometeors in the clouds. The transmitted laser 

pulse will be polarized. The polarization state of the 

backscattered signal will depend on the scattering 

properties of the medium. Precision backscatter 

measurements require detection of the complete polarization state. At a minimum, the. polarization, 

parallel and perpendicular to the polarization of the 

transmitted pulse, must be detected. From these 

measurements the degree of polarization or depolarization, can be inferred. The latter quantity, depolarization, can be used to determine the presence 

of ice and/or liquid particles in the cloud. 

Since clouds are highly inhomogeneous, the 

vertical structure information should be obtained 

with useful accuracy from single laser pulses at horizontal resolutions of less than 1 km. The most basic 

and easily obtained observation for clouds is the 

height of the cloud top. The existence of multiple 

cloud layers may be determined from lidar return 

profiles under certain conditions. 

In addition to cloud-top height, a number of 

other significant cloud parameters are derivable 

from the lidar signal return. The backscatter signal 

from clouds is a function of the single scattering 

backscatter coefficient within the cloud, the attenuation above and within the cloud, and any addition 

to the signal from multiple scattering (Sassen, 1980; 

Spinhirne et al., 1983). For dense clouds such as 

stratus, strong signal returns are received from the 

cloud top, but the signal is attenuated at fractional 

distances into the cloud and multiple scattering is 

significant. The parameters which may be obtained 

for stratus clouds are the cloud-top scattering coefficients and an inference of the water/ice phase of 

the cloud top from signal depolarization (Pal and 

Carswell, 1978). 

In the case of optically thin cirrus clouds, the 

importance of lidar for retrieval of significant cloud 

parameters has been demonstrated by ground-based 

9
18 

15 

12 

E 

LU

I-I 0 

'6 

and aircraft observations (Platt and Dilley, 1979; 

Platt and Dilley, 1981; Spinhirne et al., 1982). For 

cirrus, lidar signal returns may typically be obtained 

through the entire cloud layer, and multiple scattering may be taken into account. The cirruarameters which may be measured from lidar returns include the optical thickness and emissivity of cirrus 

layers, the presence of super-cooled droplets as a 

component of high, thin clouds, and the presence 

of subvisible cirrus. The presence, height, optical 

thickness, and relative emissivity of cirrus are possibly the most significant cloud observations that 

would result from a spaceborne lidar observation. 

Instrument Characteristics 

The various cloud measurement objectives 

listed above give rise to a broad range of instrument 

characteristics required for spaceborne observations 

of clouds. The system requirements to detect the 

height of dense clouds are modest. Next in order of 

measurement difficulty are the less-dense cirrus 

clouds and most difficult are the subvisible cirrus. 

Simulations of cirrus parameter retrievals have been 

run for selected lidar system parameters as listed in 

Table 1.

'

RELATIVE SIGNAL 

WITHOUT NOISE 

SNR 

'

\

4, 

[ci 

0.01 0.10 1.00 10.0

RELATIVE

SIGNAL-TO-NOISE RATIO (SNR) 

Figure 1. A modeled lidar return signal for the lidar

system described in Table 1. 

Table 1. System Parameters for Cloud Simulations 

Day Night 

System Altitude (km) 700 700 

Receiver Clear Diameter (m) 1.0 1.0 

Pulse Energy (J) 0.5 0.5 

Wavelength (nm) 532 532 

Detector Quantum Efficiency (%) 20 20 

System Optical Efficiency (%) 25 12 

Optical Bandwidth (nm) 0.2 1.0 

Field-of-View (mrad) 0.2 2.5 

Vertical Sampling Resolution (m) 30 30 

Background Light Intensity (W/m2-sr-nm) 0.46 5 x 10

Simulations were run for a model cirrus cloud 

of optical thickness 0.2 and physical thickness 1.0 

km to represent a thin but visible cloud. The background light intensity would correspond to cirrus 

over a bright, lower cloud layer. The model atmosphere included scattering by molecules and a background aerosol distribution with a boundary layer 

enhancement. 

A modeled lidar return signal for the system 

parameters given in Table 1 is shown in Figure 1. 

The signal return has been modeled to include the 

expected signal noise that would exist for a singleshot return. As mentioned above, for cloud parameters it is desirable that measurements be obtained 

from a single laser return without recourse to averaging. From the signal return in Figure 1, it is seen 

that a single pulse return would be sufficient to define the presence and height of the thin cirrus layer. 

The simulated signals were applied in a solution 

procedure that had been developed to derive the 

effective optical thickness of cirrus from lidar return 

data. The results for effective optical thickness may 

be presented as the number of pulses that must be 

averaged to achieve a given measurement accuracy. 

Results are given in Table 2. 

In summary, for the most part, cloud parameters of interest for visibly evident clouds could be 

quantified to useful accuracies on a single-shot basis, with the proposed spaceborne system. 

Measurement Example 

An example of lidar return data for clouds is 

shown in Figure 2. Signal returns from both cirrus 

and an underlying stratus cloud layer are shown. 

10 
0

0 20 40 60 80 100 120

IN 

FJ 

We 

50 

III 

E 

II 0 

u-I 

I

ORIGINALPW 

OF pOOR QUA11D 

DISTANCE (km) 

Figure 2. Example of lidar returns from both cirrus and an underlying stratus layer. 

Table 2. Number of Pulses That Must

be Averaged for Cloud Observations 

Day Night 

Cloud Presence 

Cloud-Top Height 

Cloud Backscatter Coefficient* 

Cirrus Optical Thickness* 

*10 percent accuracy 

The data were obtained from a lidar system operating from a high-altitude (19 km) aircraft (Spinhirne et al., 1982). The cirrus structure throughout 

the cloud layer is directly evident through the weak 

signal return, whereas a much stronger but rapidly 

attenuated signal is received from stratus. The signal measurement accuracies for the data shown are 

similar, on a single-shot basis, to those for the spaceborne system described previously. 

PLANETARY BOUNDARY LAYER

OBSERVATIONS 

Measurements of PBL height on a global basis, 

especially over the oceans and in the Southern Hemisphere, would provide the foundation for studying 

the impact of the PBL and its characteristics on 

global circulation, storm development, and climate. 

The incorporation of PBL depth data into general 

circulation models (GCM) would open opportunities to both initialize and verify the PBL portion of 

the models. The results of such tests would assist in 

establishing the validity of basic assumptions used 

in the development of climate and weather models. 

The height of the PBL is also crucial in efforts to 

infer surface fluxes of energy and water vapor from 

remotely sensed data. To adequately support these 

objectives, the LASA instrument should obtain the 

PBL depth with a vertical resolution of 50 m over a 

horizontal scale of 50 km. 

Measurement Approach 

A convectively mixed PBL, which by definition 

is coupled to the surface, is characterized by a high 

concentration of aerosols and moisture. Generally, 

both the aerosol and moisture content of the free 

atmosphere are quite low, resulting in a sharp contrast in aerosols at the PBL top. LASA measurements of the height of this aerosol contrast can be 

interpreted to infer PBL depth. Continuous measurements of the PBL depth for each Eos orbit will 

provide a unique data set. 

11
Measurement Simulations 

PBL observations can be obtained from a basic 

backscatter lidar system with sufficient sensitivity to 

resolve aerosol scattering. The characteristics of 

four lidar systems that could be used for PBL observations are given in Table 3. Basically, they consist of a single-wavelength system incorporating a 

Nd:YAG doubled laser (X = 0.532 p.m). 

Measurement simulations were performed utilizing the four lidar configurations shown in Table 

3, along with a model atmosphere consisting of a 

U.S. Standard Atmosphere molecular profile with 

a simulated PBL whose aerosol content is equivalent 

to an optical depth of 0.05. The aerosol content in 

the PBL was assumed to be constant up to 750 m. 

Between this height and 1 km, it was assumed that 

the aerosol content would fall to zero linearly. An 

example of the expected lidar return without noise 

is shown in Figure 3. 

Figure 4 shows the results of the simulations. 

The curves in Figure 4 for lidar configurations (A) 

through (D), as described in Table 3, respectively,

220 

200 

180 

160 

z 

0 140 

I-

> 100 

UJ 80 

60 

40 

20 

(1

0 2 4 6 8 10 12 14 16 18 20

HORIZONTAL RESOLUTION (km) 

Figure 4. Root-mean-square (rms) deviation of the retrieved PBL height for satellite simulated data. Calculations performed for: A = 532 nm, PBL depth = 875 m, 

and PBL optical depth = 0.05, daytime. Laser output 

energies and receiver apertures were as follows: A - 1.0 

J, 1.0 m; B - 1.0 J, 0.5 m; C - 0.1 J, 1.0 m; D - 0.1 J, 

0.5 m.

C 

Table 3. Lidar System Configurations Used in PBL Simulations 

Laser Wavelength (nm) 

Laser Output Energy (J) 

Receiver Aperture (m) 

Quantum Efficiency (%) 

Optical Efficiency (%) 

Telescope Field-of-View (mrad) 

Digitization Rate (MHz) 

Background 

A B C D 

532 532 532 532 

1.0 1.0 0.1 0.1 

1.0 0.5 1.0 0.5 

20 20 20 20 

12 12 12 12 

0.2 0.2 0.2 0.2 

10 10 10 10 

ocean ocean ocean ocean

12

BOUNDARY LAYER 

IL,] LIDAR SIGNAL 

WITHOUT NOISE 

E8 

H6 I (

W4 I

0 ! 1 1 I 

0.01 0.10 1.00 10.00

NORMALIZED SIGNAL 

Figure 3. Expected lidar return showing the effect of

aerosols trapped within the PBL. 

show the effect of averaging lidar returns as the 

spacecraft moves. Curve A, a simulation for the 

largest lidar system, indicates that for each lidar 

return (0.7 km horizontal resolution) a measure of

PBL height can be obtained to within ± 35 m. Curve 

B shows the effect of reducing the receiver aperture 

to 0.5 m. With a 0.5 in aperture, two lidar returns 

(1.4 km horizontal resolution) must be averaged to 

obtain 50 in vertical resolution. Curve C shows the 

effect of reducing the laser output energy to 0.1 J. 

With 0.1 J laser output, 15 lidar returns (10 km 

horizontal resolution) must be averaged to obtain a 

PBL height certainty of ± 50 m. Curve D represents 

the simulation for the smallest lidar system. A 

±50 in measurement of the PBL requires lidar return averaging over a horizontal distance of 21.7 km. 

In summary, the smallest system (0.1 J laser output 

and 0.5 in aperture) can more than meet the measurement objectives of 50 in vertical resolution with 

50 km horizontal averaging. 

Planetary Boundary Layer Measurement Example

PBL observations by ground-based and airborne lidar have become an established method of

boundary layer research. Figure 5 shows a measure 

12 
E 

I 

w 

I

ORIGJNAL PAC 3 

OF POOR QU<Y 

DISTANCE (km) 

Figure 5. Lidar observations of PBL structure over the Atlantic Ocean (Melfi et al., 1985). 

of the structure in a developing PBL over the Atlantic Ocean during a cold arctic air outbreak in January 1983 (Melfi etal., 1985). The data were acquired 

with a downward-looking lidar system mounted in 

the NASA Electra aircraft. The image, which was 

constructed by analyzing thousands of lidar returns, 

shows the sharp gradient in aerosol scattering associated with the PBL top. Also shown is the cellular 

convective structure that is a manifestation of strong 

PBL mixing. 

TROPOSPHERIC AEROSOL

OBSERVATIONS 

Measurements of aerosols in the troposphere 

contribute to the understanding of atmospheric 

structure and dynamics and radiative budgets. Aerosol distributions can provide valuable insight into 

vertical and horizontal transport processes in the 

atmosphere. Convective cloud activity can transport 

gases and aerosols from the boundary layer into the 

free troposphere, and from the troposphere into the 

stratosphere. Aerosols can be used as a tracer of 

this vertical transport, which can occur over horizontal scales from 1 to 1,000 km. Downward transport of stratospheric air into the troposphere has 

been followed using aerosols as a tracer in investigations of Tropopause Fold Events (TFE). Since 

there are many TFEs occurring simultaneously over 

the globe, the global impact of these events could 

be studied for the first time with LASA. 

Long-range transport of gases and aerosols over 

100 to 10,000 km ranges can take place in the free 

troposphere under undisturbed conditions. LASA 

measurements of aerosols in these layers can provide information about the large-scale air masses 

and their impact on the global troposphere. Because 

aerosols efficiently scatter solar radiation, they must 

be included in any complete study of the global radiation budget. The presence of aerosol layers in 

the troposphere can also modify measurements of 

atmospheric emission in the infrared region of the 

spectrum and deleteriously affect various remote 

sensors. A secondary consequence of tropospheric

aerosols is their effect on the microphysics of cloud 

formation and cloud radiative properties. 

The objectives for the LASA measurement of 

tropospheric aerosols are to detect the presence of 

aerosol layers in the free troposphere and boundary 

layer and to determine the optical depth of aerosols 

to the surface. Since the PBL is typically 1 km in 

depth, vertical resolution should be 0.1 km in the 

PBL and approximately 1 km in the free troposphere. The aerosol optical depth of the troposphere including the PBL is typically 0.1 to 0.2 and 

should be measured with a resolution approaching 

0.01. 

Measurement Approach 

A basic backscatter aerosol lidar system may be 

used to detect the distribution of aerosols in the free 

troposphere and boundary layer. By use of analysis 

techniques that may involve dual wavelengths or 

other techniques, the aerosol extinction coefficients 

and optical thicknesses may also be derived. 

Measurement Simulations 

A lidar system of sufficient power and aperture 

to provide signal returns that resolve aerosol scattering from background noise is required. Parameters of a lidar system that could provide the necessary accuracy are listed in Table 4. 

Simulations were performed to demonstrate the 

feasibility of tropospheric aerosol measurements. 

Table 5 presents the number of lidar shots that 

would be necessary to detect with 95 percent confidence a change in the aerosol backscatter ratio at 

various levels for two altitude ranges and under day 

and night conditions. The detectability criterion is 

given in terms of the signals from a background 

region (Sb) adjacent to the aerosol layer under investigation and from an aerosol plus background 

layer (Sa±b) and their associated signal statistics (ab 

and (ra*b, respectively). Specifically, the 95 percent 

detectability criterion is: ( Sa+b - Sb) > 3(0 a, b2 + 

crb ). A ground reflectance of 0.3 was used in the 

daytime case to simulate the background solar radiation reflected from vegetation, which exceeds the 

13
Table 4. Aerosol Lidar Parameters 

Wavelength 727 nm 

Pulse Energy 500 mJ 

Pulse Rate 10 Hz 

Assumed Lidar Altitude 700 km 

Area of Receiver (active) 1.08 m2 

FOV 0.1 mrad 

Night* Day** 

Filters Broadband 1 A 0.1 A 

Optical Efficiency 60% 32% 29% 

Quantum Efficiency 20% 20% 20%

*With interference filter 

**With Fabry-Perot etalon 

Table 5. Number of Shots Required to Detect Tropospheric Aerosols

(No Cloud Cover)* 

Atmospheric Boundary Layer Mid-Troposphere 

Backscatter Aerosols (0.5 km alt.) Aerosols (5.5 km alt.)

Ratio** 

Ila+b'flb Night Day*** Night Day*** 

2 1 3 2 10 

4 1 1 1 2 

6 1 1 1 1 

* Detectability of aerosols at 727 nm assuming 500 mJ/shot with no cloud cover and detectability criterion: 

(Sall - Sb) > 3(o.*b2 + 0b). 

**Pb is the total atmospheric backscatter coefficient of the background aerosol and 13.*b is that of the aerosol region to be detected. 

***A 0.1 A day filter and a ground albedo of 0.3 were used. 

values for loam (0.2) and the ocean surface (0.03). 

The values listed in the table are for the case of no 

cloud cover. 

It can be seen that even for the daytime case in 

the middle troposphere, an aerosol layer having a 

factor of two enhanced backscatter over the background level can be detected within 1 sec (10 shots 

or 8 km horizontal resolution). Small changes in the 

aerosol scattering can be detected with little horizontal averaging at the top of the boundary layer. 

The factor of I3a±tIIb = 2 at 0.5 km corresponds to 

a visibility of 12 km in the boundary layer compared 

to 23 km above it. These are not unreasonable values for an unpolluted rural atmosphere. Table 6, 

using the same detectability criterion as in the nocloud case, presents the results for the condition of 

a cloud cover having a one-way optical depth of 0.5. 

This is comparable to the characteristics of a moderate cirrus cloud condition. Nighttime measurements are degraded from the no-cloud case by about 

a factor of two, while the daytime measurements are 

degraded by about a factor of four. The increased 

daytime degradation is to be expected because of

the higher background levels. Even under these adverse conditions, thin aerosol layers can be detected 

with reasonable horizontal resolutions (<32 km). 

Measurement Example 

Aerosol profiles in the free troposphere can be 

used to indicate the presence of stable layers, and 

these layers can be investigated in long-range transport studies (Browell et al., 1983; Browell et al., 

1985; Shipley etal., 1984). Figure 6 shows a Saharan 

dust layer aloft at about 3 km near Barbados on 

June 21, 1984 (Talbot et al., 1986) (The data were 

taken from the NASA Electra aircraft with a downlooking lidar.) This layer was transported from the 

African continent over a period of 8 days. The marine boundary layer and some scattered clouds can 

also be seen in the figure. 

Figure 7 shows another example of lidar measurements of elevated aerosol layers made from the 

NASA Electra aircraft. In this figure, a tongue of 

stratospheric-aerosol-laden air in a tropopause fold 

is shown in the lower troposphere over the Western 

United States (Browell et al., 1986). The layer of 

stratospheric origin became perturbed by the 

14 
Table 6. Number of Shots Required to Detect Tropospheric Aerosols

(With Cloud Cover)* 

Atmospheric Boundary Layer Mid-Troposphere 

Backscatter Aerosols (0.5 km alt.) Aerosols (5.5 km alt.)

Ratio** 

I3+b/f3b Night Day*** Night Day*** 

2 2 9 5 43 

4 1 2 1 6 

6 1 1 1 3 

8 1 1 1 2 

10 1 1 1 1 

* Detectability of aerosols at 727 nm assuming 500 mJ/shot with a cloud optical depth of = 0.5, and detectability criterion as in 

Table 5. 

b is the total atmospheric backscatter coefficient of the background aerosol and *b is that of the aerosol region to be detected 

***A 0.1 A day filter and a ground albedo of 0.3 were used. 

boundary layer and eventually was fumigated into 

the boundary layer. 

STRATOSPHERIC AEROSOL

OBSERVATIONS 

Detailed information on the global distribution 

of stratospheric aerosols can directly contribute to 

studies of stratospheric motions and to radiative 

model calculations. The analytical techniques for 

these stratospheric aerosol measurements have been 

well developed during 20 years of both groundbased and aircraft-based experiments. Spaceborne 

lidar measurements of stratospheric aerosols can 

provide better horizontal and vertical resolution 

than passive sensors for some aerosol loading conditions, and can provide much more information per 

orbit of the spacecraft. Simultaneous tropospheric 

measurements are useful for subsequent stratospheric/tropospheric exchange studies. The capability of these lidar measurements presents unique opportunities for measuring transient atmospheric 

events such as volcanic plumes and noctilucent and 

polar stratospheric clouds. In addition, lidar data 

obtained after volcanic eruptions can greatly aid in 

assessing aerosol effects on other remote sensors. 

For example, the large loading caused by the eruptions of El Chichon was shown to affect many spacecraft remote sensors (Bandeen and Fraser, 1982). 

Finally, a spaceborne lidar will greatly enhance our 

ability to monitor events that occur in otherwise 

inaccessible regions of the globe. 

The objective is to determine stratospheric aerosol layering characteristics by measuring backscatter profiles at a minimum of two wavelengths and 

possibly two polarizations over each spacecraft orbit, with at least a 300-to-500 km horizontal and 

2 km vertical resolution under background aerosol 

loading conditions. For special conditions, like those 

after a recent volcanic eruption, or for the study of

stratospheric clouds and dynamical phenomena, the 

horizontal resolution will be reduced to about 100 

km. These profiles will be used to describe aerosol 

amounts and distributions in the stratosphere, developing a global aerosol climatology. Studies using 

these data will include, as examples, aerosol effects 

on atmospheric temperature fields, aerosol potential 

for climate perturbation, heterogeneous surface 

chemistry, aerosol effects on photochemistry, and 

the use of aerosols as tracers for transport and circulation studies. In addition, the measurement objectives will include the investigation of stratospheric clouds (McCormick et al., 1982). 

Satellite-based lidar measurements will provide 

a greatly expanded opportunity for global sampling 

and geographic mapping of stratospheric aerosol 

spatial and temporal distributions. Measurements 

would be performed primarily in nighttime viewing 

conditions, but daytime measurements could also be 

obtained with longer integration times (and concomitant coarser horizontal resolution). Obviously, 

a spaceborne lidar instrument would be able to respond in sufficient time to make successful measurements of transient atmospheric events like volcanic eruptions. These measurements would permit 

detailed mapping of the volcanic plumes and subsequent studies of their transport and diffusion over 

time. Such measurements would greatly enhance 

the available data base for global climate and circulation studies. In addition, unique events such as 

the seasonal occurrence of polar stratospheric 

clouds (PSC) in the arctic and antarctic regions 

could be successfully monitored with a spaceborne 

lidar aerosol measurement system. The uniqueness 

of these PSCs has been documented by observations 

with the SAM II satellite experiment and two airborne lidar missions to the arctic region. Active lidar measurements from space would provide greatly 

enhanced opportunities for a more complete mapping of the occurrence of these unusual high-altitude clouds. In addition, they would allow much 

15
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WEST LONGITUDE 

ELECTRA LOCATION 

Figure 7. Lidar observations of aerosols in a tropopause fold event located over the Western U.S. 

more complete studies of the inaccessible antarctic 

region. 

Measurement Approach 

The measurement technique utilizes elastic 

backscattering of the transmitted lidar signal, which 

is highly sensitive to the spatial distribution of stratospheric aerosols. Coordinated spaceborne lidar 

measurements with the SAGE II satellite sensor can 

also be performed. The combination of the lidar 

aerosol backscatter profile with the retrieval of aerosol extinction at several wavelengths would yield 

valuable information on the optical scattering properties of the stratospheric aerosol for deriving important aerosol radiative parameters. 

Passive radiometers flown with a spaceborne Iidar could provide coordinated data to combine with 

the high vertical resolution of the backscatter aerosol profile. These measurements would yield important information for the definition of the optical 

scattering properties of the stratospheric aerosol 

and also help in the retrieval of more accurate temperature profiles and cloud properties. 

Measurement Simulations 

Simulations have been performed to demonstrate the sensitivity in percent error for quantitatively retrieving from LASA the aerosol backscatter 

coefficient or scattering function in the stratosphere 

as a function of various vertical and horizontal resolutions and instrument parameters. 

The aerosol model used in these simulations 

was based on a background mid-latitude profile, 

while a molecular density profile was modeled from 

the 1976 U.S. Standard mid-latitude atmosphere.

Figure 8 plots the input atmosphere in terms of the 

backscattenng coefficient as a function of altitude 

for a visible wavelength of 0.71 p.m. Curve A is the 

aerosol backscatter coefficient profile, while curve 

30 

25 

E

Ly 

w 

O 

I.-

A 

I--J

10 

5 

01 1 I I 1 I 

10- 6 10 iO 10-3 

BACKSCATTERING COEFFICIENT 

(sr- 1 km-1 

Figure 8. Model atmosphere extinction coefficient plotted 

as a function of altitude for X = 0.71 jim. Curve A is the 

aerosol backscatter coefficient profile for background 

stratospheric aerosol concentrations, and curve M is the 

molecular backscatler profile for the 1976 Standard Atmosphere. 

17
M is the corresponding molecular backscatter coefficient profile. 

Table 7 describes the two lidar systems used in 

the simulations. Molecular density errors of 3 percent for z < 30 km and 5 percent for z > 30 km 

have been assumed, based on prior experience with 

the SAM II and SAGE satellite data analysis. It 

turns out that these molecular density uncertainties 

result in the largest error source when retrieving the 

aerosol scattering function (not just the total scattering function or backscatter radiance profile). The 

details of this analysis are given in Russell et al. 

(1979). Simultaneous two-wavelength lidar measurements can be performed where one wavelength 

in the UV is used to retrieve a lidar-derived molecular density profile in the stratosphere. If the result 

is employed with a visible wavelength lidar measurement, greatly improved accuracies are obtained 

for retrieving the aerosol scattering function (Russell and Morley, 1982). In addition, it should be 

realized that the simulations presented here are in 

terms of accuracy of retrieving aerosol backscatter 

coefficients or scattering functions. The measurement of the relative changes in aerosol scattering is 

straightforward and can be easily implemented with 

very few laser pulses. 

Figure 9 summarizes the simulations for the 

stratosphere using the atmospheric model described 

above for a wavelength of 0.71 p.m. As can be seen 

in the figure, the error in the retrieved aerosol backscatter coefficient (or aerosol backscatter function) 

is about 30 percent at the peak of the stratospheric 

aerosol layer (20 to 22 km) for these modeled background conditions. For volcanic conditions where 

there are enhanced aerosol concentrations such as 

those present in the stratosphere in 1986 from the 

El Chichon eruption of 1982, the retrieved accuracies at the layer peak are about 15 percent. Near the 

mid-latitude troposphere, where the aerosol concentrations are at a minimum, the retrieved accuracies are the worst, reflecting the minimal aerosol 

backscattering at those heights. 

At all heights the molecular density uncertainties represent the dominant source of error. To mm25

(1 x300km) 

20 

E

15 

w (2 x 500 km) 

I—

p 10 

—J

0 1 I 1 I I J 

0 20 40 60 80 100 

PERCENT ERROR IN AEROSOL

BACKSCATTER COEFFICIENT 

Figure 9. Simulation results for the percent error in the 

aerosol backscatter coefficient at A = 0.71 ILm, using 

model molecular density profile. Lidar characteristics are 

those listed in Table 7 for A = 0.71 gm. The numbers in 

parentheses above refer to vertical and horizontal resolutions, respectively. 

imize the uncertainty, the molecular as well as the 

aerosol backscatter coefficients can be obtained 

from a simultaneous two-wavelength lidar measurement. The second wavelength used is the frequencydoubled wavelength of 0.355 p.m. The analysis technique consists of retrieving the aerosol backscatter 

profile at the fundamental wavelength of 0.71 p.m in 

the usual single-wavelength manner. This aerosol 

profile can be modeled by a wavelength conversion 

factor to the aerosol backscatter profile at 0.355 p.m. 

The estimated aerosol backscatter profile can then 

be combined with the measured lidar signal at 0.355 

p.m to yield a relative molecular density backscatter 

profile. This relative density profile is normalized to 

absolute values at a height where the density can be 

estimated to high-accuracy a priori. The level chosen is the isopycnic layer at a height of 8 km where 

Table 7 Characteristics of Lidar Used in Two-Wavelength Simulations* 

Frequency Doubled Fundamental 

Wavelength 0.355 p.m 0.71 p.m 

Energy 0.20 J 1.0 J 

Rate 10 Hz 10 Hz 

System Optical Efficiency 0.08 0.40 

Quantum Detection Efficiency 0.20 0.15 

*Assumed Conditions: Spacecraft Altitude = 700 km 

Diameter of Receiver = 1.25 m 

Receiver Field-of-View = 5 x 10 rad 

Density Error = 3% for z < 30 km, 5% for z > 30 km

18 
yearly density variations are of the order of 1 percent. The lidar-derived density profile and its associated errors are used with the lidar signal at the 

0.71 p.m wavelength to rederive the aerosol backscatter profile. The simulated error results using 

these two wavelengths are shown in Figure 10. With 

the lidar-derived density analysis, the errors in the 

retrieved aerosol backscatter coefficient have been 

reduced greatly and, at the peak of the aerosol layer, 

the error rate is about 15 percent. 

The two-wavelength analysis of simultaneous iidar signals at 0.355 and 0.71 p.m has been shown to 

give greatly improved results for the retrievals of the 

stratospheric aerosol backscatter coefficients. In addition, the lidar-derived density profile at the 0.355 

p.m wavelength can be analyzed for temperature. It 

is anticipated that such a temperature retrieval will 

result in uncertainties of 2 K between about 40 and 

8 km. 

Measurement Example 

A long-term lidar data set of stratospheric aerosol measurements taken at a fixed site of 37N, 

76°W is shown in Figure 11 (McCormick etal., 

1984). The figure, which plots integrated aerosol 

backscattering function from the tropopause 

through 30 km at wavelength 0.6943 p.m versus 

time, clearly shows the effects of the eruptions of 

25 

OR 

15 

w 

0 

D 

I—

I-

-j

0 1 I I I 

0 20 40 60 80 

PERCENT ERROR IN AEROSOL 

BACKSCATTER COEFFICIENT 

Figure 10. Simulation results for the percent error in the 

aerosol backscatter coefficient at A = 0.71 pm, using 

lidar-derived molecular density profile (two-wavelength 

analysis). Lidar characteristics are those listed in Table 7 

for A = 0.355 p.m and 0.71 p.m.

various volcanoes (vertical arrows indicate the times 

of known eruptions) on stratospheric aerosol 

loading. 

An example of a cross-sectional plot of aerosol 

backscatter ratio versus height and latitude from 

aircraft measurements made over the Pacific Ocean 

about 1 year after the eruption of El Chichon is 

shown in Figure 12. By this time, the stratospheric 

material produced from the April 1982 eruption is 

shown to be spread throughout the latitudes 56°S to 

72°N, the latitudes covered by this flight campaign. 

WATER VAPOR OBSERVATIONS 

The study of water vapor in the troposphere 

addresses important atmospheric processes in the 

large-scale hydrological and biogeochemical cycles, 

climate, and meteorology. The hydrological cycle 

was cited by the Eos Science and Mission Requirements Working Group (SMRWG) as the primary 

scientific issue that would require high-spatial-resolution water vapor data from a spaceborne lidar 

system. The transport of water and changes in its 

physical state are key to understanding the Earth as 

a system. 

Lidar measurements of water vapor, especially 

in data-sparse regions, provide necessary inputs to 

both meteorological and climatological studies. Table 8 summarizes the science and observational requirements for the LASA measurements of water 

vapor. In general, the observational requirements 

for the global hydrological cycle are the most stringent of those listed. 

Measurement Approach 

Water vapor column content and profiles can be 

measured by the DIAL technique using water vapor 

spectral lines in the region from 725.2 to 730.9 nm. 

Using the DIAL technique, ratios are obtained of 

lidar return signals at two wavelengths, the ON and 

OFF wavelengths, which are typically separated by 

<70 pm. For the DIAL measurements to be useful, 

the ON and OFF wavelengths should be transmitted 

near simultaneously, and hence the DIAL system 

requires two laser sources. When pulsed laser 

sources are used, the range-resolved number density n of water vapor molecules can be calculated 

using the formula: 

1

[P..(Z2 Pon(zI) )

P0ff(Z2)l 

n =2&r(Z1 -In 

POff(Z,)] 

where M is the differential absorption cross section 

per molecule of the species being measured in the 

altitude region Z1 to Z,. For the column-content 

measurement, only ON and OFF signals from a 

19
w 

i- 10 

10-

cr 

LU 

Cr 

w 

Iz

10

YEAR 

Figure 11. Long-term variation of integrated stratospheric aerosol backscattering function - Hampton, Virginia (37°N).

Table 8. Water Vapor Science Needs 

Application Science Problem

Observational Requirements 

Resolution Altitude Accuracy 

Global Hydrological Cycle Precipitation, Evaporation, 0 to 10 km 1 km 100 km ± 10% 

Evapo-transpiration, 

Atmospheric Transport 

Radiative Budget/Climate Water Vapor Influence on Global 0 to 10 km 2 km 500 km ± 20% 

Radiative Budget 10 to 30 km 2 km 400 km ± 50% 

Atmospheric Composition Trace Flow of Gases and 0 to 10 km 1 km 100 km ±10% 

and Trace Gases Aerosols 10 to 50 km 2 km 500 km ±10% 

Meteorology Improve Short- and Long-Term 0 to 8 km 1 km 250 km ±10% 

Predictions and Augment 

Synoptic Observations in DataSparse Regions

strong reflecting target (like ground or a cloud) are 

needed. 

Measurement Simulations 

Predictions of LASA DIAL system performance have been made based on computer simulations. LASA DIAL system parameters assumed in 

these simulations were the same as those given in 

Table 4. The lasers used in this differential absorption technique must have a spectral purity of better 

than 99.5 percent. 

Table 9 lists the various atmospheric models for 

aerosols, water vapor, temperature, and pressure 

used in the following experiment simulation calculations. Model water vapor profiles for tropical, midlatitude summer, mid-latitude winter, and subarctic 

winter regions are shown in Figure 13. These models 

are used in water vapor DIAL simulations to show 

20 
= w L)0 

LU

I-

-J 

z 0

CLL

MC'J

0 U) 0 U) 

.-

(w) IHOIH 

21
12 

10 

E 

w 

0 

D 

FI- 4 -J

2 

0

Table 9. Model Atmospheres 

Model Type Reference 

Molecular Atmosphere U.S. Standard 1976 Mid-latitude Model 

Aerosol Model The Deirmendjian Haze M Model with a 23 

km Visibility for Background Aerosols 

Water Vapor Profile Tropical, Mid-latitude (Summer/Winter), and 

Subarctic Winter (McClatchey et al., 1972) 

Other Profiles Mid-latitude Model (McClatchey et al., 1972) 

(Pressure and Temperature) 

.:.\

-.. 

N 

- N

N

N

N

\ \ 

\

\ 

..\ \S. 

N

\ 

\

\ 

\

\\ 

.1 •

\

10 14 1015 1016 1017 1018

WATER VAPOR CONCENTRATION (cm-3) 

Figure 13. Water vapor concentration profiles for the indicated latitudinal bands and seasons: tropical, 

mid-latitude (summer), --- mid-latitude 

(winter), -- subarctic (winter). 

the effect of water vapor profile variations on DIAL 

measurement uncertainties. The tropospheric aerosol model used in the aerosol and water vapor DIAL 

calculations, the Deirmendjian Haze M model with 

phase function p,/4,n = 0.01, is shown in Figure 14 

(dashed line). Using an - off-line wavelength of 727 

nm leads to the return signal per 100 in range bin 

shown in the figure (solid line). The wavelength chosen (727 nm) is representative of off-line wavelengths in this water vapor band where the return 

signal will be relatively constant with wavelength. 

The horizontal bars represent the one sigma variation of the signal for each shot. 

Candidate water vapor absorption lines that 

could be used in LASA DIAL measurements are 

listed in Table 10. Selection of appropriate water 

vapor absorption lines for application in various latitudinal and altitude regions will produce less than 

1 percent error in the DIAL measurement resulting 

from temperature uncertainties.

The results of the simulations are estimates of 

the uncertainties which can be achieved. Table 11 

shows the measurement uncertainties in the measurement of water vapor total column content. A 

surface reflectance of 0.03 (corresponding to the 

ocean) is used in these calculations. As can be seen 

from the table, a nighttime measurement uncertainty of <10 percent is achievable for a single shot 

for all assumed water vapor profile conditions and 

for any of the three water vapor absorption lines. 

This indicates that the water vapor column-content 

measurement could be made using only one water 

vapor line for all latitudes. The daytime measurement uncertainties are more dependent on the 

choice of water vapor lines. A compromise choice 

of water vapor lines, such as one having a cross 

section of 10.0 x 10_24 cm2 , produces a water vapor 

column-content measurement uncertainty of <10 

percent over all latitudes for both day and night. 

This results in a water vapor column-content measurement every 800 in along the satellite ground 

track. 

Table 12 shows the variation in water vapor 

measurement uncertainty with different surface albedos and various water vapor models. Even with 

an absorption cross section of 70 x 10 24 cm 2, the 

results indicate that over land a measurement uncertainty of <5 percent can be achieved for all water 

vapor profiles investigated. Scanning of this system 

would provide 20 column-content measurements on 

a 30 km x 30 km grid over a ± 500 km cross-track 

scan, and an average of 10 samples would be obtained for each 100 km x 100 km resolution 

element. 

LASA has the capability to make water vapor 

profile measurements as well as the column-content 

measurements just discussed. Figure 15 shows the 

LASA water vapor profile measurement uncertainties for various water vapor absorption line cross 

sections. For the mid-latitude water vapor summer 

profile, the optimum measurement region is dependent on the selection of the water vapor 

22 
BACKSCATTERING RATIO 

6 7 8 

10 I I - 

LIDAR SIGNAL 

AEROSOL MODEL 

w 6 

O 

I--J 

<2

- 

10 20 30 40 50 

SIGNAL (Photoelectrons/100 m bin) 

Figure 14. 11opospheric aerosol model and lidar signal 

for each 500 mJ pulse at 727 run. Atmospheric backscattering ratio ((,, + corresponds to Deirmendjian 

Haze M aerosol model with phase function, pJ47r = 

0.01. 

absorption line, and it gets progressively higher with 

increasing water vapor absorption cross section. Operation on the 40.0 x 1O_24 cm2 line would permit 

measurements from about 800 in to 7 km altitude 

with <10 percent uncertainty for a 1 km vertical 

resolution (Az) and a 100 km horizontal resolution 

(Ax). To cover the entire altitude range from the 

surface to 100 km altitude would require the use of 

two water vapor lines; however, the range from 0 to 

6 km could be covered by one line. 

Figure 16 gives the results of simulations for 

various latitudinal water vapor models. These results show that the water vapor line used in this

calculation is optimum for maximum altitude coverage in tropical and mid-latitude summer regions. 

In the other two regions, the use of this line is limited to below 3 km altitude. 

Daytime measurements of water vapor profiles 

provide important information on diurnally varying 

atmospheric processes. Figure 17 presents results of 

simulation calculations for daytime water vapor profile measurements using various water vapor lines 

and a surface albedo of 0.3. The region of minimum 

measurement uncertainty is restricted to the lowest 

4 km. Measurements up to 9 km can be made with 

<20 percent uncertainty, and with additional horizontal averaging to ix = 250 km. This uncertainty 

can be reduced to near 10 percent (see Figure 18). 

The variation in daytime water vapor measurement 

uncertainties with water vapor model profiles is 

shown in Figure 19. Daytime measurements are 

more restrictive in the usable altitude range for each 

water vapor line than are the nighttime measurements. As a result, there will have to be different 

water vapor lines used for the different latitudinal 

regions of interest. The lines chosen could complement each other in improving latitudinal and altitude coverage. For example, lines having cross sections of 10.0 x 10_ 24 (see Figure 19) and 40.0 x 

1024 (see Figure 20) would complement each other 

in altitude coverage in the mid-latitude summer region; and, whereas the 10.0 x 10 24 line could be 

used for the low altitudes in the mid-latitude summer, and the 40.0 x 10 24 line could be used for the 

mid-latitude winter and subarctic regions at low 

altitudes. 

The LASA system will be able to measure water 

vapor profiles under night or day conditions with 

vertical and horizontal resolutions of <0.1 km and 

Table 10. Candidate Temperature-Insensitive Lines for LASA Water Vapor DIAL 

A (nm) (x 1024 cm- 2) E" (cm- 1 ) S° (cm'/molec/cm2) 'y (cm-') 

1 726.5594 86.40* 136.77 32.30 0.1190 

2 727.0129 9.41 325.35 2.54 - 0.0859 

3 727.5407 41.20* 212.15 14.31 0.1106 

4 727.6314 11.45 224.83 3.30 0.0919 

5 727.6549 9.70* 224.83 3.13 0.1026 

6 727.6848 12.25 222.06 4.10 0.1064 

7 727.7388 70.10 224.83 24.10 0.1097 

8 727.8078 16.79* 222.06 5.75 0.1092 

9 728.7379 43.10 300.35 14.53 0.1074 

10 728.8126 20.87* 275.52 7.05 0.1075 

11 729.1401 4.37 325.35 1.35 0.1019 

12 729.2109 14.78 315.79 4.71 0.1015 

13 730.8701 5.95 285.43 1.311 0.0702 

14 730.8782 13.00 285.43 4.100 0.1010

*These cross-section values are ± 5% of the values used in simulations. 

Definitions: r0 is the center line absorption cross section at standard temperature and pressure (STP); E" is the ground-state energy 

level; S° is the absorption line strength at STP; 'L is the absorption line half-width at STP. 

23 
Table 11. Percent Measurement Error Per Shot from LASA DIAL Water Vapor

Column-Content Simulations* for Various Water Vapor Absorption Line Cross

Sections, Acr 

Model Water Aor= 19.5 x 1O- cm' r = 10.0 x 10 cm' Ao, = 7.0 x 10 cm' 

Vapor Profile Night Day** Night Day** Night Day** 

Tropical 9.8 75 3.9 8.3 3.4 5.6 

Mid-latitude Summer 4.9 18 3.5 5.8 3.9 5.6

Mid-latitude Winter 3.9 6.6 6.5 9.3 9.3 15 

Subarctic Winter*** 1.4 1.9 2.6 3.3 3.6 4.6 

*Assumes pulse energy = 500 mJ, albedo (p) = 0.03 

**A 1 A interference filter is assumed. 

***A surface albedo of 0.6 is assumed for the snow-covered subarctic region. 

Table 12. Percent Measurement Error Per Shot from LASA DIAL Water Vapor

Column-Content Simulations for Various Surface Albedo Variations, p 

Model Water p = 0.03 (Ocean) p = 0.2 (Loam) p = 0.3 (Vegetation) 

Vapor Profile Night Day* Night Day* Night Day* 

Tropical 3.4 5.6 1.4 2.2 1.1 1.8 

Mid-latitude Summer 3.9 5.6 1.6 2.2 1.3 1.8 

Mid-latitude Winter 9.3 15 3.5 4.8 2.8 3.9 

Subarctic Winter** 3.6 4.6 3.6 4.6 3.6 4.6 

Water vapor absorption line cross section is: icr, = 7.0 x 10_ 24 cm2. 

*A 1 A interference filter is assumed. 

**A surface albedo of 0.6 is assumed for the snow-covered subarctic region. 

100 to 250 km, respectively. Nighttime measurements in a given latitudinal region can be made to 

<10 percent uncertainty over a 6 km altitude range 

using one water vapor line. To globally study the <3 

km altitude region, only one water vapor line will 

be required for nighttime measurements, and daytime global coverage measurements can be achieved 

using two water vapor lines. Investigations at midto upper-tropospheric altitudes will need line selections for the specific latitudinal region to be studied. 

Some calculations, assuming reasonable improvements in overall LASA system performance, 

indicate the possibility of measuring water vapor 

profiles with a 10 percent measurement accuracy for 

a vertical and horizontal resolution of 0.5 km and 

50 km, respectively, during night, and also permit 

10 percent measurement accuracy over the oceans 

during day with a resolution of 1 km and 100 km, 

respectively. It is also expected that with stronger

new laser sources the water vapor DIAL technique 

could be used in the 0.93 .sm region where stronger 

water vapor absorption lines are available that will 

permit measurements up to about the 12 km altitude range. 

Measurement Example 

Figure 21 shows water vapor variations in the 

marine boundary layer across the Gulf Stream Wall 

as measured on September 24, 1982, with an airborne DIAL system aboard the NASA Electra aircraft (Browell et al., 1984). The abrupt decrease in 

water vapor missing ratios can be seen at the top of 

the boundary layer on both sides of the Gulf Stream 

Wall. There are some small-scale variations in the 

water vapor mixing ratios in the boundary layer; 

however, in this example, there does not seem to be 

any significant difference in the average water vapor 

concentrations on the two sides of the Gulf Stream 

24 
----

15 

12 

E 

Ui9 

0 

I-

<3 

ill 

0 

I-

<3 

15 15 

12 12 

E 

w

- — Ui 9 

0 0 

-, -- — — FI— -J 

<3 <3 

0

10 20 30 40 50 

PERCENTAGE RANDOM ERROR 

Figure 15. LASA water vapor DIAL range-resolved 

measurement simulations. Variations with line strength 

(night filter, mid-latitude (summer) water vapor model. 

Calculations performed for: X,,.. = 727 Inn, 500 mJ pulse 

at 10 Hz, .z = 1,000 m, Ax = 100 km. Water vapor line 

absorption cross sections (so): 

90.1 x 10 cm2 , 40.0 x 10 cm2, 

0-0-0 19.5 x 10-1 cm2 , 10.0 x 10 cm2.

0! I I 1 I I 

0 10 20 30 40 50 

PERCENTAGE RANDOM ERROR 

Figure 17. LASA water vapor DIAL range-resolved measurement simulations. Variations with line strength (day 

filter (0.1 A); mid-latitude (summer) water vapor; albedo 

= 0.3). Other parameters as in Figure 15. Water vapor 

line absorption cross sections (so): 

90.1 x 10cm2 , .---. 40.0 x 10Mcm2, 

0-0-0 19.5 x 10 em', 0-0-0 10.0 x 10 cm2. 

0 1 - I I I 

40

I 

0 10 20 30 50 

PERCENTAGE RANDOM ERROR 

Figure 16. LASA water vapor DIAL range-resolved 

measurement simulations. Latitudinal variations (night 

filter). Calculations performed for: o = 19.5 x 10 

cm', A,. = 727 inn, 500 m.J pulse at 10 Hz, Az = 1,000 

m, Ax = 100 km. Latitudinal bands and seasons as follows: tropical, mid-latitude (summer),

0-0_a mid-latitude (winter), o-0—o subarctic (winter).

QI I I 1 1 I 0 10 20 30 40 50 

PERCENTAGE RANDOM ERROR 

Figure 18. LASA water vapor DIAL range-resolved 

measurement simulations. Variations with line strength 

(day filter (0.1 A); mid-latitude (summer) water vapor; 

albedo = 0.3). Calculations performed for: A,, = 727 

run, 500 mi pulse at 10 Hz, Az = 1,000 m, Ax = 100 km. 

Water vapor line absorption cross sections (g(r): 

90.1 x 10 cm2 , ----- 40.0 x 10 cm', 

0-0-0 19.5 x 10 cm2 , 

0-0-0 10.0 x 10 cm2. 

Wall. This result was confirmed with simultaneous 

in Situ aircraft measurements. 

An Alternative Measurement Approach 

Total column-content water vapor can be measured through a cw carbon dioxide laser differential 

absorption approach. The cw carbon dioxide laser 

water vapor measurement instrument could meet 

the Eos two-year lifetime requirement with 100 percent duty cycle operation, assuming only that a relatively small-scale lifetime testing program is supported. The instrument would be able to measure 

water vapor column-content abundances over land 

and water, day or night, with 10 km horizontal resolution along the suborbital track. The measurement accuracy would be in the 5 to 10 percent range. 

Water vapor measurements are valuable for radiative transfer and energy budget calculations. Often 

these applications can make use of data based on 

sampling of several geographical regions around the 

globe in a period of 1 to 10 days. Coverage rate can 

be improved by using scanning lidar. 

25
15 

12 

W 9 

D

6 

I3. 

15 I I 

12 

E 

w9 

a 

I-

<3 

- 

(1 0'-0 10 20 30 40 50

PERCENTAGE RANDOM ERROR 

Figure 19. LASA water vapor DIAL range-resolved 

measurement simulations. Latitudinal variations (day filter (0.1 A); Air = 10.0 x 10 cm'; albedo = 0.3; 

Az = 250 kin). Other parameters as in Figure 16. Latitudinal bands and seasons as follows: tropical, 

mid-latitude (summer), °-°-° mid-latitude 

(winter), — —. subarctic (winter).

10 20 30 40 50

PERCENTAGE RANDOM ERROR 

Figure 20. LASA water vapor DIAL range-resolved 

measurement simulations. Latitudinal variations (day filter (0.1 A); Aor = 40.0 x 10 cm'; albedo = 0.3). 

Other parameters as in Figure 19. Latitudinal bands and 

seasons as follows: tropical, 0-0-0 mid-latitude 

(summer), 0-0-0 mid-latitude (winter), — -. subarctic (winter). 

The measurement of water vapor depends on 

the existence of several water vapor absorption lines 

in the 850 to 1,100 cm - ' region where many carbon 

dioxide laser wavelengths exist. The measurements 

of differential absorption would be made by simultaneously transmitted co-linear radiation from two 

cw carbon dioxide lasers, one wavelength tuned to 

overlap with a water vapor absorption feature, and 

the other chosen to interact minimally with water 

vapor. To avoid atmospheric carbon dioxide absorption (due to the "hot bands" of atmospheric carbon 

dioxide that are the same as the laser bands) rare 

isotopes of carbon dioxide would be used in the laser 

gas mixes. 

Figure 22 is a water vapor volume mixing ratio 

profile for a U.S. Standard Atmosphere. (This 

shows that most of the water vapor occurs in the 

lower 5 km, and, in fact, usually drops off rapidly 

above the boundary layer.) Using this model, synthetic spectra have been computed for one-way vertical transmission through the atmosphere for a 

number of spectral regions. Figure 23 shows the 925 

to 933 cm-' region. Most of the absorption lines 

shown are due to water vapor. (Those at 927, 931, 

and 933 cm - ' are isolated carbon dioxide lines.) 

The R(20) line of the 13C'602 laser overlaps a weak 

water vapor line and can be used for water vapor 

column-content measurements in wet regions, such 

as subtropical or mid-latitude summer conditions. 

Other laser line frequencies are indicated with vertical marks, using the notation that 636, for example, is a carbon dioxide molecule with two 160 atoms 

and a ' 3C atom. (The normal isotope would be 626.) 

Figure 24 shows the transmission in the 997 to 999 

cm- 1 region. The P(22) line of the '3C'6 02 laser 

(high-frequency band) overlaps with a moderately

strong water vapor line, which appears as a relatively broad feature between two narrow ozone 

lines. Most of the sharp, narrow lines shown in this 

region are due to stratospheric ozone. (The typical 

ozone line half-widths are approximately 0.05 

cm-'.) Several other overlaps have been identified, 

allowing for measurements to be made over a wide 

variety of atmospheric moisture conditions just by 

tuning each laser to a few selected lines. 

Measurement Simulations 

The instrument characteristics and the assumptions used in deriving the instrument parameters to 

meet the measurement objectives are described in 

the following paragraphs. Using the standard lidar 

equation shown earlier, where the volume backscattering coefficient is replaced by the factor 

(p cos 0)/7, where p/u is the collinear Bidirectional 

Reflectance Distribution Function (BRDF) and 0 

is the angle of incidence of the radiation relative to 

the local normal to the surface, we can compute the 

expected return power and compare it with the 

Noise Equivalent Power (NEP) . of the heterodyne 

receiver. NEP is given by NEP = (hvl-q2)(B1FIT)'/2 

where -q is the detector quantum efficiency, hi' is the 

laser radiation photon energy, BIF is the resolving 

bandwidth of the receiver, and T is the integration 

time. We can achieve a detected SNR of 100, sampling over 10 km along-track segments, with the set 

of system parameters given in Table 13, for the water 

vapor column abundance measurement. 

It is assumed that the receiver would contain a 

minimum of 10 filter channels, each 250 kHz bandwidth, with 50 percent overlap. These system parameters are consistent with a speckle averaging efficiency that would allow for a 1 percent minimum
3550 :56,w.

36°10 3620' 3630 3640 

W 

I—

z

0 -J 

a.

N LATITUDE 

Figure 21. Airborne DIAL measurement of water vapor distribution in Gulf Stream region. 

detectable differential absorption. Independent 

speckle lobe sampling would occur every 1 in of 

along-track motion, resulting in an average of over 

10 samples in a 1.5 sec integration time. 

Measurement Example 

Using various common model atmospheres 

(McClatchey et al., 1972), measurement accuracies 

have been computed using a number of spectral 

overlaps of atmospheric water vapor lines with certain laser lines of the ' 3C16 07 isotope carbon dioxide 

lasers. The absorption cross sections for these water 

vapor line overlaps range from 4 x 10 24 cm2 to 

2.5 x 1023 cm2 . Table 14 lists simulation results, 

indicating percentage measurement errors for four 

atmospheric models that cover the range from 

warm, humid conditions to cold, dry conditions. 

These measurement errors do not include spectral 

line parameter inaccuracies and assume that the atmospheric temperature profile for the region from 

surface to 8 km altitude is known to within ±3 K. 

The spectral line parameters at present are at a level 

of uncertainty such that this would contribute an 

additional 5 to 10 percent error; however, the appropriate spectroscopic studies can reduce this source 

of inaccuracy to the 2 to 4 percent level.

100 

80 

w 60 

a

40 

I-

-J

20 

01 I 

1.OE-07 1.OE-05 1.OE-03 1.OE-01 

VOLUME MIXING RATIO FOR

WATER VAPOR 

Figure 22. Volume mixing ratio profile for water vapor

U.S. Standard Atmosphere. 

These simulations include the water vapor continuum absorption expected for each atmospheric 

model. The simulation results indicate that the intrinsic instrument measurement accuracy would be 

better than 5 percent under a wide variety of circumstances. In each case, the "off-line" wavelength 

would be selected to minimize the wavelength 

27
R(16) R18 rR(fl)T R6) 

)j 636 636 I 636 636 

P(46) ' R(2) 

828 838 

R(20) 

636 

Z 

I.--

_J U) 

wz 

cr< 

Cr 

I925 926 927 928 929 930 931 932 933

WAVE NUMBER (cm-1)

separation, and the wavelengths would be in the 10 

to 11 m region in order to minimize the silicate 

Reststrahlen band and other differential reflectance 

effects that occur mainly in the 9 to 10 pm region. 

(It is estimated that the residual spectral reflectance 

fluctuations would add a 5 percent measurement error over land, and would add negligible error over 

water.) 

A cw carbon dioxide laser system employing 

heterodyne detection has been developed and flown 

for measurements of water vapor column content 

(see Figure 25) (Englisch et al., 1983). 

Figure 23. One-way relative vertical transmission spectrum through the Earth's atmosphere (U.S. Standard Atmosphere) from 925 to 933 cm-'. Laser lines which can 

be obtained with rare isotope versions of carbon dioxide 

are noted with tic marks. Lines due to standard isotope 

carbon dioxide appear at 927, 929, 931, and 933 cm'. 

The remaining lines are water vapor features, including 

one which is blended with the 929 cm-' carbon dioxide 

line. 

Z 

w2 >c,)cn 

j U) WZ 

cc

F997 998 999

WAVE NUMBER (cm) 

Figure 24. One-way relative vertical transmission through 

the Earth's atmosphere (997 to 999 cm'). The P(22) 

carbon dioxide laser line (with the 'C and 160 isotopes) 

at 998.77 cm' is noted with a tic mark, and overlaps with 

a strong water vapor line.

ATMOSPHERIC PRESSURE

OBSERVATIONS 

The pressure field is one of the fundamental 

variables used for weather prediction and atmospheric circulation studies. Surface pressure and 500 

mb heights are used by meteorologists and other 

atmospheric scientists to describe the state of the 

atmosphere to define frontal characteristics, and to 

determine mass movement. The three-dimensional 

pressure field is also a principal variable used in 

general circulation models for mid-range and longrange forecasting. It may be noted that the pressure 

profile can be used directly to specify both the temperature profile and the balanced wind. In contrast, 

the derivation of a pressure profile from a temperature profile is not possible without specifying pressure at a reference height. 

Lidar provides the only remote-sensing technique for the direct measurement of the pressure 

profile. In addition, it also provides the only remotesensing technique for measuring surface pressure 

over both land and ocean surfaces. Moreover, the 

lack of an accurate pressure reference level hinders 

the derivation of pressure profiles from radiometrically sensed temperature profiles. As a result, measurements of the pressure profile are available only 

at radiosonde locations and where satellite 

P(22) 

636 

Table 13. Summary of Carbon Dioxide Laser Differential

Absorption Spectrometer Parameters 

Parameter Value 

700 km

1m2

5 W (2 lasers, 10 W total)

0.4

0.4

0.005 (worst case)

0.3 (including water vapor continuum)

250 KHz

1.5 sec 

Altitude 

Optical Receiver Mirror Area 

Laser Transmitter Power 

Optical Efficiency 

Photomixer Heterodyne Quantum Efficiency 

Surface "Reflectance" (phr) 

Two-Way Path Transmission 

Receiver Bandwidth 

Integration Time

28
Table 14. Water Vapor Measurement Accuracy Based on Model Simulations 

Column Laser Line Differential 

Model Abundance Wave Number Cross Section Accuracy 

(g/cm2) (cm _I) (cm') (%) 

Tropical 5.2 928.656 3.2 x 10_24 2.5 

Mid-latitude Summer 3.7 928.656 2.9 x 10_24 2 

Mid-latitude Winter 1.05 998.788 14 x 1024 3 

Subarctic Winter 0.48 948.308 4.1 x 10_23 1.5

temperature profiles and a surface pressure measurement coincide. 

Measurement Approach for Surface 

Pressure 

This experiment will accurately determine surface pressure over land and ocean regions of the 

globe for both day and night operation. In areas 

with low-lying clouds, the pressure and altitude of 

the clouds will be measured; this measurement can 

be used to derive surface pressure. 

The surface pressure experiment is a two-wavelength DIAL measurement (Korb and Weng, 1983) 

utilizing the backscattered energy from the Earth's 

surface or from low-lying clouds. The measurements 

can be made in either a crosstrack scanning or nadir-viewing mode, depending upon availability of a 

scanning configuration. Only modest laser energy is 

required since the backscattered signals are large. 

A pressure-sensitive measurement is obtained by 

locating one wavelength in a temperature insensitve 

absorption trough region. A trough region is the 

region of minimum absorption between two 

strongly absorbing lines in the oxygen A-band near 

0.76 m, or 13,150 cm-'. The absorption in the 

trough is proportional to the square of the pressure. 

A second wavelength located in a weakly absorbing 

region with a shift of 0.0001 to 0.001 tkm is used as 

a reference to normalize out the effects of surface 

I-

-J cc Z 

g/m 

w<I >ZH 2 O_ 

0.4 

w 0.2 5000 

2500

-Lj 0 

0

1400 

<3_ H E 1000 

Wj

600

0 5 10 

DISTANCE ALONG FLIGHT PATH (km) 

Figure 25. Water vapor column content measured using 

the Differential Absorption Lidar Experiment (DIALEX) (cw carbon dioxide laser system) (Englisch etal., 

1983).

reflectance. The use of an absorption trough technique reduces the sensitivity of the measurements 

to the effects of laser frequency jitter by up to two 

orders of magnitude. The integrated path absorption method used for the measurement allows high 

sensitivity to be achieved. 

Measurement Simulations 

A two-wavelength solid-state Alexandrite laser 

tunable over the 0.76 pm region is assumed. It has 

a spectral resolution of 0.02 cm-' or better, a pulse 

length of less than 100 ns, and an energy of 0.1 to 

0.5 J/pulse is used for the measurement. The two 

wavelengths are separated in time by a nominal 300 

s, which allows temporal separation and singlechannel detection of both the on-line and reference 

wavelengths. An energy monitor is needed to measure the transmitted laser energy at each wavelength 

and a wavemeter must be used to precisely measure 

and control the laser wavelength. 

The atmospheric model used for the simulations 

is a one-dimensional model with up to 50 homogeneous vertical layers. The temperature and pressure 

data are taken from the mid-latitude January U.S. 

Standard Atmosphere model and the aerosol composition data are taken from the Elterman model 

(Elterman, 1968). The simulations were performed 

using a high spectral resolution atmospheric transmission code. The transmission at a given frequency 

is calculated using the Voigt line profile and integrated vertically over the layers from space to altitude z. The simulations include detailed calculations 

of laser finite bandwidth. The effects of background 

radiation on the simulations have been considered. 

These include surface reflectance, aerosol scattering, and Rayleigh scattering. 

The simulations used the following parameters 

for measurements from a 700 km Eos platform. A 

0.5 J Alexandrite laser transmitter with a repetition 

rate of up to 10 Hz, a three-mode spectral output 

with a bandwidth of 0.02 cm', a telescope diameter 

of 1.25 m, a receiver field-of-view (FOV) of 0.15 

(0.5) mrad for daytime/nighttime experiments, and 

a surface reflectance of 0.1. An optical efficiency of 

0.59 was used for nighttime simulations, which includes the transmission of a wide-band 20 A spectral 

detection filter, and an optical efficiency of 0.38 was 

29 
used for daytime simulations, which includes the 

additional narrow-band (1.6 or 5 cm-') filters required. A quantum efficiency of 0.15 is assumed for 

the detector. The detailed parameters for nighttime 

and daytime experiments are given on the attached 

figures with the overall efficiency for each experiment. Figures 26 and 27 give the results of simulations for a crosstrack scanning experiment to measure surface pressure using the pressure-sensitive 

trough absorption at 13,150.86 cm -1 . The figures 

give the number of shots required to achieve 0.2 

percent surface pressure accuracy versus scan angle 

for nighttime and daytime measurements with a 250 

km grid. As shown, a 2.9 Hz repetition rate is sufficient for nighttime measurements and 4.5 Hz is 

sufficient for daytime measurements. For a nadir 

experiment, the required laser energy could be reduced by a factor of four and still achieve these 

26 \ 21 \ 26 \ 250 

shots 21 \ shots \shots \km 

•. shots \ 

1000 km

km 

Figure 26. Surface pressure scanning lidar simulated for 

Eos experiment at 700 km—night. Calculations performed for: 500 mJ pulse at 2.9 Hz, v = 13,150.86 cm' 

or (13,152.49 cm'), Av = 0.02 cm', 3-mode laser, 0 

= 1.25 m, PJP = 0.02%, eff. = 0.0885, FOV = 0.5 

mmd, AX, = 20 A, p = 0.1. 

accuracies. We note that for cloud-top pressure altitude measurements, an accuracy of 0.2 percent or 

better could be obtained with approximately onefifth the laser power factor. 

Measurement Approach for the Pressure 

Profile 

LASA can also accurately determine the pressure profile-the pressure and altitude structure of 

the atmosphere over land and ocean regions of the 

globe for both day and night operation. 

The pressure profile experiment is a two-wavelength DIAL measurement utilizing the backscattered energy from the clear atmosphere. In a manFigure 27. Surface pressure scanning lidar simulated for 

Eos experiment at 700 km—day. Calculations performed 

for: 4.5 Hz pulse rate, eff. = 0.057, FOV = 0.15 mmd, 

AX, = 1.6 cm-' or (5 cm-'), p = 0.3. Other parameters 

as in Figure 26. 

ner similar to the surface pressure measurements, a 

pressure-sensitive measurement is obtained by locating one wavelength in an absorption trough region in the oxygen A-band near 0.76 m or 13,150 

cm-'. A second wavelength located in a weakly absorbing region is used as a reference to normalize 

out the effects of atmospheric backscatter. The absorption trough technique greatly reduces the sensitivity of the measurements to the effects of laser 

frequency jitter, and the integrated path absorption 

method allows high sensitivity to be achieved. 

Measurement Simulations 

The instrument characteristics required are 

similar to those given for surface pressure, again 

using a two-wavelength tunable solid-state laser. 

Higher energy, 0.5 J/pulse, is needed for the profiling measurement. The profiling measurements, 

however, do not require an energy monitor 

measurement. 

The basic atmospheric model used for these 

simulations is similar to that used for surface pressure. The laser transmitter assumed for the simulations is an Alexandnte laser with a three-mode 

spectral pattern with a bandwidth of 0.02 cm '. The 

simulations include detailed calculations of laserfinite bandwidth. The effects of background radiation on the simulations have been considered. These 

include the effects of surface albedo, aerosol scattering, and Rayleigh scattering. 

The pressure profile simulations used almost 

the same parameters as those used for surface pressure. Differences were the use of a wide-band 10 A 

spectral detection filter for nighttime and a narrowband 0.12 cm-' filter for daytime simulations. Also, 

a quantum efficiency of 0.2 is assumed for the 

30
detector. The detailed parameters for nighttime and 

daytime experiments are given on the attached figures with the overall efficiency for each experiment. 

Figures 28 and 29 show the results of simulations of pressure profiling for a nadir-viewing experiment. As shown, for 1 km vertical and 250 km 

horizontal resolution, good accuracy (<0.4 percent) 

can be obtained over a large portion of the troposphere using the pressure-sensitive absorption 

troughs located at either 13,150.86 or 13,153.79 

cm'. We note that the use of a 0.15 mrad FOV and 

a 0.12 cm' spectral detection filter minimizes the 

effects of background radiation and allows high-accuracy daytime measurements.

0.6 

il 13152.49 cm-' 

Cr ir

13153.79 cm 

Uj 

cc 

in 0.2 13150.86 cm-' 

Measurement Example 

The atmospheric pressure profile has been 

measured from both ground-based (Schwemmer, 

1984) as well as aircraft platforms (Korb et al., 

1986). An example of a comparison of aircraft lidar 

measurements of the pressure profile with radiosonde measurements is shown in Figure 30. The 

lidar system used two high-resolution Alexandrite 

lasers (0.02 cm-' resolution) tuned to an absorption 

trough and a reference wavelength near 760 nm. The 

measurements were made from the NASA Electra 

aircraft at an altitude of 2,790 m, approximately 220 

km off the coast of Delaware during the nighttime 

of November 20, 1985. The lidar measurements 

were made with a 30 m vertical resolution and analyzed for a 100 shot average. The average deviation 

between the lidar and radiosonde data was less than 

0.2 percent (2 mb). 

ATMOSPHERIC TEMPERATURE

PROFILE OBSERVATIONS 

The temperature field is of fundamental importance in meteorology, climate, and for initializing 

global circulation models for all scales of weather 

forecasting. Temperature profiles are required for 

determining tropopause height, tropopause breaks, 

the depth and structure of the planetary boundary 

layer, and frontal and air mass characteristics, as 

well as numerous other meteorological phenomena. 

In the mid- and high-latitudes, temperatures are of 

added importance since over oceanic areas in these 

latitudes wind fields are derived from temperature 

fields using balance relationships. 

General circulation models have now developed 

to the point where data at as many as seven independent tropospheric levels are required. The most 

advanced passive sounders currently under study 

cannot accurately provide vertical resolution at this 

scale. 

Lidar differential absorption measurements can 

provide direct measurements of the temperature 

profile with extremely high vertical resolution. The 

use of the lidar technique will allow the temperature

ALTITUDE (km) 

Figure 28. Pressure profiling accuracy for simulated Eos 

experiment at 700 km—night. Calculations performed 

for: 500 mJ pulse at 10 Hz, Av = 0.02 cm', 3-mode 

laser, D = 1.25 m, eff. = 0.118, Ax = 250 km, Az = 1 

km, FOV = 0.5 mrad, AX, = 10 A, p = 0.1. 

0.6

13152.49 cm'

3153.79 cm-1 

13150.86 cm-1 

0.2 

Cl) 

Ui 

cr 

a0

ALTITUDE (km) 

Figure 29. Pressure profiling accuracy for simulated Eos 

experiment at 700 km—day. Calculations performed for: 

eff. = 0.076, FOV = 0.15 mrad, Av, = 0.12 cm-'. Other 

parameters as in Figure 28. 

2000

11/20/85 (ANF8) 

100 SHOT AVERAGE 

1500 30 METER RESOLU 

X RADIOSONDE 

• LIDAR 

1000 

< 500

800 900 1000 1100 

PRESSURE (mb) 

Figure 30. Lidar measurements of atmospheric pressure

profile as a function of altitude (aircraft - 2,790 m). 

PIZ 

cc

0.4 

Cr 

LU 

LU 

cc 

31
cc 

CD 

cc cc 1.0 

w 

uJ 

F0.5 

LU 

CL 

LU

13010.81 cm 

= 3 k 

12999.95 cm-1 

Az = 2.5 km 

to be measured in a direct manner, rather than requiring the use of inversion techniques as must currently be done in order to use the data from passive 

profilers. The simulations below indicate that a lidar 

temperature profile experiment could achieve 1 K 

temperature accuracy while simultaneously obtaining 1.25 to 2.5 km vertical resolution. 

Measurement Approach for the 

Temperature Profile 

LASA can accurately determine the atmospheric temperature profile over land and ocean regions of the globe for both day and night operation. 

The temperature profile measurement uses a 

two-wavelength DIAL technique (Korb and Weng, 

1982) utilizing the backscattered energy from the 

clear atmosphere. The measurement is made in a 

differential ranging mode to obtain vertical resolution. A measurement highly sensitive to temperature is obtained by locating one wavelength on a 

high j line in the oxygen A-band near 0.77 pm, or 

13,000 cm-', where the variation of absorption 

coefficient with temperature is as high as the sixth 

power of temperature. A second wavelength located 

in a weakly absorbing region with a shift of 0.02 to 

0.5 nm is used as a reference to normalize out the 

effects of atmospheric backscatter. 

Measurement Simulations 

The instrument characteristics for temperature 

profiling are the same as those given for pressure 

profiling. Also the return signals are measured with 

a nominal 100 in range cell. The atmospheric model 

and the laser transmitter used for these simulations 

are the same as those used for the surface pressure 

and pressure profile experiment. 

The simulations used the same parameters as 

those used for pressure profiling: a 0.5 J Alexandrite 

laser transmitter with a repetition rate of 10 Hz, a 

three-mode spectral output with a bandwidth of 

0.02 cm - ', a telescope diameter of 1.25 m, and a 

receiver FOV of 0.15 (0.5) mrad for daytime (nighttime) experiments. An optical efficiency of 0.59 was 

used for nighttime simulations, which includes a 

wide-band 10 A spectral detection filter, and an optical efficiency of 0.38 was used for daytime simulations, which includes a narrow-band 0.12 cm-' 

filter. A quantum efficiency of 0.2 is assumed for 

the detector. The detailed parameters for nighttime 

and daytime experiments are given on the attached 

figures with the overall efficiency for each 

experiment. 

Figures 31 to 33 show the results of simulations 

of temperature profiling for a nadir-viewing experiment. As shown for the line at 12,999.95 cm-', 2.5 

km vertical resolution allows high-accuracy measurements with less than 1 K error for altitudes up 

to 10 km. In addition, the use of the stronger absorption line at 13,010.81 cm' with 3 km resolution

1.5 

2 12990.46 cm-1 

Cr Az= 2 5 km

13010.81 cm-1 

CD 

CC 

!1•O 99.95cm1 Az = 3 km 

0.5 

uJ 

W E,z=2.5km I0

I 

0 5 10 15 20 25

ALTITUDE (km) 

Figure 31. Temperature accuracy for simulated Eos experiment at 700 kin-night. Calculations performed for: 

Ax = 175 km. Other parameters as in Figure 28. 

1.5

12990.46 cm z = 2.5 km 

01 I I I I I 

0 5 10 15 20 25 

ALTITUDE (km) 

Figure 32. Temperature accuracy for simulated Eos

experiment at 700 km-day. Parameters as in Figure 29. 

1.5 

C

1.0 13010.81 cm 

w Az = 3 k 

W 

I- 12999.95 cm' 

0.5 Az = 2.5 km 

LU 

LU 12990.46 cm' Az = 2.5 km 

0 1 I I 0 5 10 15 20 25

ALTITUDE (km) 

Figure 33. Temperature accuracy for simulated Eos experiment at 700 km-day. Calculations performed for 

p = 0.3. Other parameters as in Figure 29. 

allows high-accuracy measurements to be extended 

to 15 km altitude. The horizontal resolution is 175 

km for nighttime and 250 km for daytime measurements. The effects of background are small for the 

32
12,999.95 cm-' measurement but become significant for the 13,010.81 cm-' measurement. 

Figures 34 to 36 show the results of temperature 

profiling accuracy simulations for a 1.25 km vertical 

resolution nadir-viewing experiment. As shown, 

high temperature accuracy and high vertical resolution can be obtained in the lower portion of the 

troposphere. 

Measurement Example 

Differential absorption measurements of atmospheric temperature have been made over a 1 km 

horizontal atmospheric path to better than 1 K using 

an ultrahigh spectral resolution cw dye laser system 

(Kaishoven et al., 1981). Recently, range-resolved 

differential absorption lidar measurements were 

made with a ground-based upward-viewing lidar 

system (Korb et al., 1985). The system used dual 

high-resolution Alexandrite lasers with a spectral 

resolution of 0.02 cm-. The measurements were 

made over the altitude range from 800 to 2,700 m. 

For a 300 m vertical resolution and a 300 shot average, the accuracy was better than 2 K. 

OZONE OBSERVATIONS 

The ozone layer determines the ultraviolet 

transmission properties of the atmosphere. The 

amount of ultraviolet radiation reaching the surface 

of the Earth has direct effects on living systems. 

Ozone is formed in the stratosphere by ultraviolet 

photolysis of oxygen, and some ozone is transported 

from the stratosphere providing one source of 

tropospheric ozone. Ozone is a key compound in 

the atmospheric chemistry of trace constituents. 

Ozone in the troposphere is an oxidant directly affecting the surfaces of living organisms. A global 

data base, sufficient to produce a coarse resolution 

ozone climatology, is needed for the field of tropospheric chemistry. It would enable production of a 

1.5

12990.46 cm-' 

= 1.25 km 

LU 12999.95 cm-1 Ui

LZ = 1.25 km 

cc 0.5 Uj 

LU

0! I I 

0 2 4 

ALTITUDE (km) 

Figure 34. Temperature accuracy for simulated Eos experiment at 700 km—night. Calculations performed for 

p = 0.3. Other parameters as in Figure 31.

1.5 

Cr 12999.95 cm-' 

D 

Cr 10 Az 125km.-f 

Cr 

LU 

LU 

Cr 0.46 cm-1 

I— Az = 1.25 km 

CC 0.5 LU 

0 

LU 

I0'-0

ALTITUDE (km) 

Figure 35. Temperature accuracy for simulated Eos

experiment at 700 km—day. Parameters as in Figure 29. 

1.5 12999.95 cm-1 

= 1.25 km ,Z/ 

cc 

cc 1.0- - 

LU

12990.46 cm-' 

- Az = 1.25 km 

CC 0.5

- 

LU 

LU

4 

ALTITUDE (km) 

Figure 36. Temperature accuracy for simulated Eos experiment at 700 km—day. Calculations performed for 

p = 0.3. Other parameters as in Figure 29. 

global distribution of hydroxyl radical, the principal 

oxidizing species in the various elemental cycles, 

and it would also provide a framework for tropospheric photochemistry as a whole. Due to the high 

variability of ozone in the troposphere, relative 

standard deviations in the range of 25 to 100 percent, a realistic global data base will require relatively high spatial and temporal resolution, which 

can best be obtained from an orbiting laser DIAL 

measurement of global ozone profiles. 

There is a continuing concern that the total column and vertical distribution of atmospheric ozone 

will be affected by changing concentrations of a 

number of trace species such as chlorofluorocarbons, carbon dioxide, oxides of nitrogen, and methane. If ozone were to be seriously depleted by these 

compounds, increased amounts of solar ultraviolet 

radiation would be allowed to reach the ground and 

could have serious impact on human health and agricultural and oceanic productivity. The ozone absorption of solar energy also provides the main 

4 

33
source of heating for the middle atmosphere; without it there would be no middle atmosphere. Thus, 

the measurement of the vertical distribution of 

ozone is important in determining the temperature 

structure and dynamics of the middle atmosphere. 

The distribution of ozone is also critical in determining connections between stratospheric and 

tropospheric weather and climate that can occur 

through radiative, dynamical, and chemical 

mechanisms. 

Both the Dobson network and satellite observations (Solar Backscatter Ultraviolet (SBUV) and 

Total Ozone Mapping Spectrometer (TOMS)) provide a global field of total ozone measurements. Unfortunately, approximately 90 percent of the total 

ozone resides in the stratosphere, and existing techniques are not able to accurately measure the small 

fraction that resides in the troposphere. It has not 

been possible to use these two existing passive 

methods for trend analysis of ozone. This has been 

in part due to lack of precision in these techniques 

and drift in their calibrations. For example, the 

Nimbus 4 Backscattered Ultraviolet (BUV) total 

ozone data show a drift of about 0.5 percent per 

year relative to the ground-based measurements. 

This instrument also showed wavelength-dependent 

drifts in radiance data. These instrument artifacts 

are sufficiently large to overwhelm trends of the size 

predicted for chlorofluorocarbon chemistry-induced 

ozone destruction. The longest data base available 

for altitude-resolved ozone measurements is that 

from the Umkehr measurements, which provide vertical ozone profiles up to 48 km. Derivation of ozone 

profiles from Umkehr data requires an indirect inversion method and suffers from limited vertical resolution and nonuniqueness of the profiles, and the 

absence of simultaneous measurements of aerosols 

and other factors which affect the inversion. 

The best existing tropospheric ozone data set is 

provided by individual ozonesonde stations that are 

now measuring or have ueasured vertical profiles of 

ozone from the ground to the middle stratosphere 

on a more-or-less regular basis. Unfortunately, a 

number of different types of sensors have been 

used, many of which were never accurately intercalibrated. Serious doubts have recently been raised 

about the absolute accuracy of the ozonesonde 

measurements in the troposphere particularly for 

the older types of sondes that are no longer in operation or available for intercomparisons with current devices. Previous intercomparisons of operational devices alone have raised serious doubts 

about combining measurements from different research groups or devices into a single data set. 

Three key issues involving atmospheric ozone 

concentrations are to determine: (1) the climatology 

of ozone (i.e., the concentration distribution including mean values and higher moments); (2) the process or combination of processes that exert dominant control on ozone climatology; and (3) the

possible existence of long-term changes in the mean 

and extremes of concentration, and the causes of 

such trends if they do exist. 

Measurement Approach for Ozone 

Profiles 

A satellite lidar system using the ultraviolet 

DIAL measurement method based on the XeCI excimer laser is a promising approach to obtain global 

profiles at high vertical resolution of both stratospheric and tropospheric ozone. Recent groundbased measurements (Uchino et al., 1979; Werner 

et al., 1983) of tropospheric and stratospheric ozone 

profiles using an XeC1 excimer-based lidar have 

demonstrated that precise high-resolution measurements of ozone can be obtained on a routine basis. 

The XeCI excimer-based DIAL system for 

ozone measurements transmits two laser lines in the 

ultraviolet: one at 308 nm and the other at either 

338 or 353 run. It is straightforward by further Raman shifting to propagate an additional laser line in 

the visible region at 500 to 600 nm. By using the 

backscattered return signal from 308 and either 338 

or 353 nm, the ozone concentration profiles can be 

obtained. The backscattered data from 338 or 353 

nm is a measure of the Rayleigh (molecular) scattering, and the visible return at 500 or 600 nm provides a measure of the Rayleigh and Mie (aerosol) 

scattering return. From this data, a density profile 

of the atmosphere from 10 to 50 km can be calculated and a temperature profile can be extracted 

from the density profile. 

A more precise lidar determination of the tropopause height derived from the Rayleigh and Mie 

lidar backscattering signals converted to temperature profiles would improve the inversion methods 

for passive temperature instruments. There is also 

an important correlation between the tropopause 

height and total ozone, which can be measured simultaneously using a spaceborne lidar system. For 

example, when the tropopause height is low, there 

is little exchange between the tropopause and the 

stratosphere, and when there is a high tropopause 

or no tropopause, a jet stream occurs. 

In the stratosphere the distribution of ozone is 

strongly affected by the advection of ozone and energy, which together with vertical motions influence 

the temperature distribution. The photochemistry 

of ozone is highly temperature dependent. The motions, in turn, are strongly influenced by radiative 

sources and sinks of thermal energy, while ozone is 

the principal absorber of solar radiation in the stratosphere. Chemistry, radiation fields, and dynamics 

are all closely coupled in the stratosphere. The synergistic lidar measurement of aerosols, temperature, and density along with the ozone profiles will 

aid in understanding the complex interactions of 

these processes on ozone concentration. 

34
The XeC1 fundamental at 308 nm, and the first 

Stokes shift in hydrogen at 353 nm, can be used to 

obtain ozone concentration profiles using the DIAL 

technique. Since the demands for the laser transmitter are virtually identical for the measurement of 

ozone profiles or column content, there is no advantage in simplicity in just measuring the column 

content. 

Measurement Simulations 

Simulations for ozone profile observations have 

been calculated using the lidar system parameters 

listed in Table 15.

of the laser and telescope FOV was unity over the 

range of 50 to 0 km altitude. The receiver was assumed to have an aperture of 1 m diameter and an 

overall efficiency of 10 percent. For these preliminary estimations, no attempts were made to include 

wavelength-dependent parameters such as quantum 

efficiency, etc. The photon returns, as a function of 

altitude and range from the observing platform, are 

shown in Figure 39 for all of the wavelengths that 

can be easily obtained from the 308 nm output of 

the XeCI laser by stimulated Raman shifting in hydrogen; and the backscattered return for the 308 

nm wavelength, which is absorbed by ozone, is 

Table 15. Lidar System Parameters Used for Ozone Simulations 

Day Night 

System Altitude 700 km 700 km 

Receiver Clear Diameter 1.0 m 1.0 m 

Laser Pulse Energy 1.0 J 1.0 J 

Laser Wavelengths 308 nm, 353 nm 308 nm, 353 nm 

System Optical Efficiency 10% 10% 

Vertical Sampling Resolution 1 km 1 km 

Optical Bandwidth 0.2 nm, 0.001 nm 0.2 nm 

FOV 0.1 mr, 1 mr 0.1 mr 

Albedo 0.1, 0.8 0.8 

The basis for the ozone DIAL simulations is the 

U.S. Standard Atmosphere and the ozone profile is 

shown in Figure 37. Also required is an aerosol distribution model; that used in these simulations corresponds to a particularly clean atmosphere. It is 

shown in Figure 38. Scattering cross sections conforming to a Deirmendjian Haze L model were used 

and these are listed together with the other relevant 

cross sections in Table 16. 

The lidar equation was used to evaluate the 

magnitude of the return signal. The transmitted 

beams were each assumed to have an energy of 1 J/ 

pulse and to have a divergence such that the overlap 

( 

I I •

io' 7 1018 1019 

OZONE CONCENTRATION (rn-3) 

Figure 37. U.S. Standard Atmosphere ozone profile.

50 

40- .• - 

w30- - 

1-20- - 

I-

<10- ••. - 

0

I I I I• 

2 3 4 5 6 7 8 9 

LOG(10) AEROSOL DENSITY (rn3) 

Figure 38. Composite aerosol distribution (Stanford

Research Institute model). 

shown on an expanded scale in Figure 40. In order 

to estimate the precision of the measurement, the 

magnitude of other sources contributing to the observed signal must be considered. By far, the largest 

of these is the background caused by scattering of 

solar radiation. 

Procedure For Estimating Solar 

Background 

Each of the simulation calculations at wavelengths of 1,064 nm and less requires estimates of 

w 30 

20 

I-

-J 

<.10 

[i

35 
1100 

1000 

w 900 

'_j 

2 500

400

o 300

EL 200 

100 

0

(1 m Telescope) 

308 nm 

- 353 nm 

• 414 nm 

• 500 nm 

o 631 nm 

• 856 nm 

1 J/Pulse (All) 

650 660 670 680 690 

RANGE (km) 

50 40 30 20 10 

ALTITUDE (km) 

Figure 39. Multiwavelength lidar: signal returns

(composite aerosol, Haze Model L).

Table 16. Scattering Cross Sections 

Scatterer Wavelength 

308 nm 353 nm 

Ozone 1.28 x 10 2.00 x 10_22 cm2 

Rayleigh 5.54 x 10_27 3.21 x 10_27 cm2 

Haze L 1.35 x 10 -10 1.10 x 10 - 11 cm2 

the scattered sky radiance. For scattering at the surface, we assume Lambertian scattering such that 

1(X) = 10(X) a cos®/lr 

where a is the albedo and 0 is the solar angle from 

JJ zenith. Ignorng any wavelength dependence of the 

albedo, the typical albedo values for some representative surface conditions are shown in Table 18. 

The magnitude of the received background so700 lar radiation is given by: 

MSL

SBG = I(X)1A2 (zRIc) fi (X) dX 

I(X)A2 (zRIc) TI (X) AX 

Note: fl is the solid angle subtended by the telescope. For small angles, 

fl _ IT 4)2/4 

100

(1 m Telescope) 

ul 80 1 J/Pulse 

60 

40 

0 

I 

a0 r I I I I 

650 660 670 680 690 'LI'S]

RANGE (km) 

50 40 30 20 10 i!MI

ALTITUDE (km) 

Figure 40. Ozone lidar: signal returns at 308 nm

(composite Haze Model L). 

backscatter solar radiance. The backscattered solar 

radiance is directly proportional to the wavelengthdependent solar irradiance. Values given by Thekaekara (1974) may be used for the solar irradiance 

(Ia) and they are listed in Table 17 for several selected wavelengths. 

To a first approximation, it can be assumed that 

the attenuation by the atmosphere is balanced by

where 4) is the full plane angle of the telescope FOV 

Thus, the level of background solar radiation detected by the lidar receiver increases with the square 

of the telescope FOV. 

An Example: 

Sun at zenith, 0 = 0, over land, a = 0.2, and 

at 500 nm

1(500) = 1,947 x 0.2 x i/IT 

= 124 MW m 2 nm 1 sr' 

Table 17 Solar Irradiance 

Wavelength Irradiance 

X (nm) 10 (mW m 2 nm1) 

308 653 

353 787 

414 976 

500 1,211 

532 1,213 

631 1,300 

724 1,570 

760 1,773 

856 1,836 

1,064 1,947

36
Table 18. Typical Albedo Values 

Surface Albedo (a) 

Fresh Snow 0.9 

Cloud 0.8 

Old Snow 0.5 

Sand 0.25 

Soil 0.2 

Ocean 0.1

For a telescope with the following 

characteristics, 

FOV 1 mr fl = 785 x 10 sr 

Diameter 1.25 m A = 1.25 m2 

Range Element 1 km AR = 1,000 m 

Bandpass 0.1 nm AX = 0.1 nm 

Throughput 10 percent r(X) = 0.1 

the level of solar background actually counted by 

the detector is 16,000 photons per pulse per range 

bin. This is unacceptably high for virtually all measurements. There are only two parameters in the 

receiver system that can be changed to ameliorate 

this situation. Either the telescope FOV can be reduced or the spectral bandpass of the filter system 

can be reduced. 

The latter solution is not applicable to all lidar 

systems since some may require a large bandpass to 

obtain the desired information. Clearly, the only 

universal solution is to reduce the FOV of the telescope. However, the telescope FOV should normally 

be somewhat larger than the laser spot size in order 

to ensure overlap of the FOVs of both the transmitter and receiver systems. In reducing the ground 

spot size of the laser, the question of whether the 

laser will still meet eye safety requirements must be 

addressed. 

The computer model follows the error analysis 

given by Russell et al. (1979) and allows variation of 

the lidar system parameters such as receiver FOV, 

laser pulse energy and repetition rate, and spectral 

bandpass of the filter. The position of the sun and 

the surface albedo can also be varied. For the sun 

angle, two limiting conditions were used with the 

sun at zenith for worst-case daytime measurements, 

and at the horizon for nighttime measurements. The 

number of pulses that can be averaged to improve 

the SNR is limited by the horizontal resolution required and by the repetition rate of the laser. While 

the excimer laser is capable of operation at high 

repetition rates, up to 1 kHz, the system is limited 

by the power available on the platform and by lifetime considerations for the laser components. It has 

been assumed that a rate of 10 Hz will meet these 

operational restraints. Thus, for a horizontal resolution of 250 km, with a ground-track velocity of 7

km s', the maximum number of pulses that can be 

averaged is 350. This causes severe limitations for 

stratospheric measurements but does not constrain 

the tropospheric measurements. 

Figures 41 to 45 show the calculated error and 

the inverted ozone profiles for a number of different 

conditions, as marked on the figures. As can be seen 

from these simulations it is difficult to obtain precise 

concentration measurements above about 30 km for 

the 250 km horizontal resolution due to the velocity 

of the satellite and the low pulse rate of the laser. 

However, good measurements are possible in the 

lower stratosphere and in the troposphere. For the 

nighttime measurements the uncertainty is signal 

limited, and thus only methods to increase either 

the laser pulse energy or repetition rate will improve 

this measurement. The incorporation of very narrow bandwidth filters could enable daytime measurements with good SNR, but this increases the 

complexity of both the laser transmitter and receiver 

filter.

LIDAR SCANNING 

By scanning, greater geographic coverage (left/ 

right of orbit plane) can be obtained between successive orbits, but with the concomitant loss of data 

along the track for a fixed laser pulse rate. 

There is a great desire to enhance the global 

coverage of LASA measurements through scanning, 

40 

35 

30 

E 

.e 25 

w

20 

F_

15 

10

in i 70 9r, in 15 40 45 sn 5 

RANDOM ERROR (%) 

Figure 41. Ozone DIAL precision. Random error only. 

Does not include systematic errors due to large AX for 

the two DIAL wavelengths. Calculations performed for: 

telescope area = 1.23 m2, FOV = 0.1 mr, solar zenith 

angle = 0, 1,000 shots, throughput = 10%, albedo = 

0.8, filter width = 0.1 nm, Az = 2 km, pulse energy = 

1 J at 308 am and 0.1 J at 353 nm. 

37 
40 

35 

30 

E 

w O 

II--J 15 

10 

0 1 I I I I I I I 

0 4 8 12 16 20 24 28 32 36 40

RANDOM ERROR (%) 

Figure 42. Ozone DIAL precision. Parameters as in

Figure 41 except ifiter width is 0.001 nm. 

40 

35 

30 

E 

. 25 

Ui

20 

II-'15

40 

35 

30 

E 

Ui

20 

15 

10 

5 

01 I 1 1 I I I 

0 2 4 6 8 10 12 14 16 18 20

RANDOM ERROR (%) 

Figure 44. Ozone DIAL precision. Parameters as in 

Figure 41 except nighttime observations with albedo 0.1 

and ifiter width 0.2 am. 

40 

35 

30 

25 

Ui 0 

II-

-j 15 

10 

5 

I II I I 

0 -4 8 12 16 20 24 28 32 36 40

RANDOM ERROR (%) 

Figure 43. Ozone DIAL precision. Parameters as in

Figure 41 except albedo is 0.1. 

but there will be space, weight, and power limitations on the Eos initial orbiting configuration 

(IOC). A scanning mode will require these changes: 

• Provisions to compensate for the momentum 

changes induced by moving parts of the scanfling device. 

• Provisions to accommodate the additional instrument packages required for a LASA scanner.

0 1 I I Z I I I I I I I I 

0 1 2 3 4 5 6 7 8 9 10

RANDOM ERROR (%) 

Figure 45. Ozone DIAL precision. Parameters as in 

Figure 41 except nighttime observations with albedo 0.1, 

ifiter width 0.2 am, and Az = 5 km. 

• Provisions for increased power demand for a 

LASA scanner. 

• Increased pulse rate commensurate with increased ground coverage. 

A further consideration is this. A nadir-looking 

lidar with a FOV of 0.1 mr from 700 km orbit maps 

out a 70 m wide ribbon swath across the globe. 

38
Because of limited available power for the lidar 

from the Eos platform and because of lifetime consideration for laser components, one is limited to a 

pulse repetition rate of about 10 Hz. Therefore, one 

cannot consider total global coverage from a lidar 

experiment unless a coarse average over a large area 

is taken. Essentially, the lidar provides for a series 

of detailed sets of atmospheric measurements across 

the globe with wide swath gaps between the ribbons 

of lidar data. The addition of a scanning arrangement would fill the large gaps between the swaths 

obtained by the nadir instrument; the larger area 

covered, however, would have to assume a largearea horizontal homogeneity for chemical species 

for the expanded area to provide an accurate representation of global profiles. Essentially, one is averaging the same number of laser shots required for 

a single profile over a larger area of the globe. The 

use of scanning would require more laser energy, 

because the slant path encounters more atmospheric 

absorption and scattering, and would entail a 

greater R2 loss than the use of nadir viewing. 

SYNERGISTIC SCIENCE 

One of the fundamental objectives of Eos is to 

integrate to the fullest extent instrument systems to 

contribute complementary and/or mutually reinforcing data sets to the various disciplines. 

Because of the highly interdisciplinary nature 

of Eos, this challenging goal can best be met if it is 

incorporated in the initial planning efforts. 

To this extent, the LASA Panel has identified 

several areas of synergistic measurements, whereby 

one particular laser package can satisfy other measurement needs within LASA or where data sets mutually obtained by LASA, and by passive sensors or 

active sensors operating at wavelengths beyond infrared, strengthen the individual measurement sets 

by their complementary nature. 

A few examples demonstrating the synergistic 

potential of LASA measurements are listed below: 

• The LASA water vapor DIAL system could 

be used to make measurements - in addition 

to its primary function of obtaining water vapor profiles - of cloud-top heights, PBL 

heights, stratospheric aerosols, and tropospheric aerosols. This is possible because the 

information content of the "off-line" pulsed 

laser used in the DIAL measurements of 

water near 0.73 m is also near optimum for 

measurements of aerosols, cloud distributions, and aerosol properties in the troposphere and stratosphere. Frequency doubling 

of the off-line laser output into the 0.365 pm 

wavelength region provides the desired second wavelength for the two-wavelength 

tropospheric and stratospheric aerosol measurements. There are many advantages in conducting simultaneous measurements of water 

and aerosol/cloud distributions in atmospheric investigations with a lidar system. The 

small-scale (vertical resolution, Az < 100 m, 

and horizontal resolution, ix < 10 km) atmospheric structure can be determined using 

the off-line lidar returns at a 10 Hz repetition 

rate, while the on-line lidar returns are being 

acquired for the larger-scale water vapor 

measurements (z < 1 km and Ax < 100 km). 

• The measurement of water vapor will assist 

radiation studies when combined with other 

optical depth measurements of aerosols and 

clouds. Investigation of the atmospheric energy budget requires both passive and active 

techniques to observe seasonal and interannual variations in cloud properties, aerosols, 

radiation, liquid water, and water vapor. It is 

the complementary aspect of the LASA 

DIAL measurements to the data provided by 

passive remote sensors that permits a more 

complete understanding of important atmospheric processes on a global scale. While passive water vapor measuring devices provide 

global coverage, they are limited in spatial 

resolution and information content from 

lower atmospheric regions, and they depend 

on inversion techniques for obtaining water 

vapor concentration. 

• Measurement of PBL height using LASA, 

along with passive microwave measurements 

of atmospheric moisture using the Eos HighResolution Multifrequency Microwave Radiometer (HMMR), would provide an improved measure of moisture distribution with 

height. These measurements together would 

also allow for an estimate of moisture in the 

PBL. It is generally assumed that most of the 

moisture in the atmosphere (80 to 90 percent) 

is trapped within the PBL. Passive measurements of total atmospheric moisture, along 

with the height of the PBL together with the 

above assumption, are sufficient to derive the 

approximate moisture concentration in the 

PBL. Moisture concentration near the Earth's 

surface has generally been used as one term 

in the bulk aerodynamic approach to estimating surface moisture fluxes. Clearly, surface 

moisture flux is an important term in the hydrological cycle. 

• The measurement of aerosol optical characteristics and distributions can provide needed 

information for interpretation of measurements with passive satellite systems. Aerosol 

measurements with lidar will assist in removing aerosol-related artifacts from passive 

measurements of surface reflectance and atmospheric radiance. In particular, these 

39
measurements can assist in the interpretation 

of data from Eos passive instruments such as 

MODIS and HIRIS. The combination of aerosol profile information with water vapor profiles obtained from HMMR at low vertical 

resolution (>3 km) and from LASA at high 

vertical resolution (<1 km) greatly enhances 

the science content of each independently. 

• Carefully planned ground-based correlative 

measurements would yield valuable ancillary 

information to combine with the lidar aerosol 

backscattering measurements. These data can 

be used to construct reliable models for converting the aerosol backscatter to other desired radiative parameters such as aerosol extinction and optical depth. This procedure has 

already been used with good success in the 

case of correlative measurements of the stratospheric aerosol for the SAM II and SAGE 

satellite measurements. 

• Clouds often interfere with passive observations of the Earth's surface or the retrieval of 

atmospheric parameters. For example, thin 

cirrus clouds may often be present and obscure Earth imagery, leading to errors in the 

observation of surface characteristics. Also, 

clouds are not always entirely transparent to 

microwave radiation. Small but significant residual errors may therefore arise, due to 

clouds, in the retrieval of atmospheric and 

surface parameters from passive microwave 

observations. The addition of lidar measurements to detect and quantify cloud presence 

could be useful in improving passive observations. These measurements may also be 

used to calibrate cloud-top heights obtained

indirectly from infrared temperature 

determinations. 

The measurement of temperature profiles 

with LASA will provide information that will 

substantially enhance the interpretation of 

data from passive instruments as well as complement the data from other active experiments. The lidar will provide direct measurements of the temperature across given 

atmospheric layers without the need to use 

inversion techniques to retrieve the temperature profile, as required by passive sounders. 

This will allow comparisons of the measurements using two independent techniques. The 

high accuracy and vertical resolution of the 

lidar measurement will also supplement the 

passive sounding experiments by providing 

the shape of the temperature profile, which 

can then be used as a first guess by the passive 

systems. 

The measurement of ozone with LASA can 

be used to help recalibrate passive ozone column-content instruments. The multiwavelength lidar concept will simultaneously provide for density and temperature profiles in 

the 10 to 50 km altitude range, along with the 

ozone concentrations. 

As Eos moves from concept to implementation, 

the LASA Panel envisions that considerable additional efforts will be devoted to better define the 

mutual interdependence of instrument packages. 

These ongoing efforts will guarantee that all data 

obtained from Eos are consistent in space and time 

so that they can be integrated into a mutually supportive data set to the benefit of all. 

40
III. LASA SCIENCE: ICE-SHEET DYNAMICS

AND SURFACE ALTIMETRY 

LIDAR ALTIMETRY APPLICATIONS 

The lidar altimeter provides precise range measurements to the Earth's surface by acquiring data 

on laser pulse time-of-flight and reflected pulse energy. The primary scientific need is for topographic 

mapping of polar ice-sheet surfaces. Altimetry data 

on terrain and ocean surfaces will provide additional 

useful data sets for the 90 percent of each orbit that 

lies outside the ice-sheet regions. When cloud cover 

obscures the Earth's surface, the altimeter will provide an accurate measure of cloud-top altitude. 

Analysis of the amplitude distribution of the altimeter pulse will also provide information on the 

surface roughness within the laser footprint for studies of glacier and sea-ice roughness and bare terrain 

microtopography. 

Measurements of ice-sheet topography to the 

precision provided by lidar altimetry are not possible with radar altimeter instruments. Radar altimeters have a pulse-limited footprint that is kilometers in extent and is biased over the ice-sheet 

surface by the presence of crosstrack slopes (Griffiths, 1981). In addition, the radar altimeter depends 

on multiple pulse averaging because of target-induced speckle modulation and the consequent low 

SNR for an individual pulse measurement. The optical wavelength of the laser altimeter permits even 

a relatively small transmitter to provide a submilliradian beam divergence and beam-limited footprint 

of less than 100 m. This, together with a short, highpower pulse and freedom from speckle modulation, 

results in high-accuracy range measurements for individual laser pulses. The lidar altimeter also has 

the capability of a relatively large range gate, which 

will provide measurement of discontinuities in range 

(e.g., ice-shelf boundaries) with the full horizontal 

resolution provided by the altimeter repetition rate. 

Over terrain features the small footprint, beamlimited geometry, and short-pulse measurement 

provide additional useful data sets. Most benefit is 

obtained when the precise laser-altimeter data are 

used for reference and intercomparison with wide 

survey topographic-mapping data from photographic and radar instruments. For certain types of 

terrain where precise knowledge of topography, and 

absolute roughness, and dynamic phenomena is required, lidar altimetry would provide the only 

space-based measurements with the appropriate 

vertical and spatial resolution. Examples of these 

types of terrain include volcanic areas, desert sand 

sheets, crustal flexure regions, impact craters, and 

flood plains. In addition, a unique application for 

the laser altimeter is found in the assessment of 

tropical biomass by the measurement of forest canopies and timber volume (Nelson etal., 1984). Here, 

the short, beam-limited laser pulse can penetrate to 

the bare Earth, a measurement not possible with 

photographic techniques. 

MEASUREMENT OBJECTIVES 

A discussion of the science role of a lidar altimeter in ice-sheet measurements published by 

Zwally et al. (1981) documents the special needs of 

a laser altimeter for ice-sheet measurement. The 

principal requirements are for a small (70 m) footprint on the ice-sheet surface, a high SNR for accurate individual pulse (no averaging) measurements, a moderately high pulse repetition rate of at 

least 20 Hz over the ice sheets, and highly accurate 

pointing knowledge for each laser-pulse data point. 

The combination of the 70 m footprint requirement 

and the 700 km altitude of the Eos platform results 

in a requirement for a 0.10 mrad divergence angle. 

The pointing requirement results from the mapping of angular offsets with respect to nadir into 

range measurement biases. Extremely accurate 

knowledge of altimeter attitude (to the arcsecond 

level for each laser pulse) is needed in order to compute altimetry angular offset from nadir. The scale 

factor relating altimetry range error to laser pointing 

uncertainty for a flat target surface is 3 cmlarcsec 

for a 700 km orbit altitude. Sloping target surfaces 

aggravate the effect of pointing error on altimetry 

accuracy. In addition to laser altimeter attitude, it 

is very important to know spacecraft position relative to Earth coordinates. The horizontal position 

of each laser-pulse footprint on the Earth's surface 

must be known to an accuracy of the order of 10 m. 

This can be accomplished by incorporating a global 

positioning system (GPS) receiver in the LASA instrument electronics package. Additional refinement of vertical position knowledge to the 10 cm 

level required for altimetry data can result from use 

of ground-based laser-ranging measurement to a 

retroreflector array located on the spacecraft. 

Laser-range measurement during flight by one or 

more of the existing stations of the Laser Tracking 

Network can define the spacecraft orbit to the centimeter level. The Geodynamics Laser-Ranging System (GLRS), described in the next chapter, ranging 

to a number of ground-based retroreflectors, can 

also provide a powerful solution to orbital parameters. Analysis of altimetry data to locate orbital 

crossing points provides the final step in reduction 

of spacecraft position errors. 

Over the oceans, the laser altimeter will also 

provide a high-resolution verification of range 

41
measurement for other more general survey instruments. In particular, the reduced dependence of optical range on the unknown column content of atmospheric water vapor suggests calibration of the 

radar altimeter with laser data. The short pulse and 

the small footprint of the laser altimeter can also be 

very useful in resolving ocean wave structure. Vertical resolution of 10 cm or better coupled with a 70 

in footprint size results in data unavailable with radar altimeters. 

SYNERGISTIC SCIENCE 

One of the best synergisms for laser altimeter 

data is derived from simultaneous operation of laser 

and radar altimeters. In the case of the radar altimeter, it is assumed that the device available for 

flight on the Eos platform would be a derivative of 

the pulse compression 13.6 GHz radar altimeter first 

flown on Seasat. The relatively large footprint (several kilometers) of this device has the advantage of 

always including the satellite nadir point in its FOV. 

In ice-sheet altimetry, the laser data would define 

surface topography to high precision every 350 in 

along the nadir track, remove the surface-slope ambiguity characteristic of radar altimetry, and provide 

the only altimetry data in regions where the surface 

slopes are too large or changing too rapidly for the 

radar altimeter. 

Over ocean areas, the radar altimeter has a 

clear advantage in providing accurate, all-weather, 

wide-coverage measurements of sea-surface topography. Bursts of high repetition-rate laser data (approximately 100 in spatial resolution) over ocean 

areas could, however, serve to calibrate the radar 

altimeter data by removing uncertainties in the microwave propagation delay due to atmospheric water 

vapor. At present, the lack of precise knowledge of 

the water vapor content contributes up to several 

tens of centimeters in range error, but only at microwave frequencies where there is significant water 

vapor effect on refractive index.

The instrument design parameters (large aperture size and large laser pulse energy) assumed for 

the simulations described in the previous chapter 

were considered necessary for atmospheric lidar investigations. This results in an excess capability for 

altimetry that can be used to ensure that each individual laser pulse results in a high SNR range 

measurement. No averaging of sequential pulses is 

necessary or, in fact, desired. This is especially important for the ice-sheet topography data where a 

relatively small portion of each orbit offers an opportunity for data acquisition. Repetition rates of at 

least 20 Hz for laser operation are required to resolve kilometer scale horizontal structure on icesheet surfaces. 

A block diagram showing the interrelationship 

of components in the altimeter mode of operation 

of the LASA instrument package appears in Figure 

46. Two lasers are used in this concept for 

redundancy. 

It may be advantageous to consider a simple 

form of crosstrack scanning for laser altimeter measurements. In general, the relatively low repetition 

rates of laser devices, even the 20 Hz of a burstmode operation, are not sufficient to support scanning data acquisition. This is because the rapid 

movement of the satellite nadir point at approximately 7 km/sec requires obtaining all data along 

the nadir track in order to obtain sufficient horizontal resolution. If a sacrifice of along-track resolution 

is possible, the crosstrack data could provide additional information to resolve the surface-slope ambiguity of the radar altimeter over ice sheets. A 

major problem with crosstrack scanning beyond 1.5° 

is the degradation in precise range measurement 

due to pulse spreading. Any proposed use of scanning must also be approached cautiously because of 

the increased demands placed on the measurement 

of altimeter pointing attitude as the instrument optical system is in constant motion. Further studies 

are required to determine if the scientific benefits 

from scanning outweigh the degradation in horizontal resolution and range precision. 

ALTIMETER INSTRUMENT

DESCRIPTION 

The lidar altimetry technique consists, in essence, in the precise measurement of the time of 

flight of the laser pulse from the transmitter to the 

surface and then back to the satellite detector. The 

timing precision should give an equivalent range resolution of 1 cm. Altimetry output data include pulse 

time-of-flight and transmitted and received pulse 

energy for each laser shot. The ratio of received and 

transmitted energies provides information on atmospheric transmission and target albedo 

variations.

MEASUREMENT SIMULATIONS 

The set of altimeter instrument parameters assumed here for simulation studies is included as Table 19. These parameters are significantly different, 

and in some respects more demanding on LASA 

design, from those given in Chapter II. Also note 

that the LASA maximum laser pulse energy of 1 J 

is not necessary for altimeter operation, and a transmitted energy value of 100 mJ has been assumed. 

This would conserve laser lifetime, conserve electrical power, and make possible the maximizing of 

laser repetition rate in a burst mode that may be 

required over the ice-sheet surfaces. 

42
POLAR PLATFORM DATA &

THERMAL CONTROL 

SYSTEM 

500 kbit/sec 

STAR I LASA COMPUTER I FREQUENCY TRANSLATION 

TRACKERS

I ALTIMETER ELECTRONICS I AND TUNING OPTICS 

INERTIAL

REFERENCE

UNIT

FILTERS 

1.25 METER DIA. AND

RECEIVER TELESCOPE I DETECTORS

BEAM r 

ANGLE LASER 11 

AND 

ENERGY LASER #2 

SENSOR

100 mJ/Pulse 

COLLIMATING

OPTICS

POWER 

SUPPLY

500 W

AVERAGE 

PLATFORM

POWER 

SPACECRAFT

NADIR 

Figure 46. LASA atmospheric sounder and altimeter concept. 

Table 19. LASA Altimeter Parameters 

Parameter Value 

Laser Energy 100 mJ/pulse 

Repetition Rate 20 Hz burst mode, 

1 Hz to 5 Hz continuous 

Laser Pulsewidth 5 ns 

Laser Divergence 0.1 mrad 

Receiver Optical Efficiency 0.38 

Receiver Telescope Diameter 1.25 m 

Optical Bandpass Filter 5 A 

Detector Quantum Efficiency 0.25 at 0.50 Rm, 

0.25 at 0.75 p.m, 

0.60 at 1.00 .tm 

Detector Bandwidth 50 MHz

Altimetry operation at three laser wavelengths 

of 0.5 p.m, 0.75 p.m, and 1.0 pm is indicated in this 

table. They represent the practical range of values 

that would provide good ice and snow reflectance, 

and are long enough in wavelength that atmospheric 

transmission is adequate. A plot of snow and ice 

reflectance from O'Brien (1975) is included as Figure 47. Note that reflectance begins to fall off near 

1 p.m wavelength and goes rapidly to near zero at 

1.5 p.m. These three laser wavelengths are also practical choices based on current and foreseeable laser 

technology. The wavelengths of 1.0 p.m and 0.5 p.m 

are close, respectively, to the fundamental and frequency-doubled wavelengths of the solid-state 

Nd:YAG laser and its derivatives. The 0.75 p.m 

wavelength represents the spectral region where 

water vapor DIAL measurements are possible and 

the tunable, vibronic crystal lasers of Alexandrite, 

emerald, titanium-doped sapphire, and others are 

practical choices. Detector quantum efficiency is a 

43 
1.0 

0.8 

LU 0

0.6 A 

I0.4 

-J 

Uw 

Cr

0.01 I 

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 

WAVELENGTH (jim) 

Figure 47. Reflectance (r) of old snow (relative to BaSO4) 

versus wavelength. Curve A - original old cold snow, 

density = 0.347 g1cm3; Curve B - wet melting snow, 

density = 0.403 g/cm3 ; Curve C - refrozen snow, density 

= 0.403 g1cm3. 

variable among these wavelengths. In the 0.5 p.m to 

0.75 p.m region a photomultiplier tube is currently 

the best choice and provides nearly noiseless gain 

with a quantum efficiency of about 0.25 at 0.5 p.m 

up to 0.4 at 0.75 p.m. Near 1.0 p.m the Si avalanche 

photodiode (APD) is the detector of choice. Its 

quantum efficiency is high, approximately 0.6, but 

the detection process suffers from excess noise in 

the APD and in its matching to a low-noise preamplifier. Detector performance at this wavelength is 

best specified by NEP, assumed here to be about 

3 x 10' W/Hz112. 

A final consideration specific to the needs of the 

laser altimeter is choice of laser pulsewidth. The 

assumption in Table 17 of a 5 nsec full-width halfmaximum (FWHM) laser pulsewidth is based on the 

use of a 0-switched solid-state laser and is necessary in order to provide the required range resolution in the altimetry processing electronics. A 5 nsec 

pulse represents a range resolution increment of 

0.75 m. Actual range precision can be much better 

than this because of timing measurements made to 

a small fraction of the pulsewidth. The use of such 

a short pulse stands in contrast to the needs of atmospheric remote sensing, where much less vertical 

resolution is required. 

Laser-altimeter data quality is determined primarily by the strength of backscattered laser radiation from the target surface, but is also affected by 

pulse-to-pulse variability, dynamic range considerations, and pulse spreading. The basic time-of-flight 

or range measurement deemed necessary for altimetry is, in essence, a threshold measurement. 

Only those pulses which trigger the discriminators 

enter into the data stream. This effect acts to suppress background noise and detector noise up to the 

point where they and their pulse-to-pulse variability 

have a significant probability of triggering the timeof-flight measurement. The laser-altimeter concept 

described above is based on a very high SNR environment so that each individual pulse can be depended upon for a unique range measurement. 

Thus, the altimetry performance analysis is primarily one of verifying that an adequate SNR is present 

at most times, and describing those circumstances 

that can lead to marginal detector SNR. 

Target reflectance specification is a first step toward calculation of received signal strength. For discrete targets, as found in reflection of laser pulses 

from the ice-sheet surface, ocean surface, or terrain 

features, the specification of target backscatter is 

conveniently described by the term nfl where r is 

target reflectance (hemispherical) and ft is the solid 

angle of backscatter. For a Lambertian (diffuse) target backscatter at normal incidence, fl = 'rr. The 

ice-sheet surface is well modeled by such a Lambertian reflectance and has values of r/[1 in the range 

of 0.4/7r to 0.8/IT, dependent on laser wavelength 

and snow/ice composition and age. Soil and vegetation are somewhat more variable and lower in reflectance with nfl estimated in the range of 0.1/7n to 

0.4hr. The ocean surface is a special case since it 

exhibits a specular rather than a Lambertian reflectance. This is especially true near normal incidence 

(nadir-viewing) where a mirror-like reflectivity is 

one component of the backscatter. Fresnel reflectance of ocean water from 0.5 p.m to 1.0 p.m is only 

about 0.02 but the solid angle of backscatter can be 

significantly smaller than a Lambertian surface. 

Ocean backscatter is typically concentrated into a 

specular reflection of 10° to 30° width. Actual width 

of the backscatter pattern is determined by the orientation of the smallest wave slopes within the laser 

footprint. These slopes act as mirror surfaces and 

are distributed in tilt angle in accordance with wave 

and wind statistics. Recent airborne laser measurements and a full explanation of these phenomena 

are presented by Bufton et al. (1983). The range of 

variability of ocean surface backscatter is taken 

from these measurements to be 0.1/7n to 0.3hr within 

a few degrees of nadir. 

Cloud tops are diffuse targets that require characterization in terms of a backscatter function PAZ, 

where 13 is backscatter coefficient in rn- 1 sr' and 

Z is scattering pathlength in the cloud. Typical 

values of PAZ for thick cloud layers can be derived 

from the 13 and extinction (E) values reported by 

Cato et al. (1967). They are in the range 0.01 to 0.1 

for the LASA range of wavelengths. 

It should be obvious that the presence of cloud 

cover will disrupt laser altimetry data and at the 

same time enable the measurement of cloud-top 

height. Some thin cloud layers will permit continued 

altimetry operations at reduced SNR margins. This 

is the case for thin cirrus layers. Cirrus cloud models 

show that mid-latitude and polar regions have 40 

percent to 50 percent cirrus cloud cover at all times, 

while the equatorial region has an 80 percent incidence of cirrus. Typical cirrus cloud thickness is 

about 1 km and 75 percent of all cirrus is less than 

44
2 km thick. Cirrus layers are almost invariably the 

highest layers (5 to 15 km) and are, thus, a factor in 

attenuation of laser radiation for both the altimetry 

and cloud-top data sets. An analytical and empirical 

model for cirrus transmission is reported by Kneizys 

et al. (1983). The total path loss for the two-way 

propagation path through cirrus, as calculated from 

this model, is TA = exp(-0.28 L2), where L is 

cirrus layer thickness in kilometers. Polar stratospheric clouds have also been shown by Hamill and 

McMaster (1984) to be a significant factor in the 

polar night where sufficiently low stratospheric temperatures lead to a high incidence of a broad, optically thin cloud layer. 

A more serious degradation of two-way optical 

transmission in the 0.5 pm to 1.0 Rm altimetry 

wavelength region is imposed by molecular scattering, and both scattering and absorption by lowertropospheric aerosols. The transmission models 

developed for these effects by the Air Force Geophysics Laboratory (AFGL) represent a convenient 

way of parameterizing the effects of atmospheric 

loss on altimeter performance. There is little variability in molecular scattering loss versus season or 

latitude in the altimetry wavelength region, and total two-way extinction is small, especially at the long 

wavelength end (1.0 pm) due to the inverse fourthpower Rayleigh scattering. Aerosol extinction is significant in the atmospheric surface layer. In the analysis presented here, the AFGL Rural, Tropospheric, Maritime, and Urban aerosol models were 

used. These models are fully documented by McClatchey et al. (1970) and Shettle and Fenn (1979). 

Table 20 lists values of two-way atmospheric transmission for each of these models under clear (C) 

and hazy (H) conditions and the three altimetry 

wavelengths under consideration. The small effect

of molecular scattering is included in each transmission value. 

For nadir-viewing altimetry operation, the lidar 

or link equation for received laser pulse energy ER 

can be written 

ER = E AR- TOTCTA2 (nfl) (1) 

where ET = transmitted laser energy, AR = receiver area, T0 = receiver optical transmission, T 

= cirrus cloud transmission, TA = atmospheric 

molecular and aerosol transmission, [exp( - 2XT)], 

XT = total atmospheric extinction, and nfl = target 

backscatter. 

Calculations of E R were performed for high 

(H), typical (TYP), and low (L) values of nfl from 

Table 21 for each of the four target surfaces: ice and 

snow, ocean surface, soil and vegetation, and cloud 

top. The ratio of ER to the quantity hv/q (energy 

per photoelectron) yields the magnitude of the laser 

backscatter signal in photoelectrons. In this expression, hi' is energy of one photon at frequency v and 

q is detector quantum efficiency. At 0.5 m and 0.75 

rim, the altimetry detector is assumed to be a photomultiplier tube. At 0.5 pm a quantum efficiency 

of 0.25 is assumed, while at 0.75 jkm a quantum 

efficiency of 0.4 is possible. These values lead respectively to energies of 1.6 x 10' J/photoelectron 

and 6.6 x 10' J/photoelectron. At 1.0 jim, the Si 

APD detector has quantum efficiency of at least 0.6, 

which results in 3.3 x 10-' J/photoelectron. Tables 

22, 23, and 24 present the expected number of photoelectrons per laser pulse and the signal-to-background ratio E RIEB . Each table is produced for one 

of the three proposed altimeter wavelengths and includes the results of calculations for clear and hazy 

Table 20. Two-Way Nadir-Viewing Atmospheric Transmission 

Laser Altimeter Wavelength 

Aerosol Model Visibility 0.5 p.m 0.75 p.m 1.0 p.m 

XT

'i'2 

T 1A T 1A T 

C 0.21 0.66 0.090 0.84 0.039 0.93 Tropospheric H 0.39 0.46 0.22 0.64 0.12 0.79 

Maritime C 0.28 0.57 0.16 0.73 0.13 0.77 

H 0.88 0.17 0.73 0.23 0.81 0.20 

C 0.38 0.47 0.21 0.66 0.13 0.77 Rural H 0.95 0.15 0.65 0.27 0.43 0.42

Urban C 0.44 0.41 0.26 0.60 0.18 0.70 

H 1.44 0.056 1.07 0.12 0.75 0.22 

XT = Total atmospheric extinction 

TA' = Two-way atmospheric transmission, exp (- 2XT)

45 
Table 21. Target Backscatter Properties 

Laser Altimeter Wavelength 

Target Type Target

Reflectance 0.5 p.m 0.75 p.m 1.0 p.m 

RT nfl RT n/fl RT nfl 

H 0.8/ir 0.7hr 0.6/7r 

Ice and Snow TYP 25 0.7hr 23 0.6/IT 16 0.5/,Tr 

L 0.6/,Tr 0.5/ir 0.3hr 

H 0.3/7r 0.3Iir 0.3/7r 

Ocean TYP 2 0. 1/7r 3 0.1/'rr 4 0. 1/7r 

L 0.02/,Tr 0.02hr 0.02/7r 

H 8 0.5frr 8 5.2 Soil and TYP 6.5 0.23/rr 7 0.24/ir 6.5 0.251rr Vegetation L 5.5 0.1/ri 6.1 7.3 

H 25 0.10 23 14.5 

Cloud Top TYP 0.01 0.01 0.006 

L 19 0.001 19 11 

R1 = Daytime radiance (mW/m2srA) at solar zenith angle = 60

nfl = Target backscatter factor 

conditions for each of the aerosol models deemed 

applicable to altimetry data for each target. Note 

that better altimetry performance is indicated as the 

wavelength increases. This is a result of the increase 

in number of photons for a given E R as wavelength 

increases and small improvements in atmospheric 

transmission (despite a reduction in target backscatter cross section) at 1.0 pm. In nearly all cases, 

the number of signal photoelectrons (SPE) is quite 

large. The only significant reduction in received 

photoelectrons below 500 occurs under hazy conditions with maritime or urban aerosols at 0.5 .tm 

wavelength. 

Calculations have also been made for received 

optical energy (EB) fesulting from daytime background radiance of the various target surfaces. For 

these calculations of energy, a time interval of 15 

nsec was assumed. This is appropriate for detection 

of the full backscattered (undistorted) laser pulse 

and is three times the laser pulsewidth. The values 

of En are calculated from, 

EB = RT R0 FB AR 15 x 10 sec, (2) 

where RT = target radiance (WIm2srA), R0 = 

receiver FOV (sr), FB = filter bandpass (A), and 

AR = receiver area (m2). 

Values of RT from Wolfe and Zissis (1978) are 

listed in Table 21 as a function of wavelength and 

target type. The other parameters are given in Table 

19. In Tables 22, 23, and 24, the ratio of ER/EB is 

included to show the margin of laser backscatter

signal strength over the background of a solar-illuminated target. Note that most ratios are on the 

order of 100 to 300 and quite consistent at the three 

wavelengths. The only exceptions are ratios of 10 to 

15 for altimetry detection of daytime cloud tops. 

Even in these more marginal cases, the SNR is adequate to permit an altimetry threshold setting that 

will eliminate the mean background photoelectron 

level from triggering the time-of-flight and energy 

measurement electronics. Background-induced 

quantum noise (E B)' will also be dominated by 

laser signal quantum noise. The amount of background radiance from moonlit clouds for nighttime 

operations is about 10_6 less than the daytime 

values. 

For the quantum-noise-limited photomultiplier 

tubes, the noise level is simply the square root of 

the number of signal photoelectrons, (ER) 1t2 . In 

the case of the Si APD, the NEP estimated from 

state-of-the-art devices is 3 x 10_ 14 W/(Hz)1. 

This leads to a noise equivalent energy (NEE) of 

3.2 x 1018 J for the assumed 50 MHz detection 

bandwidth and 15 nsec interval, or about 10 photoelectrons. Hence, a value of 10 photoelectrons must 

be added to ER before the square root is taken to 

calculate noise. With the strong laser backscatter of 

several hundred to several thousand photoelectrons, 

predicted in Tables 22, 23, and 24, this effect of 

detector noise is negligible. 

Such strong laser backscatter for individual 

pulses means that altimetry signal processing can be 

done with pulse-current waveforms, and photon 

46 
Table 22. SPE and E,,/E. for 0.5 ttm Altimetry Wavelength 

Aerosol Model 

Target Type Tropospheric Maritime Rural Urban 

C H C H C H C H 

Ice and Snow SPE 6,900 4,600 5,600 2,100 4,600 1,500 4,000 550 

E./E. 210 170 190 130 180 100 160 60 

Ocean SPE 940 620 810 240 690 210 - - 

E,/E, 380 290 350 190 320 180 - - 

Soil and SPE 2,100 1,500 - - 1,600 490 1,300 180 

Vegetation E./E. 260 220 - - 230 130 210 77 

Cloud Top SPE 290 210 - - - - - - 

E,/E,, 12 10 - - - - - - 

Table 23. SPE and E,,/E. for 0.75 urn Altimetry Wavelength 

Aerosol Model 

Target Type liopospheric Maritime Rural Urban 

C H C H C H C H 

Ice and Snow SPE 18,000 13,000 15,000 4,800 14,000 5,600 13,000 2,600 

ER/ER 230 190 200 120 200 120 190 85 

Ocean SPE 2,900 2,300 2,600 800 2,300 940 - - 

E,,/E. 280 250 270 150 250 160 - - 

Soil and SPE 7,100 5,300 - - 5,600 2,300 5,000 1,000 

Vegetation E,/E, 290 250 - - 260 170 250 110 

Cloud Top SPE 920 700 - - - - - - 

E,/E, 14 12 - - - - - -

counting may not be necessary. The major source of 

difficulty in acquisition of altimetry data is likely to 

be the dynamic range necessary to accommodate 

variability in ER. Two sources of expected variability 

in ER are target reflectance variations from pulse to 

pulse and atmospheric transmission fluctuations due 

to aerosols or thin clouds. Atmospheric turbulence 

and target-induced speckle noise will be negligible 

for telescope apertures of 0.5 m or larger. It is beyond the scope of this report to quantify ER variability, but this study needs to be done. 

As mean atmospheric attenuation increases, or 

for operation at the low end of backscatter variability, the smaller number of received signal photoelectrons results in increased range error. This effect 

is readily quantified by use of Poisson statistics for 

the rate of emission of both signal and background

photoelectrons. It is shown by Abshire (1978) that 

the rms theoretical ranging precision AR for an optimum matched filter receiver for Ns signal photoelectrons and a laser pulsewidth AT is given by 

AR = (thT/2ir)/(N) 112 (3) 

For 100 photoelectrons this expression yields a 

AR of 1.2 cm (0.08 nsec). This precision will be 

further degraded by the presence of background 

photoelectrons according to 

AR = (thT/2Tr)/ (4)

{NB + N - [NB (NB + 2N)]112}1'2 

This increases AR to 2.3 cm (0.15 nsec) when 

Ns and NB are both at the 100 photoelectron level 

47
Table 24. SPE and E,/E,, for 1.0 pin Altimetry Wavelength 

Target Type Tropospheric 

C H

Aerosol Model 

Maritime Rural 

C H C H

Urban 

C H 

Ice and Snow SPE 33,000 28,000 28,000 7,200 28,000 15,000 25,000 7,800 

ER/Es 290 260 250 130 250 180 250 130 

Ocean SPE 6,600 5,600 5,600 1,400 5,600 3,000 - - 

ER/E. 220 200 210 100 210 150 - - 

Soil and SPE 17,000 14,000 - - 14,000 7,500 13,000 4,000 

Vegetation ER/EB 340 330 - - 310 240 310 170 

Cloud Top SPE 1,300 1,100 - - - - - - 

ER/E,, 15 14 - - - - - -

(i.e., a unity SNR). Practical laser-ranging receivers 

are usually within a factor of ten of these theoretical 

results. Their additional errors result from timing 

jitter in both detector and processing electronics 

and possible biases due to laser transverse mode 

structure. By minimizing these practical difficulties, 

it should be possible to obtain rms range precision 

under 10 cm rms fora 5 nsec laser pulse. 

A more dramatic increase in altimeter range 

uncertainty AR (than that due to low signal levels 

or strong background) results from pulse spreading 

since both Equations (3) and (4) are proportional 

to AT. The interaction of a finite laser beamwidth 

a, an angular offset 4) from nadir, and a surface 

slope S can produce significant spread ATi beyond 

the nominal laser pulsewidth AT. This spread can 

easily be estimated from the expression derived 

from the geometry illustrated in Figure 48. 

= (2/c) tan (4) + 5) Z (5) 

where c is the speed of light and Z is the slant range 

to the surface. Note that surface slope can either 

add a pulse spread or reduce it depending on slope 

polarity with respect to the angular offset from nadir. For the LASA laser altimeter, Z = 7 x 10 m, 

a = 1 x 10 rad, 4) < 10 rad, and, for ice-sheet 

altimetry, S 10_2 rad, 82 percent of the time. As 

a result, ATi :!s 9.3 nsec and the original laser pulse 

of 5 nsec is spread by a factor of two. This will 

require, according to Equation (3), a factor of four 

increase in the number of signal photoelectrons to 

achieve the same range precision. 

When the angular offset 4) is not known exactly 

(i.e., there is satellite attitude error) the uncertainty 

A4 maps directly into a range error AR, = Tic/2. 

This effect can be calculated from Equation (5) with 

A4) substituted for a. The work of Zwally et al.

(1981) documented these angular uncertainty effects 

on range error as a function of S for various 0 

values. Their results are repeated in Figure 49 and 

show that LASA attitude knowledge must be kept 

to about 5 arcsec rms for 10 cm precision or better 

altimetry measurements of the ice-sheet surfaces. 

The range measured to the Earth's surface from 

a space-based laser altimeter exceeds the actual 

range due to a slowing (propagation delay) of the 

optical pulse by the refractive index of the atmosphere. The amount of this delay depends on meteorological conditions and is about 2.4 m at nadir 

for the two-way atmospheric path from space to sea 

level and return. The delay is also an inverse 

SATELLITE 

POSITION 

H I \ \ ALTIMETER 

\ \BEAMWIDTH 

PA 

EARTH'S

SURFACE / s 

Figure 48. Geometry for laser altimeter ranging error in

off-nadir pointing. 

48 
•

4' / /7 

-82%

of

'% 

/ / 0 Surface 

- I 

-

1 (degrees) 2 

I ii I i 

0 0.01 0.02 0.03 

S (radian) 

Figure 49. Laser ranging error (AR) versus surface slope (S) for various values of satellite pointing error (44). 

8

OR 

E 

cr

Ii 

function of wavelength. Hopfield (1972) points out 

that the propagation delay is simply the integral over 

(n —1) along the propagation path where n is the 

local atmospheric refractive index (proportional to 

atmospheric density). For vertical observations, the 

integral over atmospheric density is just the surface 

pressure. Thus, at nadir the propagation delay is 

independent of surface temperature and temperature structure in the atmosphere. Marini and Murray (1973) report a formula for range correction to 

account for the propagation delay. It is given approximately by 

AR = 0.002357(0.9650 + 0.0164fL2)P (6) 

where L is laser wavelength in micrometers, P is 

surface pressure in millibars, and AR is range correction in meters. At 0.5 i.m wavelength, AR is 

2.46 m and decreases to 2.34 m at 1.0 p.m. The 

derivative of Equation (6) with respect to surface 

pressure yields a range correction sensitivity of 2.3 

mm/mb. Thus, a 43 mb uncertainty in P would result in a 10 cm range error. This illustrates that P 

must be known or modeled to about 2.5 percent in 

order to keep the uncertainty in atmospheric propagation delay less than the required range precision.

EXAMPLES OF LASER-ALTIMETER

DATA 

Laser-altimeter data useful for determination of 

ice-sheet topography will necessarily require a stable space platform, careful correction for laserpointing angle and orbital position, and an accumulation of multiple data sets over months of time 

with many opportunities for orbital-track crossing 

points. Approximately 1 year of space-based observations will be required in order to provide an adequate data product for the polar ice sheets. It is 

not possible to perform quick, suborbital mapping 

of ice-sheet areas. Fortunately, the time constant of 

the ice sheet and other surface topographical processes is sufficiently slow that a gradual accumulation of high-precision data is possible. At present, 

only demonstration data sets of laser altimeter operation from aircraft are available. They serve to 

illustrate the type and quality of data that need to 

be accumulated. 

Figure 50 is an example of a continuous profile 

of laser-altimeter data acquired over the sea ice in 

the 1970s from the NASA C130 aircraft. These data 

were acquired with a laser-geodimeter ranging device that is the cw analog of the pulsed laser altimeter planned for LASA. This data set illustrates 

centimeter-level range precision from an aircraft 

platform and demonstrates the need for correction 

of low-frequency biases or drift in altitude due to 

aircraft motion. The typical ice-sheet surface roughness with peak-to-peak variations of a few meters

IRCRAFT MOTION 

I I I I I I I I I I I I 

0 200 400 600 800 1000 1200 1400 

DISTANCE (m) 

Figure 50. Continuous profile laser altimetry over sea

ice in the 1970s. 

and horizontal scales of tens-to-hundreds of meters 

is evident in the figure. The planned laser footprint 

size of 70 m for the LASA altimeter would integrate 

over some of the fine-scale structure seen here and 

would resolve the large-scale structure. Sea-ice 

roughness would be revealed by a slight spreading 

of each LASA altimeter laser pulse. 

Figure 51 is an example of aircraft laser-altimetry data on a profile crossing the California 

coast just south of Big Sur. In this case, the laseraltimeter instrument was a pulsed Nd:YAG system 

constructed for cloud lidar observations and operated on the NASA ER-2 aircraft in 1983. The significance of this data set is the 10 cm resolution in 

range measurement with over 1,000 m change in 

surface topography as the aircraft flight-track 

crossed the coastal mountain range. Over the ice 

sheets, the same vertical resolution is required and 

I-=4 

(D 

w2 I 

49
1400

CALIFORNIA COAST

1200 SOUTH OF BIG SUR 

1000 

800 

I 

(-9 600 

w 

' 400 10cm RESOLUTION 

ALTIMETRY DATA 

12 

DISTANCE (km) 

Figure 51. Altimetry mode data, Goddard Space Flight 

Center (GSFC) ER-2 cloud lidar system, October 10, 

1983. 

total vertical excursions can reach several kilometers. During this laser profile, the altimeter instrument maintained a high SNR measurement on each 

laser pulse as it constantly followed large excursions 

in the mean surface height. All data were acquired 

from an aircraft altitude of 25 km, beyond a significant portion of the Earth's atmosphere. The NASA 

ER-2 cloud lidar system that made these measurements is shown in Figure 52. It is a forerunner of 

the space-based LASA altimeter by virtue of its 

autonomous operation, compact size, and ability to 

withstand reduced atmospheric pressure (4 psi) and 

wide variations in the thermal and vibration environment. This cloud lidar is functionally identical to 

the instrument required for ice-sheet measurements, with the exception of no correction for platform pointing angles or position. 

A further example of airborne data acquisition 

with a pulsed laser altimeter is shown in Figure 53. 

These data were acquired from a relatively low altitude, 300 m, by the NASA Airborne Oceanographic Lidar (AOL) system in the terrain-mapping 

mode (Nelson et al., 1984). The significance of this 

data set lies in the high repetition rate (400 Hz) of 

pulse measurements while maintaining a 10 cm-level 

range precision. Note the returns from both the top 

of the deciduous tree canopy and the forest floor. 

This illustrates the use of laser altimetry for an assessment of tree height relevant to biomass determination and the ability of the small, high SNR laser 

pulses to penetrate to the Earth's surface for a 

"bare-Earth" range measurement. 

The evolution of laser-ranging technology toward shorter laser pulses and higher resolution 

"Mm", 

=low -MMMMM—W 

Figure 52. ER-2 lidar experiment. 

50 OR(AL PAGE 13 

OF POOR QUALITY
.30 

F- i 20 

0 

w 

Z: 10

)ME - 

LOCK ELECTRA AIRCRAFT 

ORIGINAL rOF POOR QUALITY 

40 

0 1 1 1 I I I I L I 0 1 2 3 4 5 6 7 8

TIME (sec) 

Figure 53. Airborne laser altimeter data (terrain

backscatter). 

receivers has resulted in the present achievement of 

30 psec pulses and millimeter-level range precision. 

The addition of two-color capability (simultaneous 

range measurement at two laser wavelengths) permits measurement of atmospheric dispersion over 

the range path to high precision. This in turn yields 

an accurate measure of surface pressure for vertical

propagation paths. Recent laser-altimetry measurements of the ocean surface from an airborne platform (Abshire et al., 1985) have demonstrated this 

technology and precision in the field. A summary of 

their technology, method, and data is presented in 

Figure 54. The key features of the instrumentation 

are a cavity-dumped Nd:YAG laser and a streakcamera receiver. These data serve as an extreme 

example of high-resolution laser altimetry from a 

remote platform. 

A final example of laser-altimeter data presented in Figures 55 and 56 is the space-based data 

provided by the Apollo Lunar Laser Altimeter program. The data coverage illustrated in Figure 55 

shows the beginnings of a topographic map of the 

lunar surface obtained in the Apollo 15, 16, and 17 

missions. Data in Figure 56 show the laser altimeter 

and a radar altimeter working together to map a 

portion of the lunar surface in the Apollo 15 mission. Range precision in all the laser data was 1 m. 

These data have been of extreme importance in lunar geological and geophysical studies (Kaula et al., 

1974), particularly in high-resolution studies of 

I 

TERVENIN

C 

WALLOPS LOG 

/

INTERVENING

AIR \ _______ 

/ MASS LASER BAR 

/CREST \

\

APRIL 3, 1985 FLIG1 

TROUGH 

OCEAN SURFACE 

BASIC WAVE WITH WIND DRIVEN 

SURFACE CAPILLARY WAVES SUPERIMPOSED

ACCOMPLISHMENTS 

41 03185 TWO COLOR DIFFERENTIAL

DELAY VS LOCATION

923.4 

930.9 

9383 

164

TWO COLOR WAVEFORMS 945.8 9 

TRANSMITTED RECEIVED 

A I CREST 

532 flO-H 

—..A.-3opsec

"

T'l-TROUGI4 

o -r,----.j I..— 

A2 CREST TROUGH 

1355 nml

psec TA 

o TIME

90 

- 80 

70 9532 

60 3537 

1 2 3 4 5 6 7 8 9 

FLIGHT LINE LOCATION 

ATMOSPHERE INDUCED TIME DELAY 1 GIVES SURFACE PRESSURE 3 hI

RMS DEMONSTRATED: I millibar RMS FEASIBLE) 

TIME INTERVAL T BETWEEN CREST AND TROUGH GLINTS GIVES OCEAN NAVE 

AMPLITUDE TO ONE CENTIMETER 

FOURIER TRANSFORM OF RECEIVED WAVEFORMS PROVIDES CENTIMETER 

RESOLUTION MEASUREMENTS OF SURFACE ROUGHNESS WHICH CAN BE RELATED 

TO SURFACE WIND SPEED 

Figure 54. Office of Aeronautics and Space Technology/Goddard Space Flight Center (OAST/GSFC) demonstration

of two-color picosecond pulse laser altimetry measurements of atmospheric surface pressure and sea state. 

51
C 

C 

LU

0 

oc cfl 

0 

cc

0 

LU

I CczD 

0 

LU 

C C 

no S.

OFUG:L 

OF POOR QIJALr(( 

52
E 

z 

0 

> 

FLU 

-J 

w 

<-4 

z 

-J

-8

-5 Wi 35 55 75 

LUNAR LONGITUDE (deg) 

Figure 56. Maria Crisium and Serenitatis; the squares represent the laser altimeter measurements. 

lunar-basin topography. These results, both simu- onstrate the technical maturity of laser altimetry, 

lations and actual observations, illustrate the capa- and foreshadow the unique scientific results possible 

bility of a space-based altimeter instrument, dem- with the LASA altimeter. 

53
IV. LASA SCIENCE: GEODYNAMICS AND LASER-RANGING

APPLICATIONS 

INTRODUCTION 

Historically, two principal space technologies 

have emerged as the primary sources of data that 

support crustal dynamic studies - ground-based satellite laser ranging (SLR) and very long baseline 

interferometry (VLBI). In the SLR technique, ultrashort laser pulses are reflected off orbiting satellites and measurements of their time of flight are 

analyzed to yield centimeter-precision range information. Utilizing SLR data from a global network 

of ground-based stations and accurate models of the 

Earth's geopotential field to propagate the orbit of 

a high-altitude satellite such as the Laser Geodynamics Satellite (LAGEOS), the relative positions 

of the stations have been determined with centimeter accuracies. In the passive VLBI approach, 

the incoherent emission of radio stars is observed

simultaneously at two widely separated ground locations. Cross-correlation analysis of the signals observed at the two sites yields a differential time delay 

proportional to the component of the baseline between the two stations in the direction of the astronomical source. 

Comparative measurements of baselines between sites separated by thousands of kilometers, 

utilizing both techniques, have shown agreement to 

within a few centimeters (Kolenkiewicz etal., 1985). 

Table 25 gives the results of these measurements and 

comparisons for some sample baselines. For the 22 

baselines considered, the rms difference about 0 was 

5.2 cm and the mean difference was 1.0 ± 1.1 cm. 

Repeated measurements of baselines across fault 

lines over several year time intervals have yielded 

highly accurate values for the relative velocities of 

the tectonic plates, that are on the order of a few 

Table 25. SLR and VLBI Baselines and Their Differences: Ordered by Length (cm) 

From To Length SLR VLBI SLR - VLBI 

Westford Pasadena* 405,702,200+ 3 7 —4 

Owens Valley 392,972,500+ 83 77 6 

Goldstone 390,059,500+ 76 76 0 

Fort Davis 313,863,600+ 10 16 —6 

Platteville 275,321,900+ 14 18 —4 

Fort Davis Quincy 184,183,700+ 85 79 6 

Owens Valley 150,127,300+ 80 76 4 

Pasadena* 138,369,700+ 73 63 10 

Goldstone 129,433,300+ 100 98 2 

Platteville Owens Valley 122,172,600+ 61 67 —6 

Fort Davis Monument Peak* 119,828,300+ 84 76 8 

Platteville 105,568,500+ 89 85 4 

Quincy Monument Peak* 88,360,100+ 80 79 1 

Pasadena* 68,574,300+ 28 32 —4 

Goldstone 63,970,200+ 98 94 4 

Owens Valley Monument Peak* 51,088,100+ 1 8 —7 

Quincy 38,206,700+ 79 71 8 Pasadena* 33,604,200+ 82 79 3 

Goldstone Monument Peak* 26,365,300+ 71 76 —5 

Owens Valley 25,828,900+ 76 78 —2 

Pasadena Monument Peak 21,828,200+ 80 80 0 Goldstone* 17,137,400+ 82 79 3

*Baselines crossing tectonic plate boundary

54 
centimeters per year. Table 26 lists some baseline 

rates that have been measured using ground-based 

SLR techniques. 

Because of the high cost of fabricating and operating the SLR and VLBI ground stations and the 

scientific imperative to monitor as many baselines 

as possible, new fixed stations are rarely built; 

rather, various sites are occasionally occupied by a 

limited number of mobile systems. Nevertheless, because of the high fabrication and operational costs 

of these options, data taking is still necessarily limited to a small number of sites. The density of sites 

is not adequate to provide the spatial and temporal 

resolution desired, for example, by scientists studying regional crustal deformation and strain.

The retroreflectors can also be attached to ice 

sheets to monitor their horizontal and vertical movements. The system would also be capable of providing additional data for gravitational field studies. 

These, in turn, yield information on the Earth's internal mass distribution, moments of inertia, and 

solid earth and ocean tidal components (NASA, 

1983). An example of solid earth scientific results 

derivable from laser-ranging data is shown in Figure 

57, which gives a time history of the "wobble" in the 

Earth's pole as determined by years of groundbased ranging to the LAGEOS satellite. Figure 58 

illustrates the variation in length of day as measured 

by SLR and its high correlation with independent 

estimates of atmospheric angular momentum after 

Table 26. Preliminary Observations of Tectonic Plate Motion* 

Measurement Baseline Laser Results Geological Record 

North America to Pacific 4 cm/yr ± 1 cmlyr 2 cm/yr 

North America to South America - 1 cm/yr ± 4 cm/yr - 1 cm/yr 

North America to Australia - 1 cm/yr ± 2 cmlyr —3 cm/yr 

Australia to Pacific —7 cm/yr ± 1 cmlyr —6 cm/yr 

Australia to South America 6 cm/yr ± 3 cm/yr 2 cm/yr 

South America to Pacific 5 cm/yr ± 3 cmlyr 5 cm/yr

* From 3 years of laser tracking of LAGEOS 

A Geodynamics Laser-Ranging System 

(GLRS) instrument on the Eos platform would invert the current SLR measurement scheme. The sophisticated laser-ranging and optical-tracking 

equipment would be placed on the satellite and 

would range to inexpensive passive retroreflectors 

on the ground. This would permit orders-of-magnitude greater density of sites to be installed at selected locales throughout the world and provide, in 

combination with ground-based systems, a truly 

global, centimeter-accuracy, geodetic data base for 

the study of plate tectonics and regional crustal deformation and strain. Most importantly, the spaceborne laser-ranging system offers a permanent oncall surveying capability that can be scheduled for a 

variety of geophysical observations throughout the 

world. Thus, the operation of a laser-ranging instrument from Eos to ground-based retroreflectors 

would permit dramatic improvements in the temporal and spatial resolution with which geodynamic 

processes can be studied. The measurements would 

expand current capabilities to measure tectonic 

plate motion and provide unique opportunities to 

search for time variations in the plate velocities. 

Such a system could also provide a capability to 

observe the precursory geodetic motions believed to 

occur prior to the onset of large earthquakes and, 

in a broad sense, provide significant information on 

the evolution of the Earth.

corrections for known gravitational effects have 

been made. The system would also permit centimeter-level knowledge of the Eos coordinates in an 

Earth-based reference coordinate system in support 

of other Eos missions such as altimetry. The GLRS 

instrument also has potential as a centimeter-accuracy altimeter for terrain profiling, biomass (tree 

height) studies, ice-sheet thickness, and marine 

geoid and sea-state (wave height, period, and direction) measurements. Based on recent aircraft experiments (Abshire et al., 1985), it also 'looks feasible to relate sea surface roughness to surface wind 

velocity with precisions greater than currently available from radar altimeters. 

In the retroranging or geodynamics mode, the 

GLRS instrument measures the times of flight between the instrument origin and an array of groundbased retroreflectors. This mode is illustrated in 

Figure 59 for ranging to a number of passive targets 

distributed in the vicinity of the San Andreas Fault 

in Southern California. In the altimetry mode, 

GLRS measures the time of flight between the instrument origin and a diffuse scattering surface such 

as the ocean, a cloud top, an ice sheet, tree canopy, 

or solid terrain. In the geodynamics or retroranging 

mode, the total data set of individual range measurements from Eos to the ground array can be processed to yield the three-dimensional relative coordinates of each target with centimeter accuracies 

55 
MAY 

16-c

81 

0.6 0.5 0.4 0.3 0.2 0.1 

Y (arcsec)

-0.3 

-0.2 

-0.1

a) 

0.0 Co 

>( 

0.1 

0.2 

0.3 

0.0

Figure 59. Spaceborne geodynamics

laser-ranging system. 

Figure 57. LAGEOS polar motion measurements. 

a) LAGEOS LENGTH OF DAY 

ATMOSPHERIC ANGULAR MOMENTUM 

>-

LL3 

0 

I—

w

"0 200 34 234 69 269104304138338173 

1976 1977 1978 1979 1980 1981

DAY OF THE YEAR 

Figure 58. Comparison of atmospheric angular

momentum with LAGEOS variations in length of day. 

(Kahn et al., 1980). If selected targets in the global 

array are collocated with known fiducial monuments 

in an Earth-based geodetic coordinate system, such 

as ground-based SLR or VLBI sites, then the absolute positions of all of the targets are known in 

that reference system. This permits centimeteraccuracy geodesy on a global scale. By resurveying 

the target sites at periodic intervals on subsequent 

orbits, relative motions can be observed. These relative motions might be indicative of tectonic plate 

or ice-sheet movement, or the buildup and release 

of strain in the Earth's crust (e.g., in regions of 

earthquake risk). 

The system can be upgraded at a later date to 

utilize two laser wavelengths and picosecond resolution streak camera receivers when the latter units 

reach a space-qualifiable status. This would permit

a substantial reduction in the atmospheric refraction 

error, which limits the absolute range measurement 

accuracy in geodynamics studies to between 5 and 

15 mm for a single wavelength system (Abshire and 

Gardner, 1985). This technology would also permit 

the measurement of surface pressure over oceans 

with submillibar accuracies (Gardner et al., 1983; 

Abshire and Kalshoven, etal., 1983). Table 27 summarizes the capabilities of the basic GLRS instrument in its two modes of operation (retroranging 

and altimetry) and its upgraded versions. In the system descriptions that follow, the discussion is limited 

to the basic instrument configuration. 

MEASUREMENT OBJECTIVES AND

STRATEGY 

As discussed in the LASA science issues section, the geodynamic measurement objectives focus 

primarily on major questions in plate tectonics. The 

major objectives include: 

• Measurement of crustal deformations in the 

vicinity of seismically active zones at tectonic 

plate boundaries at spatial scales of 25 to 500 

km. 

• Determination of the contemporary relative 

velocities between tectonic plates and tectonic 

plate interior deformation using a globally distributed set of targets located hundreds to 

thousands of kilometers apart. 

• Measurement of spatial and temporal variation in crustal movements associated with regional scale processes (e.g., earthquake-related processes, land subsidence). 

Additional measurement objectives include, 

among others, terrain mapping, geoid determinations, and precison orbit determination for Eos. 

56
Table 27 Applications of the GLRS Instrument on Eos 

Basic Instrument: Basic Instrument: Two-Color Upgrade High-Performance: 

Ranging Mode Altimeter Mode Atomic Clock Upgrade 

Precise Orbit Centimeter-Precision Submillibar Accuracy Subnanosecond Global 

Determination Altimetry Surface Pressure Over Time Transfer 

Oceans 

Centimeter-Precision Ice-Sheet Thickness Long-Term General 

Global Geodesy Improved Ranging and Relativity Experiments 

Marine Geoid Altimetry 

Tectonic Plate Motion Measurements 

Sea State (Ocean Wave 

Regional Crustal Height and Frequency) 

Deformation and Strain 

Earthquake Prediction 

Polar Wobble 

Earth Rotation Rate 

Gravitational Field 

Studies 

Earth Internal Mass 

Distribution and 

Moments of Inertia 

Earth and Ocean Tidal 

Components 

Ice-Sheet Movement

The multiple objectives of the geodynamics 

laser-ranging measurements are satisfied by a strategy that involves ranging to grids of cube-corner 

retroreflectors arranged with various intersite separations and undertaking resurveys at selected epochs for different locales on the Earth. In the vicinity of a major fault zone where significant strain is 

accumulating and earthquake occurrence is likely, a 

greater density of sites and more frequent surveying 

is required. 

In California, the San Andreas fault system 

marks the boundary between the North America 

and Pacific plates. Retroreflectors would be placed 

at sites separated by several tens of kilometers with 

the intersite separation increasing slowly with distance from the fault. Resurveying of the sites several 

times a year should be adequate for monitoring 

strain accumulation and plate motion during times 

between major seismic events if no significant temporal variability is noted. When significant temporal 

variability and change in the rate of strain accumulation are detected, more frequent resurveying 

would be required to study the possibility of a forthcoming earthquake or a seismic deformation process. Surveys would be made on a weekly basis, or

more often in certain cases. Similarly, frequent observations would be made subsequent to an earthquake to study how stress changes diffuse into a 

plate interior. Such studies would contribute to the 

knowledge of the driving mechanisms for plate motions and earthquakes and the rheology of the subsurface layers of the Earth. 

Other candidate locations for high-density grids 

can be found in the major fault zones in Japan, New 

Zealand, the Mediterranean, East Africa, the Caribbean, the west coasts of South America, China, 

and elsewhere. 

Away from major fault zones, regional crustal 

deformations and plate interior deformations will be 

studied by placing targets at distances ranging from 

102 to 10 km. An average strain rate as small as one 

part in 108 per year, produces a baseline change of 

1 mmlyr for a 100 km baseline and 1 cm/yr for a 

1,000 km baseline. Thus, yearly resurveys would be 

appropriate on these baselines. A worldwide network of sites would be desirable, but specific candidate locations for intraplate deformation studies 

include most of the United States (across the western Cordillera and such physiographic provinces as 

the Basin and Range, the Colorado Plateau, the Rio 

57 
Grande rift, and the Appalachian belt), the rest of 

North America, Europe, and most of the regions 

mentioned in the preceding paragraph. 

The data obtained from both short- and longbaseline measurements will be used to develop kinematic models of tectonic plate motion. One of the 

basic science objectives of the measurement program will be to compare these tectonic plate models 

based on contemporary geodetic data, with existing 

models based primarily on long-time-scale geologic 

data.

One of the major advantages of spaceborne geodetic surveying is that rescheduling of observations 

on selected sites can quickly respond to the needs 

of the scientific community. This has already been 

discussed with reference to preseismic and postseismic observations. In general, however, the duty 

cycle for the ranging observations to cube-corner 

reflectors is low. Thus, the laser is also available for 

terrain profiling over continental structures (e.g., 

mountain belts), over oceans (for mapping both 

plate tectonic and oceanographic features), and over 

ice sheets. 

The ability of a GLRS instrument to determine 

spacecraft ephemeris at the centimeter level in the 

retroranging mode greatly enhances the absolute accuracy of any collocated higher-power altimeter by 

removing the meter-level uncertainties in spacecraft 

position. An upgraded dual-wavelength version of 

GLRS can provide remote surface pressure measurements over oceans as well as parameters related 

to sea state such as wave amplitude, frequency, and 

direction (Gardner etal., 1983; Abshire etal., 

1985). 

Other important geological and geodetic applications of GLRS include the monitoring of lake levels, providing survey control in remote regions of 

the Earth or for large-scale engineering projects 

(highways, pipelines, etc.), and monitoring ground 

subsidence caused by oil or water withdrawal. Under certain circumstances, data from the basic 

GLRS instrument may also be used in the study of 

polar motion, variations in length of day (Earth rotation rate), solid earth and ocean tidal components, atmospheric angular momentum, and gravitational field studies. The inclusion of an accurate 

atomic time standard permits the real-time transfer 

of time at the subnanosecond level between widely 

separated clocks eliminating the need for portable 

clock trips between stations. Such time transfer experiments are also fundamental to experimental verifications of the General Theory of Relativity (Alley, 

1983). 

A block diagram of the proposed instrument is 

given in Figure 60. The system consists of three 

major subsystems: (1) a navigation and attitude determination subsystem; (2) a high-speed, high-accuracy optical-tracking subsystem; and (3) a centimeter-accuracy laser-ranging subsystem. Functional 

engineering prototypes of the laser-ranging and optical-pointing subsystems, shown in Figure 61, have 

been designed, fabricated, integrated and tested 

successfully under an Advanced Laser Ranging 

project funded by the Office of Aeronautics and 

Space Technology during the period 1975 to 1983 

(Degnan, 1984). Some additional development will 

be required in selected subsystems, notably the 

laser transmitter, to achieve fully space-qualified 

status for the system prior to deployment on the Eos 

platform. 

Navigation and Attitude Determination 

Subsystems 

The navigation and attitude determination subsystem consists of the following components: navigation computer, Inertial Reference Unit (IRU), a 

GPS receiver, and twin standard NASA star trackers. Updated estimates of spacecraft position, at the 

few-meter level, are provided periodically by the 

onboard GPS receiver. The spacecraft position between updates can be obtained by integrating the 

equations of motion using a simple geopotential 

field as is currently done in ground-based ranging to 

satellites. The laser beam divergences planned for 

GLRS are on the order of tens of arcseconds and 

illuminate areas on the order of 100 in at the ground. 

Thus, the few-meter absolute positional accuracy of 

GPS and the few-arcsecond absolute angular accuracy provided by the star trackers should permit 

open-loop pointing to the "grapefruit-sized" ground 

retroreflectors. Nevertheless, the GLRS concept 

provides for an acquisition sequence if the system 

fails to detect range returns in the open loop mode. 

Furthermore, following acquisition of the ground 

retroreflectors, real-time knowledge of the spacecraft position can be improved to better than a meter by applying a Kalman filter estimation algorithm 

to the laser range data. An optional monopulse 

tracker, which determines the location of the image 

of the return pulse in the receiver focal plane, can 

be used to generate pointing error commands which 

keep the target image in the center of the transmitter and receiver fields-of-view. 

The requirement for accurate real-time spacecraft position and attitude information is not unique 

to GLRS. Such information is required by LASA as 

well as by several passive sensors being considered 

for inclusion on the Eos platform. Thus, the proposed navigation and attitude determination subsystem can be shared by many instruments. 

Optical-Pointing Subsystem 

Ground target locations are stored in the computer memory. New target positions or operational 

instructions can be added via telemetry as required. 

Knowledge of the spacecraft and target positions 

and spacecraft attitude permits calculation of 

azimuth and elevation pointing angles in the 

58
TARGET

ATMOSPHERE 

FROM UPS 'N 

SATELLITE TRACKING 

CONSTELLATION MOUNT

POINTING ERROR 

EVENT TIMER 

MONOPULSE 

TRACKER DISCRIMINATO

PMT 

TELESCOPE 

FIXED

SPATIAL 

FILTER

STOP 

PULSE

TIME OF FLIGHT 

50 MH 

CLOCK 

NARROWBAND

SHORT 

ALTER

PULSE

LASER XMTR 

FIRE COMMAND _F 

SERVO COMMANDS 

UPS V T F F ME T R 

RECEIVER

MICROCONTROLLER 

SIC POSITION

EPOCH TIME UPDATES

SYSTEM COMPUTER 

I NAVIGATIONI POINTING COMMANDS I I 

OUAL NAS

ACC OUTPUT REFERENCE

INPUT DATA FLOW 

COMPUTER S/C POSITION AND ATTITUDE 

ATTITUDE

GYRO OUTPUT ERG:: TIME UPDATES

RECORDER MEMORY 

STAR TR A C K ER S

Figure 60. Centimeter-accuracy GLRS spaceborne laser ranging/altimetry. 

instrument coordinate system. When a target is 

within the operating angular range of the instrument (i.e., within 70° of nadir), the system computer 

enables the firing of the subnanosecond pulse laser 

transmitter at the desired repetition rate (nominally 

10 Hz). 

During the pass over the target grid, the instrument typically ranges in a preprogrammed fashion 

to a given target three times, each for a three-second period: once at low-to-medium elevation on approach, once at high elevation, and once at a medium-to-low elevation as it exits the grid. This 

procedure, illustrated in Figure 62, strengthens the 

overall geometry of the measurement and leads to 

improved baseline accuracies. The optical-pointing 

subsystem requirements for GLRS are, therefore, 

somewhat unique. Since the time spent over the 

target grid is on the order of 10 minutes for suggested Eos altitudes, the pointing subsystem must 

be capable of slewing rapidly between widely separated ground targets and settling into an arcsecondlevel tracking mode within a few seconds of time. 

The computational burdens can be greatly reduced through the use of an intermediate microprocessor-based, all-digital controller to drive the optical tracking mount. The GLRS computer provides

updates every second or so to the controller in the 

form of angular position, velocity, and acceleration 

commands. The controller, in turn, provides detailed commands to the drive motors at rates up to 

512 Hz. A prototype of this system was also developed earlier and met all functional design goals including a 200°/sec maximum slew rate, a 500'/sec' 

maximum angular acceleration, and arcsecond 

tracking accuracy (Zagwodski and White, 1986). In 

these tests, the microprocessor-based controller 

drove a 12.5 by 7.5 inch elliptical beryllium flat mirror, which is representative of the telescope receiver 

aperture required by the GLRS experiment. The 

small receiver aperture is a consequence of the large 

lidar cross section (4 x 106 m2 ) of the ground retroreflectors. Although the retroflectors are only several centimeters wide, their large lidar cross-section 

results from their highly directional reflectance, 

which returns the intercepted signal beam along the 

incident beam path. 

Laser-Ranging Subsystem 

The prototype laser transmitter consists of a 

mode-locked. 0-switched, Nd: YAG laser oscillator, 

double pass Nd:YAG amplifier, and KDtP 

59
H 

CL 

a

CLH H1! 

a 

N 

Ia C 

> 

4 

4 

U 

0 

= 

I0 

U 

a 

a 

4 

a 

a 

4 

a 0.

2

2 

4 

a 

H 

'C 

2 

I

Iz 

I-0 

6. 

I6. 

0 

0 

I. 

6. 

2 

C 

0 

2 

a 

0 

0. 

C 

0. 

2 

x 

C 

I 

olmob 

4 

a 

a 4 

2 

2 

a C 

0 

0 

a 4 

H 

C 2 

C 

a C

IVA 

C 

Ll

60 OF POOR QIJALIT( 
Figure 62. Sequence of events as spacecraft passes over

a network to accomplish the ranging mission. 

frequency doubling crystal, which produces a single 

200 picosecond (ps) pulse containing 10 mJ of energy at a wavelength of 0.532 p.m. The prototype 

laser can be operated at repetition rates up to 20 

Hz (Degnan, 1984). 

Referring to Figure 60, a small fraction (<1 

percent) of the outgoing laser pulse is deflected by 

beam splitters into the range receiver to start the 

time-of-flight measurement. The remainder is directed to a high-speed, arcsecond-accuracy opticalpointing mount, which directs the beam at the desired ground target. The pulse propagates through 

the atmosphere to a passive retroreflector (or altimetry surface). The reflected energy from the 

target travels back through the atmosphere and is 

reflected from the pointing mirror into a fixed 7-

inch diameter Cassegrain collecting telescope, 

which images the return onto the photocathode of 

a high-gain, microchannel plate/photomultiplier 

tube (MCP/PMT). The range receiver, described in 

more detail elsewhere (Degnan, 1985), measures 

signal amplitudes and pulse time of flight. The principal elements of the laser range receiver are a narrow-band (1 nm) spectral filter, a high-speed microchannel plate photomultiplier with a 450 ps impulse 

response, a low time-walk constant fraction discriminator, and a 20 Ps resolution event timer. The signal

amplitude information is used to correct for small 

signal-dependent biases introduced by the 

discriminator. 

Representative GLRS Configuration and 

Characteristics 

Table 28 gives a list of the system components 

and abbreviations to be used in subsequent figures. 

Figures 63 through 66 give various views of a representative package. The package includes all of the 

necessary components with the exception of the 

GPS receiver. It also includes a redundant transmitter head to double the expected lifetime of the 

instrument. Note that packaging of the dual laser 

transmitters and the dual NASA Standard FixedHead Star Trackers (FHST) requires essentially the 

full use of the space available on the 36" x 69" 

optical bench. The FHST are mounted on the optical bench with the same orientation to each other 

and the coordinate planes as they are when mounted 

on the optical bench of the Modular Mission Spacecraft (MMS) Modular Attitude Control System 

(MACS). Table 29 lists the baseline GLRS instrument electrical components along with their 

weights, operating prime power requirements for a 

10 Hz or 1 Hz system, and operating temperature 

limits. Table 30 gives the standby and operational 

prime powers required by the system as a function 

of repetition rate and choice of system computer. 

Figure 67 shows a representative orbit for a combined geodynamics and ice altimetry mission. Ice 

altimetry is performed for North and South latitudes 

in excess of 60°. Geodynamics is performed primarily over tectonically active areas near the plate 

boundaries. Since the Eos platform will be over the 

ocean a large fraction of the time and geodynamic 

processes have a long time scale, the geodynamics 

application has a very low duty cycle, which helps 

to conserve laser lifetime. Figure 68 gives the average power per orbit consumed by GLRS as a function of laser repetition rate and duty cycle. The duty 

Table 28. GLRS Optical Bench Components 

IRU - Inertial Reference Unit SHA - Shutter Housing Assembly 

T - Telescope P - Pump 

BA - Beam Aligner PS - Power Supply 

FHST - Fixed-Head Star Tracker CR - Corner Reflector 

LA - Laser CP - Cold Plate 

SH - Shutter SS - Sun Shade 

RD - Range Detector PMA - Pointing Mirror Assembly 

AS - Angle Sensor (Tracker) OB - Optical Bench 

AT - Attenuator KM - Kinematic Mount 

FL - Filter DPTD - Dual Peak Timing 

BE - Beam Expander Discriminator 

M - Motor LS - Calibration Light Source 

LIX - Heat Exchanger PFN - Pulse Forming Network

61 
Figure 64. GLRS instrument package view towards

- Z().

AT (s/c) 

1 No

I

MIRROR +X (S/0 

ELEC 

PUMP\ (s/c) 

\

i.- .

+X(5/c)

(s/c) • 1 p Y 

'r 

II w&AllLi A( +Z(s/c) 

kn

t"(s/c) 

STID 

COMPUTER CAMAC 

MIRROR ELECTRONICS 

E LEC 

Figure 63. GLRS instrument package view towards 

-

'76.26°

PROJECTED* 

730 TRUE LINE—OF—SITE BETWEEN STAR TRACKERS 

Figure 65. GLRS instrument package view towards

+ Z() (upper plate). 

POWER 

SUPPLY CAMAC 

ELEC 

Figure 66. GLRS instrument package view towards

+ Z( , ) (lower plate). 

cycle for a typical geodynamics/ice altimetry mission 

is about 2 percent, during which the total standby 

power is about 118 W and the peak is 789 W. This 

corresponds to an average power of about 132 W 

per orbit at a 10 Hz rate. Table 31 summarizes the 

physical and operating characteristics of GLRS. 

DEMONSTRATION EXPERIMENTS

AND PERFORMANCE SIMULATIONS 

Ground-Based Satellite Laser Ranging 

The GLRS instrument is unique among other 

proposed laser instruments in that laser ranging between the Earth and artificial satellites was first accomplished in 1964 (Plotkin et al., 1965) and the 

experimental techniques have been highly refined 

and improved over the years. Today, a worldwide

network of over 20 stations routinely ranges to LAGEOS, which is in a 6,000 km high orbit (Degnan, 

1985; Cohen and Smith, 1985). The assumed Eos 

orbital height of only 700 km implies an order-ofmagnitude reduction in the range and, hence, four 

orders-of-magnitude increase in signal level relative 

to current ground measurements if all other factors 

remain constant. This permits the use of relatively 

low laser energies (tens of millijoules), small optical 

telescopes (7 to 10 inches), modest transmitter divergence cone angles (few tenths of a milliradian), 

and relatively small and inexpensive ground targets. 

Ranging Subsystem Performance 

Ground tests of the combined transmitter/range 

receiver over kilometer-length horizontal ranges 

62 
Table 29. Baseline GLRS Instrument Electrical System Components 

Components Wt. 

(Ib)

Operating 

Power 

(W)

Operating 

Temperature 

Limits 

(°C) Max/Mm 

Optical Bench 

Laser Power Supply 40 121/12.2* 50/10 

Laser XMTR (2) 108 182/18.3* 25/15 

PFN 15 54/55* 50/10 

**DRJRU 35 21 55/15 

Laser Cooling System 28 31 50/0 

Range Detector 9 8 30/10 

Angle Tracker 5 2 35/5 

Two Axis Gimbal Assembly 27 19 50/0 

**fluid Loop Pump 4 9 5010 

* *Star Trackers (2) 42 36 45/-5 

**DPTD 3 16 30/0 

316 499/178*** 25/15 

Support Module 

Pulse Amp AV7T 4.6 0.25*** 551-20 

Pulse Amp AV813 4.5 0.25*** 55/20 

Sample Hold 3.5 0.10*** 70/0 

Dual ADC 1.5 0.06*** 5010 

RF Attenuator 4.0 0.06*** 551-20 

**Event Timer 8.8 12 (1 W standby) 5010 

**Frequency Standard 22.0 24 61/-28 

Power Supply 25.0 15 (1 W standby) 5010 

Elect./Mech. Drive Circuits 6.6 0.28*** 50/0 

* * * *Computer 28.0/2.0 170/20 551-54 

Computer I/F Electronics 2.6 11 5010 

Mirror Electronics 13.0 17 50/0 

Digital I/O Circuits 4.6 16 50/0 

Matched Filter 4.8 0 851-54 

PM Power Tap 1.5 15 5010 

**fluid Loop Pump 4.0 9 50/0 

139/113 290/140**** 50/0

*10 Hz laser/1 Hz laser 

on continuously 

* * *2% duty cycle assumed 

****Norden PDP 11/34 M/Norden LSI-11 M computer 

typically yield subcentimeter precisions. The sample 

data presented in Figure 69, obtained with a modern range receiver similar to that being proposed 

for GLRS, have a single shot rms value of 5 mm. 

Figure 70 shows typical results obtained in groundbased field experiments to the LAGEOS satellite, 

carried Out in 1981 with an earlier and inferior receiver configuration, which demonstrated a singleshot precision on the order of 1 t 2 cm. If one forms 

50-point averages to reduce the random errors, the 

rms scatter of the average points about the LAGEOS orbit is reduced to about 3 mm with peakto-peak variations of less than ±8 mm. Thus, the 

performance of the ranging subsystem has been 

demonstrated over slant ranges almost 10 times 

greater than will be encountered by the GLRS instrument on Eos. 

Typical signal levels encountered in the LAGEOS experiments have varied from about three 

photoelectrons to several tens of photoelectrons. 

Figure 71 shows that the average signal level expected from the GLRS instrument, based on the 

63 
Table 30. GLRS Standby and Operational Prime Powers 

Optical Bench Components Subsystem Support Module Components 290/140 W* 

Standby Power (82 W) Standby Power (36 W) 

Star Trackers 36 W Frequency Standard 24 W 

IRU 21W Pump 9W 

Discriminator 16 W Event Timer/Power Supply 3 W 

Pump 9W 

Additional Power for Ranging Additional Power for Ranging 

(417/96 W)** (254/104 W)* 

Laser System 388/67 W* * 

Two Axis Gimbal 19 W 

Sensors, Etc. 10 W 

Norden PDP 11/34 MlNorden LSI-11 M computer 

10 pps laser/pps laser

Computer 170/20 W** 

Electronics 60 W 

Event Timer/Power Supply 24 W 

80 

60 

40 

w20-

0 

! 0-

I-

-40 

-60-

-80

-160 -120 -80 -40 0 40 80 120 160

LONGITUDE 

Figure 67. Typical twenty-four hour coverage provided by a sun-synchronous satellite at 700 km altitude. 

64
demonstrated performance of the prototype instrumentation, falls within the same range even if one 

uses a relatively wide (15 X diffraction limit) trans789 W 

639 W 

468 W 

318W 

"0 10 20 30 40 50 60 70 80 90 100

RANGING DURATION

(% OF ORBIT TIME) 

Figure 68. GLRS orbit average power consumption 

versus ranging duration and laser repetition rate.

mitter beam divergence of one arcminute (0.3 

mrad). The calculation assumes a 700 km orbit and 

a target cross section of 106 (an order-of-magnitude 

less than LAGEOS). At this altitude, the beam diameter on the ground at nadir is 210 m. 

Optical-Pointing Subsystem Performance 

Figure 72 displays the measured rms tracking 

accuracy of the prototype pointing system. Except 

E 

61 

C,) 

.) 

0

C,) 

) LU 

cc 

U.' -6 

0 80 160 240 320 400

DATA POINT NUMBER 

Figure 69. Ranging subsystem performance over 1 km 

horizontal path. The single-shot standard deviation is 

5 mm. 

900 

800 

w

700 

0

600 

Uj

500 

uj 400 

> 

< 300 

200 

;isii 

n

- 400 

200 

-200 

LU 

cD -400 

Table 31. Characteristics of GLRS* on Eos 

Instrument Component Component Measure 

Instrument Volume 

Instrument Envelope 

Instrument Mass 

Telescope Diameter 

Laser Energy Per Pulse 

Laser Pulsewidth 

Repetition Rate 

Instrument Peak Operating Prime Power 

Instrument Standby Prime Power 

Orbit Average Power 

Operational Lifetime 

Operating Temperature 

Ranging or altimetry modes

55.4 ft' (1.6 m3) 

69" x 36" x 45 "(can be reconfigured) 

306 kg (includes redundant laser head) 

7 in (18 cm) 

10 mJ 

200 ps 

10 Hz 

789 W 

118 W 

485 W (for representative combined 

geodynamics and ice altimetry mission) 

Two to three years for representative 

combined geodynamics and ice altimetry 

mission 

15°C to 25°C (determined by actively range 

mode-locked laser transmitter) 

65
12 

10 (a) 

8 

6 

4 

E 2 

C)

a 

CD -2 

-4 

-6 

-8 

-10 

-12 

0125 01E 013500 014000 0145M 015 0155M IQOIS 021MD 02150 

TIME OF DAY - HHMMSS

'ar 

05 

C-, 

D 0.0 

-0.5 

_10t I I I I I I 

01250 01.ff 0135(5 014 0145(8) 0150M 01550 02 0208)) (Q1m 02150 

TIME OF DAY - HHMMSS 

Figure 70. Centimeter-precision LAGEOS range data obtained on October 20, 1981 at 01:29 GMT. (a) Short arc orbital fit 

of single shot data, observed minus calculated (OMC) range single shot rms: 1.68 cm, number of measurements: 3808, 

edited: 101 (2.6%); (b) Short arc orbital fit of normal point data (50 Pt. average), OMC normal point nns: 0.30 cm. 

near the maximum tracking rate of 2°/sec, the tracking jitter is at the subarcsecond level in both the 

pitch and roll axes. Figure 73 is a plot of the time 

required by the pointing subsystem prototype to 

slew between targets and settle with arcsecond accuracy as a function of the angular separation between the targets. From the figure, we see that only 

0.7 sec is required for a typical GLRS slew angle of 

200. A more complete written description of the full 

range of pointing system test results is in preparation (Zagwodski and White, 1986). 

Performance Simulations for Eos 

As pointed out previously, retroranging data 

from the GLRS instrument can be used in a number 

of scientific, engineering, and general surveying ap90 

U) 

z 0 cr 60 I0 

U.' 

-J 

w 

0 

H 

0 

I 

0

plications. Simulations performed to date, however, 

have been directed primarily to the estimation of 

crustal strain and strain rate. This application is 

particularly well suited to the laser retroranging 

technique because the principal error contribution 

is a gravity error. The latter error can be largely 

removed in estimating changes in baseline length 

from a fixed orbit. Simulations indicate that centimeter-level changes in baseline length can be observed within a 6-day data-taking session. 

The results to be discussed are preliminary and 

are based on some recent simulation results combined with extrapolation from a very extensive study 

carried out previously (Kahn etal., 1980). It is 

10 

11.0 

Sr 

w 

II-

-, 0.1 

U.) 

Cr ri 

DI

0 30 60 90 

ELEVATION ANGLE (deg) 

Figure 71. Expected average signal for Eos orbital height 

of 700 km and 60 arcsec transmitter beam divergence.

0.011 1 I I I II! I I I I I I I ii 

0.01 0.1 10

TRACKING RATE (deg/sec) 

Figure 72. Roll (x) and pitch (y) tracking jitter. 

We
2.8

2.4

2.0

. 1.6 

w

1.2 

I0.8 

0.4 

0.01 

0.01 0.1 1.0 10.0 100.0 

STEP SIZE (deg) 

Figure 73. Roll (x) and pitch (y) axes step times. 

assumed in the simulation that 150 retroreflectors 

are distributed over Southern California with 50 km 

spacings and that cloud cover obscures the targets 

50 percent of the time. Table 32 gives the baseline 

accuracy as a function of baseline length for an Eos 

platform at an altitude of 700 km and in a nearpolar orbit of 100° inclination. As one can see from 

the table, an absolute measurement accuracy of 3 

cm is achieved over baselines of about 1,200 km in 

length. Gravity uncertainty is the dominant error 

source. In a long-duration mission, it may be possible to reduce the gravity uncertainty to about 1 

cm, and this possibility is currently being studied. 

In spite of the effects of gravity uncertainty on 

the absolute measurement of baseline length, it is

possible to detect subcentimeter changes in baseline 

length over 1,200 km baselines as illustrated by the 

precision numbers in Table 33. It is the changes in 

baseline length that lead to estimates of tectonic 

plate velocities and strain rates. Figure 74 shows the 

elongation rate precision for a 50 km baseline as a 

function of the measurement program duration T 

(Kahn etal., 1980). For the calculation of elongation 

rate precision, a survey period T5 of 6 days was 

assumed, and an interval between surveys, the resurvey period TR , of 6, 14, and 30 days was chosen. 

As can be seen from the figure, GLRS has the capability of determining elongation rates on individual lines to a precision of better than one part in 108 

per year over a 2.5- to 3-year mission and one part 

in 10 over 10 years. In a region of strain accumulation, surveys on a network of retroreflector sites 

can be used to determine the components of the 

strain rate tensor to a greater level of precision 

(Cohen and Cook, 1979). The precision is not increased significantly by making more frequent 

measurements (i.e., reducing TR), implying that the 

instrument can operate at a low duty cycle, thereby 

extending the mission lifetime without sacrificing 

the science objectives. 

SYNERGISTIC SCIENCE 

The measurements made by GLRS are complementary to those made using other techniques. The 

crustal movement measurements provide a densification of the existing network of ground-based SLR 

and VLBI sites. They would extend well beyond the 

Table 32. Standard Deviation of Baseline Estimation Error*

(California Simulation Accuracy 700 km Orbit) 

Baseline Noise Length (cm) (km)Atmospheric Gravity Refraction (cm) (cm)

Measurement Solar 

Radiation Bias Pressure (cm) (cm)

Drag 

(cm)

Root 

Sum of 

Squares 

(cm) 

131 0.1 0.6 0.1 0 0 0 0.6 

303 0.1 1.1 0.1 0 0 0 1.1 

591 0.2 1.6 0.15 0 0 0 1.6 

847 0.2 2.0 0.2 0 0 0 2.2 

1,204 0.3 3.0 0.2 0 0 0 3.0 

*Assumptions: 

Orbit Atmospheric Error Model 2 parameter 

Mean Altitude 700 km Pressure Noise ± 1.0 mbar 

Inclination 1000 Bias 0.33 mbar 

Observation Period 6 days Temperature Noise ± 1.4°C 

Cloud Cover 50% Radiation Pressure 33% error 

Number of Retroreflectors 150 at 50 km spacing Atmospheric Drag 22% error 

Measurement Noise ± 2 cm single pulse, 10 pulses/sec Area/Mass 0.0046 m2/kg 

Measurement Bias 0.3 cm 

Gravity Uncertainty Goddard Earth Model (GEM) 

10 covariances (I, m = 22)

67 
Table 33. Precision in Determination of Changes in Baseline Length*

(California Simulation Accuracy 700 km Orbit) 

Baseline Noise Length (cm) (km)Atmospheric Gravity Refraction (cm) (cm)Measurement Solar 

Radiation Bias Pressure (cm) (cm)Drag 

(cm)

Root 

Sum of 

Squares 

(cm) 

131 0.2 0.1 0.12 0.01 0.00 0.00 0- 

303 0.3 0.1 0.15 0.02 0.00 0.00 0.4 * 3 

591 0.3 0.1 0.21 0.03 0.00 0.00 0.5 

847 0.4 0.5 0.25 0.04 0.01 0.00 0.7 

1,204 0.4 0.8 0.33 0.05 0.03 0.00 1.0 

*Assumptions: 

Orbit Atmospheric Error Model 2 parameter 

Mean Altitude 700 km Pressure Noise ± 1.0 mbar 

Inclination 100° Bias 0.33 mbar 

Observation Period 6 days Temperature Noise ± 1.4°C 

Cloud Cover 50% Radiation Pressure 33% error 

Number of Retroreflectors 150 at 50 km spacing Atmospheric Drag 22% error 

Measurement Noise ± 2 cm single pulse, 10 pulses/sec Area/Mass 0.0046 m2/kg 

Measurement Bias 0.3 cm 

Gravity Uncertainty GEM 10 covariances (I, m = 22) 

capabilities of conventional local surveys, which are 

very labor intensive and of limited utility over long 

baselines or in mountainous regions. The GLRS tar-

_C7 > lo-7 0 •TR = 6 DAYS 

OTR = 14 DAYS 

z \

OTR =3ODAYS 

o 

0 

w 

0 

LU 108 

cr 

z 

0 

o .-.-...--..--

z 

o

io- 0

4 6 8 10 

T (yrs) 

Figure 74. Elongation rate precision versus total measurement periods, where T = time period between first 

and last measurement (years), and T R = resurvey period (days). Assumptions: Survey period (T): 6 days, 

baseline distance (D): 50 km, baseline precision: 0.25 

CM.

get grids should be designed to complement, rather 

than compete with, those that will be surveyed with 

highly mobile receivers operating on microwave signals from the constellation of GPS satellites. The 

altimetric measurements would provide high-resolution topographic mapping to complement the 

coarser data obtained from microwave altimeters. 

Gravity and magnetic data available from current 

sources and from the planned GRM satellite can be 

used in combination with GLRS data to provide a 

more complete picture of the geodynamics of the 

Earth than could be inferred from either data set 

taken by itself. 

The centimeter-accuracy ephemeris knowledge 

provided by GLRS also enhances the absolute measurement capabilities of other onboard altimeters, 

such as the high-power LASA laser altimeter or 

collocated microwave altimeters, by removing fewmeter-level uncertainties in the satellite orbit. 

The capability of the GLRS instrument for centimeter-level range precision on arbitrary slant paths 

to specific Earth targets can provide extremely important synergism with the LASA altimeter and its 

nadir-only, moderate-resolution range measurements. The GLRS range precision based on shortpulse laser and subnanosecond receiver technology 

is up to one order-of-magnitude better than that of 

the LASA altimeter. As such, it can provide occasional calibration of the LASA altimeter during simultaneous burst-mode tracking of the same icesheet, terrain, or ocean surface targets. The objective is removal of biases to the centimeter level in 

LASA altimeter data sets. The precise off-nadir 

pointing capability of GLRS can also provide crosstrack or limited scanning to supplement nadir-only 

68 
altimeter data over ice sheets. For example, the use 

of GLRS to acquire range measurements to a small 

set of surface grid points on sequential orbits can 

provide significant improvement in knowledge of the 

spacecraft orbital position. This in turn is essential 

for removal of biases in LASA altimetry data in 

order to produce an accurate map of ice-sheet topography or land targets of high interest. 

The conversion of GLRS time-of-flight measurement to a centimeter-accuracy, absolute geometric range with the basic, single wavelength GLRS 

instrument requires the application of atmospheric 

models to correct for atmospheric refraction effects. 

The inputs to these models are provided by surface 

measurements of atmospheric pressure, temperature, and relative humidity. In many seismically interesting regions of the Earth, such as Southern California, this data can be provided by existing 

meteorological stations. In other less-developed regions of the world, it may be necessary to augment 

the existing meteorological data through the use of 

a small number of automated meteorological stations located within the target grid. The use of atmospheric models presently limits the absolute 

range accuracy of the basic GLRS instrument to

about 1 cm (Abshire and Gardner, 1985). However, 

vertical profile data on atmospheric pressure, temperature, and water vapor obtained by a collocated 

LASA or other atmospheric sounder instrument 

could be used to correct for the effect of atmospheric refraction and eliminate the need for groundbased meteorological stations. 

Advanced versions of the GLRS instrument are 

currently under development. They utilize dualwavelength transmitters and picosecond-resolution 

streak camera range receivers and will be capable 

of directly measuring the atmospheric delay and 

achieving few-millimeter absolute range accuracies 

(Degnan, 1985). Such an upgraded GLRS instrument would not require ancillary meteorological 

data. The upgraded system could also provide surface pressure over oceans with millibar accuracy 

(Abshire and Kalshoven, 1983; Gardner et al., 

1983). The feasibility of this technique has been 

firmly established by a recent aircraft experiment 

(Abshire et al., 1985) in which surface pressure was 

measured with a resolution of 3 mb. The system 

also detected centimeter-level capillary waves that 

can be related to surface wind. 

69
V. HERITAGE, CONCEPTS, AND LASER TECHNOLOGY

ASSESSMENT 

LIDAR HERITAGE 

The development of the pulsed ruby laser in 

1960 made it possible to use lasers to study the atmosphere and to determine range to a target. Historically, lasers made an extraordinarily rapid transition from their laboratory demonstration in 1960 

to their application in lidar atmospheric measurements made in 1963. The following section provides 

a brief history of this development up to the present. 

For lidar-ranging applications, measurements to 

Earth satellites equipped with retroreflector arrays 

rapidly became an important application of remotesensing technology. As early as 1964, measurements 

of distance from a ground-based laser-ranging station to the Beacon-Explorer series of satellites were 

routinely accomplished with a high-power, longpulse, low-repetition rate ruby laser. These early 

distance measurements achieved precisions of several meters. The following years produced a rapid 

improvement in ranging precision of two orders of 

magnitude to the present value of 1 to 2 cm. This 

was accomplished through development of improved, short-pulse Nd:YAG lasers, faster timing 

electronics, better satellite tracking mounts, higher 

repetition rates, and increased detection sensitivity 

to the level of the single photoelectron. 

During this same period, the potential of the 

laser ranging capability to observe the atmosphere 

was also recognized. The earliest atmospheric lidar 

measurements utilized elastic backscatter of the 

laser pulse to range to clouds and aerosol layers. 

Early efforts explored high-altitude stratospheric 

and mesospheric layers. The influxes of meteoric 

and volcanic material were easily recorded and studied. More advanced lidar techniques were soon developed to include resonant fluorescence scattering 

from mesospheric sodium and potassium layers, for 

example, which provided feasible measurements of 

high-altitude dynamics. The differential absorption 

technique was applied to the measurement of water 

vapor by temperature tuning a pulsed ruby laser 

through the water absorption region near 0.6943 

m. Raman scattering was also applied to tropospheric water vapor measurements, nitrogen density 

measurements, and aerosol extinction measurements. At the same time, carbon dioxide lasers provided the means for coherent Doppler lidar wind 

measurements. 

The original ruby lidar laser source emitted at 

0.6943 pm. This was soon complemented by dye 

lasers that allowed measurements such as sodium 

resonance to be made. Neodymium:glass and YAG 

lasers also became available, emitting at 1.06 p.m. 

Electro-optical crystals to double and triple these

frequencies allowed routine operation at 0.53 and 

0.3472 p.m, among others, for many new applications. More recently, solid-state lasers have been 

used to pump various dyes to produce other wavelengths for various constituent measurements such 

as water vapor, nitrogen dioxide, and ozone. By the 

1970s, these measurements were being made by various researchers throughout the world, mostly from 

fixed sites. The Apollo Program of lunar landings 

in 1969 and the early 1970s provided a unique opportunity to further the role of laser ranging in space 

applications. A laser-altimeter instrument was included in the complement of instruments in the 

Apollo Command and Service Module in lunar orbit. Successful operation of this first space-based, 

remote-sensing instrument was achieved on the 

three missions attempted: Apollo 15, 16, and 17. 

This laser-altimeter instrument was based on the 

solid-state, flashlamp-pumped, 0-switched ruby 

laser that is the forerunner of present-day Nd:YAG 

technology. The Apollo Program also resulted in 

the) placement of several retroreflector arrays during 

the first lunar landing. This was a prime experiment 

on the first landing (Apollo 11). The result has been 

16 years of continuous laser-ranging data for the 

Earth-to-Moon link and a major milestone in the 

progress of laser-ranging techniques and 

applications. 

As lidar technology continued to progress and 

expand, airborne measurements began. Aerosol, 

cloud, and DIAL water vapor and ozone measurements were among the earliest airborne applications. Figure 75 depicts the evolution of altimetry 

and surface-ranging lidar, and Figure 76 depicts the 

evolution of lidar for atmospheric measurements 

from ground-based measurements in the 1960s to 

Eos in the 1990s. The high-altitude programs on the 

WB-57 and ER-2 aircraft have shown the feasibility 

of autonomous lidar operations and modular concepts important to space measurements. The Cloud 

Lidar System (CLS) has already flown on the WB57 and ER-2 to map clouds. This has been done 

simultaneously with radiometers and other instruments aboard the ER-2 to provide synergistic data 

sets. A balloon-borne frequency-doubled dye laser 

system, operating at 0.282 p.m and pumped by a 

frequency-doubled Nd:YAG laser, has autonomously measured stratospheric OH radical profiles 

using the resonant-fluorescent technique, and ozone 

via the DIAL technique. The Lidar Atmospheric 

Sensing Experiment (LASE) program is scheduled 

for flights beginning in 1988-1989. LASE will make 

DIAL measurements of water vapor and aerosols. 

The obvious next step in lidar evolution is to 

show its feasibility in space, verifying on-orbit 

70
OPERATIONAL 

SPACECRAFT 

LUNAR 

RETRO REFLECTORS 

APOLLO LAI3EOS 

LASER 

-, ALTIMETER 

GEOS-11 

LASER 

DETECTOR 

-

C,

LASA 

SHUTTLE 

1 

LASER / / GLRS 

IMETE R

SKYLAB EARTH I ICE-SCIENCE 

LASER BEACON- • ALTIMETER 

- BALLOON-BORNE 

OH/OZONE LIDAR 

CLOUD LIDAR 

WB-57

AUTONOMOUS OPERATION 

400 Hz TWO-COLOR 

DATA RANGING 

TRANSPORTABLE

SYSTEMS 

SPACE-BASED 

EXPERIMENTS 

BALLOONS 

AIRCRAFT 

GROUND-BASED

AIRBORNE SCANNING 

OCEANOGRAPHIC DATA 

LIDAR 

L_a_^Smyo

Q-SWITCHED SHORT-PULSE PSEC 

LASERS LASERS RANGING 

(RANGE PRECISION) 2m 20 cm 10cm 2 c 

1960L 1970 _J 1980 1990 2000 

Figure 75. NASA laser-ranging evolution. 

autonomous lidar operations, data acquisition, and 

data processing. An experiment called LITE (Lidar 

In-Space Technology Experiment) is being readied 

for flight aboard the Shuttle in 1990-1991. LITE will 

utilize three wavelengths at 1.06 i.m, 0.53 i.m, and 

0.355 lim to measure clouds, stratospheric and tropospheric aerosols, tropopause height, the PBL 

height, and temperature profiles from cloud top to 

about 40 km. The above chronology provides the 

heritage for the LASA measurement objectives. 

The following sections of this chapter present the 

discussion of lidar instrument concepts and a discussion of their technological feasibility. The final 

section of this chapter shows the match between the 

capabilities of the lidar systems and the LASA science objectives. 

INSTRUMENT CONCEPTS 

Key technical criteria for an Eos-based lidar system with an orbit of 700 km and a 1-year to 2-year 

time period between servicing operations are: (1) 

high electrical "wallplug" efficiency (i.e., the laser 

and all its ancillary equipment should have a total 

efficiency >2 percent); (2) the laser system should 

be fault-tolerant and capable of pulsed operational 

lifetime >108 shots; (3) for several atmospheric 

sounder experiments, the laser must operate at high 

pulse energies in a narrow spectral band; and (4)

the laser must be able to produce the wavelengths, 

pulsewidth, and frequency bandwidth required for 

a particular lidar measurement without compromising criteria (1) through (3). As a note of caution, 

one can find reported performances from a laser 

satisfying criteria (1) through (3) separately, but 

when the whole laser system is considered in a configuration for the LASA experiments including criterion (4), severe compromises may be necessary to 

meet Eos platform requirements. 

The instrument requirements for atmospheric 

sounding and altimetry and for retroranging are significantly different and are, therefore, presented 

separately. As indicated in Chapter II, the atmospheric measurements will require a nadir-looking 

telescope of approximately 1.25 m diameter and a 

laser pulse energy range of 0.1 to 1 J. Figure 77 is a 

preliminary drawing of a concept for the atmospheric sounding and altimeter package (LASA). 

This conceptual view of LASA consists of a 1.25 m 

telescope and several tunable lasers. LASA would 

also include thermal control and signal conditioning 

electronics, as well as electronics for interfacing with 

the Eos data system. 

Figures 63 through 66 depict a preliminary 

drawing of the retroranging package (GLRS). The 

retroranging package will consist of three major 

subsystems: (1) a navigation and attitude determination subsystem; (2) a high-speed, high-accuracy 

optical-tracking subsystem; and (3) a centimeter71 
FIRST MEAS. A's LASER (Tunable*) MEAS. SPECIES 

GND Based 1970 2 Ruby Aerosols/Molecular 

48 in. (694 nm, 347 nm) Backscatter, Water Vapor 

Aircraft 1978 3 Ruby, YAG, YAG/Dye Aerosols/Water Vapor/ 

Electra/990 (1060,720-,694,600-, Ozone 

530, 347, 300* nm) 

LASE/ER-2 1988 3 Alexandrite (720*) Water Vapor/Aerosols 

LITE/Shuttle 1988 3 YAG Aerosols/Density/T 

(1060, 530, 355 nm) 

LASA/Eos 1994 >3 - Aerosols/Clouds/Altimetry 

Density/Water Vapor/T 

Figure 76. NASA lidar atmospheric measurements evolution.

accuracy laser-ranging subsystem. The optical subsystem includes a rapidly articulated telescope with 

18 cm diameter aperture. The laser required for this 

application would have 10 mJ/pulse with a 10 Hz 

repetition rate. 

There is a technology requirement for long-lifetime lasers (5 x 10 to 108 shots) with high reliability 

and a waliplug efficiency in excess of 2 percent. For 

example, a lidar system operating for 1 year at 5 Hz 

and a 25 percent duty cycle requires 4 x 10 shots. 

(Wallplug efficiency is the ratio of laser power at the 

specified wavelength and bandwidth to the input 

electrical excitation power. In this context, it excludes power requirements for subsystems such as 

coolers, calibration systems, power conditioning 

units, or other auxiliary subsystems for a lidar transmitter requiring electrical energy.) A major power 

drain for a space laser is the pumping mechanism to 

create the population inversion necessary for obtaining laser operation in either the gas, liquid, or solidstate laser media. For gas lasers, the conventional 

excitation mechanism is electrical discharge in the

gaseous medium. Waliplug efficiencies of 3 to 5 percent are feasible. For solid-state laser materials, optical pumping with fiashlamps is the conventional 

excitation mechanism. Wallplug efficiencies in 

pulsed Nd:YAG, the most advanced solid-state material, have been reported to be 1 percent at 1.06 

m.

In the following sections, an overview of the 

status of laser technology will be presented. The 

focus will be on those classes of lasers with potential 

to operate at moderate laser powers (approximately 

0.1 to 1 i/pulse at 10 Hz repetition rate) for the Eos 

lidar experiments described in this document. Laser 

sources that appear to be too early in technology 

development for the Eos era are excluded, although 

it must be recognized that laser technology is 

emerging rapidly. Three different classes of lasers, 

in principle, have potential to serve as laser sources 

for LASA experiments: gas, liquid, and solid-state 

lasers. Eos platform requirements for overall wallplug efficiencies, lifetime, and other considerations 

(weight, volume, cooling) will eventually dictate 

72 
OPTICAL 

BENCH 

THERMAL 

ENCLOSURE

'-f COOLING FLUID 

I LINES 

J AND PUMP 

- LASER 

POWER SUPPLIES 

•1. 

OF HJR QUAL1I I 

- STAR - 

TRACKERS

LASERS 

TIMING

ELECTRONICS

KINEMATIC 

SUPPORTS 

DATA PROCESSING

ELECTRONICS 

COMPUTER 

RECEIVER TELESCOPE

0.25 m diameter) 

ER OUTPUT 

PULSES 

Figure 77. Preliminary drawing of the LASA package. 

selection of those Eos laser transmitters that meet 

the scientific performance requirements of the 

experiments. 

LASER TECHNOLOGY ASSESSMENT 

FOR ATMOSPHERIC MEASUREMENTS 

The laser technology required to measure atmospheric structure and composition parameters 

differs from that required for altimetry and retroranging applications. Several of the LASA science 

objectives addressing atmospheric measurements 

use the DIAL technique. This technique utilizes 

laser radiation at two wavelengths, one situated at 

the specific wavelength of an atmospheric absorption feature and the second at a specified nearby 

"window" wavelength. To accommodate these requirements necessitates lasers which operate at the 

wavelengths of the absorption features and are accurately tunable remotely to specific lines. Wavelength tunability is required at very high precision 

for two reasons. Firstly, the accuracy of the DIAL 

determination of an atmospheric parameter is dependent upon the strength of the absorption feature. Laser sources used in these DIAL applications 

need to be selectively tunable to selected line

strengths for measurements in different altitude 

bands of the atmosphere. Secondly, due to minute 

changes in system characteristics, these laser systems may need to be returned with great precision 

to their proper spectral positions. On the other 

hand, altimetry and retroranging applications simply require lasers which operate in any one of a 

number of atmospheric window regions in the visible or near-infrared. The only tunability required in 

the latter applications is that necessary to minimize 

atmospheric absorption. The altimetry and retroranging applications require, however, pulse durations on the order of 10 ps as opposed to the 10 ns 

required for any of the atmospheric measurements. 

The current sources of tunable narrow-band radiation over the wavelength region of interest to the 

LASE experiments are: 

• Liquid lasers 

• Solid-state lasers 

• Gas discharge lasers 

Liquid Lasers 

Dye lasers are liquid lasers that can be pumped 

with flashlamps or optical pumping with other laser 

sources. Typical energy conversion efficiencies for 

73
dye lasers run from 5 to 40 percent, depending upon 

the dye material, the bandwidth, and the optical 

cavity configuration. Current flashlamp lifetimes 

have been demonstrated to 10 shots. Dye lifetimes 

are typically on the order of 10 shots, which means 

that even if laser pumping is used (e.g., excimer 

laser, which has a demonstrated lifetime of 108 

shots), dye lasers will not be suitable for Eos without new developments that maintain the integrity of 

the dye scrubber and replenishment concepts that 

maintain the integrity of the dye lasing medium. 

Furthermore, the spectral purity required (better 

than 99.5 percent) cannot currently be met by highpulsed-energy dye lasers. 

Solid-State Laser Technology 

Solid-state lasers use either crystalline lattice or 

glassy materials as the lasing substance. Solid-state 

lasers may be divided for the purposes of this discussion into two classes: non-tunable and tunable. 

The non-tunable lasers of interest are Nd:YAG and 

Nd:glass. 

The Nd:YAG laser is a key technology for use 

by LASA experiments, both as a primary source for 

non-tunable applications and as a pump source for 

tunable wavelength energy sources. Its attractiveness is heightened by its ability to be pumped by 

long-life semiconductor lasers. 

The high optical quality of the YAG crystal coupled with the high gain of the 1.064 m transition 

has made Nd:YAG the solid-state laser of choice. It 

has been demonstrated that Nd:YAG can be 

pumped with pulsed and with continuous wave (cw) 

flashlamps, that it can be 0-switched and modelocked and operate cw in a single axial mode. Efficient harmonic generation has been accomplished 

for both cw and pulsed Nd:YAG sources. 

Progress in Nd:YAG has overshadowed the 

progress in other solid-state laser media. However, 

there have been efforts to develop other laser crystal 

hosts and other laser ions for various new wavelengths. Recent examples of new laser host and laser 

ion development programs include YLF, Alexandrite, and chromium-doped garnet lasers. At this 

time, it is not clear to what extent any of these new 

solid-state lasers will compete with the well-established Nd:YAG technology. 

The Nd:YAG or Nd:glass laser sources operate 

at 1.064 Rm and at 1.055 j.m, and they meet the 

requirements of several LASA experiments not requiring tuning to specific absorption wavelengths. 

For other experiments, the fundamental laser frequency must be converted to the harmonic frequencies. At present, this can be achieved with secondharmonic generation at 50 percent energy conversion efficiency. For operation of the laser source at 

a high peak power with 0-switching, KDP (or 

KD*P) is usable as the nonlinear crystal. For harmonic generation at either lower power pulses or for

harmonic generation of a cw source, KTP or 

MgO:LiNbO3 crystals must be used. These nonlinear oxide crystals are stable and have long intrinsic 

lifetimes. However, processes at the surface, enhanced by intense radiation, may lead to shortened 

useful lifetimes. Fortunately, nonlinear devices using both LiNbO3 and KDP have been operated in 

commercial laser sources for over 8 years and lifetime data have been accumulated. In addition, new 

nonlinear crystals such as BaB2O4 have been studied 

and show promise for harmonic conversion of the 

1.06 pm source to the third and fourth harmonics. 

New, more rapid growth techniques have been discovered for KTP, which should yield higher quality 

and larger crystal cross sections. The accumulated 

experience and recent advances in nonlinear frequency conversion reduce the risks of applying this 

technology for LASA experiments. 

Tunable solid-state lasers include Alexandrite, 

titanium-doped sapphire and Cr:GGGS. Titaniumdoped sapphire and Alexandrite are the most advanced members of a recently developed class of 

tunable solid-state (vibronic) laser materials. Alexandrite is tunable in the near-infrared portion of the 

spectrum between 0.7 and 0.8 pm, and titaniumdoped sapphire is tunable in the range 0.68 to 0.95 

pm. Alexandrite lasers pumped by flashlamps have 

been demonstrated with energies up to several 

hundred millijoules. Titanium-doped sapphire has 

a short energy storage time, and can be pumped by 

a Nd:YAG laser operating at 1.06 pm and doubled 

to 0.532 pm. The long-term prospects for Alexandrite are uncertain due to relatively poor wallplug 

efficiencies. In addition, Alexandrite lasers cannot 

be directly pumped with currently available semiconductor lasers, and, if long lifetime is to be 

achieved, breakthroughs in flashlamp development 

or semiconductor laser technology must be realized. 

The measurement of water vapor profiles and 

measurements of temperature and pressure in the 

atmosphere require the use of a laser transmitter 

tunable from 0.72 to 0.94 pm. In addition to Alexandrite and titanium-doped sapphire, CR:GGGS 

and related garnet crystals can also operate in the 

same infrared region. These lasers can be pumped 

either with flashlamps or in a few cases by the second harmonic of a Nd:YAG source. From an overall 

system viewpoint, second harmonic pumping should 

be considered because it is more efficient, and uses 

the modular approach of the semiconductor laser 

pump source. 

Tunable solid-state laser systems configured 

with an "all solid-state laser" as the basic optical 

pump source and used to pump a variety of solidstate tunable crystals could provide multiple wavelengths covering a wide range of the electromagnetic 

spectrum and, in particular, the wavelength ranges 

needed by LASA experiments. This modular configuration has the potential to provide, either simultaneously or sequentially, a large number of 

74
wavelengths to execute DIAL experiments from 

space. 

Wavelength shifting of fixed-frequency laser 

emission also represents a technique to obtain tunable laser radiation. There are technologies for 

achieving wavelength shifting with reasonable efficiency. These include primary Raman shifting in 

gases and nonlinear crystals. Raman shifters can 

produce a number of output wavelengths, which are 

tuned from the input frequency by a multiple of the 

characteristic Raman shift of the converter material. 

Output frequencies can be lower (Stokes) or higher 

(anti-Stokes) than the input frequency, and efficiencies to the first or second Stokes lines can be quite 

high (10 to 40 percent). The most frequently used 

Raman shifters are H2 , D2 , and CH4 . Liquids and 

solids also exhibit useful Raman shifts but are subject to optical damage, and have seen limited 

applications. 

Nonlinear crystals are also used for frequency 

doubling and tripling and sum and difference frequency generation. Most commonly used materials 

are KDP with deuterated and nondeuterated analogs (KD*P, KDA, KD*A, CDA, CD*A, ADP, 

AD*P, ADA, etc.). Others, including lithium niobate and potassium pentaborate have also found application. Frequency summing at high conversion 

efficiencies has also been observed, but is dependent 

on input pump power and optical beam quality. The 

phase-matching condition within the crystal requires that the propagation velocities of the three 

wavelengths maintain phase as they propagate 

through the crystal. The conditions for phase matching can be controlled by temperature, pressure, and 

orientation of the crystallographic axis. 

An alternative approach for tunable laser 

sources that should also be further explored is the 

optical parametric oscillator (OPO). The parametric oscillator offers the potential advantages of 

high efficiency, excellent frequency control, wide 

tuning range, and high-gain amplification to high 

power levels. Research is underway to demonstrate 

improved parametric oscillator performance using 

the recently developed MgO : LiNbO 3 crystal. 

Optical parametric oscillators are solid-state energy conversion devices that convert a photon at a 

given frequency to an output of two photons whose 

sum frequency equals the input frequency. Conversion efficiency depends on input energy and input 

bandwidths; to date, optical conversion efficiencies 

as high as 40 percent have been observed. Early 

experiments using lithium niobate were hampered 

by material damage problems, but newer crystals, 

including MnF-doped lithium niobate and urea, 

may yield satisfactory devices. Operation at the 

joule-per-pulse level at 5 Hz has not yet been 

demonstrated. 

The development of AlGaAs laser diode arrays 

as pump sources for Nd:YAG and Nd:glass in a rod 

or slab configuration has progressed at a rapid rate

over the past several years. At 0.532 m, this laser 

system is attractive as a pump source for some of 

the tunable vibronic lasers. In particular, Ti:Al203 

pumped by either a Nd:YAG or Nd:glass laser, 

which are in turn pumped by semiconductor diode 

arrays, appears to be an attractive "all solid-state" 

tunable laser system that incorporates many of the 

system performance characteristics necessary for 

spaceborne missions, particularly in efficiency and 

lifetime. (Wallplug efficiencies in Nd:YAG up to 10 

percent at 1.06 i.m have been projected.) Currently, 

there is an effort to increase the power of the 

Ti:Al 2 03 laser at wavelengths needed for Eos 

LASA experiments to the order of 1 J/pulse at a 10 

Hz repetition rate. Based on the rapid progress in 

the development of laser semiconductors, and with 

cognizance of historical 5-year development cycles, 

the economic use of semiconductor pump sources is 

projected to be available by the early 1990s. 

Gas Discharge Lasers 

Gas discharge lasers consist of carbon dioxide 

lasers and excimer lasers. Carbon dioxide lasers 

have the longest heritage of the gas discharge lasers. 

They operate on a number of fixed wavelengths in 

the 10.6 p.m region in both pulsed and continuous 

modes. Output pulse energies in the multiple joule 

range have been achieved at wallplug efficiencies in 

excess of 5 percent. High energy and high average 

power carbon dioxide lasers are in wide use in industry for cutting, welding, drilling, engraving, etc. 

Extended lifetime (operation to 108 shots) has not 

been demonstrated. However, recent advances in 

catalytic conversion for room temperature materials 

hold promise for extended operation with a closed 

cycle system. 

Figure 78 is a block diagram of a pulsed gas 

discharge laser system. It consists of a power source, 

a converter of power to high voltage, a primary capacitive energy storage device, and a high voltage, 

fast switch. Before the discharge is initiated, a separate source of preionization is used to provide a 

low level of conductivity to the laser gas to ensure a 

uniform glow discharge. Because the laser is a very 

inefficient energy converter, a means to remove the 

excess heat from the gas mixture is necessary as well 

as a means to circulate the gas mixture in order that 

each discharge occurs in a gas volume whose temperature and density are homogeneous. A gas 

cleanup system is required for high-power gas lasers 

to remove impurities and to regenerate any molecular component that may be chemically altered during the discharge. Finally, optical and electro-optical 

elements are needed to provide for lasing, frequency 

and bandwidth control, and beam routing. A particular advantage of gas lasers is that the gain medium has a high degree of fault tolerance compared 

to a solid-state gain medium, and excess heat is 

easily removed from a recirculating gas. However, 

75
HV PRIMARY 

CONVERTER 1_IENER6'fST0RAGEI_0I HVSWITCH 

PREIDNIZER 

OPTICAL BEAM _

I INTERMEDIATE I LASER PROCESSING & I GAN TRANSMISSION I MEDUM 

HEAT REMOVAL

GAS RECIRCULATION AND REGENERATION 

Figure 78. Pulsed gas discharge Laser system

components. 

all of these items must be considered in the overall 

energy and reliability budget of a laser system. 

Table 34 shows the number of accumulated 

shots versus time on all the electrical components 

and the gas mixture of the laser at various repetition 

rates. When data in Table 34 are compared to lifetimes of the laser components as a function of the 

number of shots contained in Table 35, it is apparent 

that high pulse energy, low repetition rate operation 

is necessary for a free-flying spaceborne lidar to 

meet the needs of adequate SNR and laser lifetime. 

Pulsed gas discharge laser electrical components can 

be operated reliably to the 108 to 10 shot region, if 

the components are carefully chosen and operated 

at derated voltage levels. Additionally, all high-voltage circuits should be immersed in oil or a highpressure electronegative gas. Finally, incorporation 

of magnetic switches is necessary to achieve long 

operational life. Although the magnetic switch concept was originally developed for excimer lasers, its 

use can improve the performance of any high-voltage pulsed power circuit such as is required for carbon dioxide lasers. 

Figure 79 shows the various means commonly 

used to preionize a gas discharge laser. For longlived space operation, only X-ray preionization or 

radioactive isotopes are suitable. Further development of the X-ray sources is necessary for long unattended operation of a discharge gas laser from 

space. Figure 80 is a functional diagram of a tunable 

injection-locked gas discharge laser system. Other 

important lifetime issues for LASA lasers include:

(1) damage of optical coatings from the high energydensities of the laser beam, (2) aging of the optical 

coatings leading to loss in reflectivity and in overall 

laser output efficiency, (3) reduction in the laser 

window transmission due to impurities formed in 

the discharge and deposited on the windows, and 

(4) degradation of the laser gain medium due to the 

production of impurities and the chemical transformation of the laser gas mixture. 

Excimer Laser Technology 

Excimer lasers are the newest of the gas discharge lasers, and operate with mixtures of noble 

gases and halogens; output wavelengths are dependent on the choice of halogen gas (e.g., ArF: 0.193 

pm, KrF: 0.248 pm, XeC1: 0.308 pm). Energies in 

excess of 1 J/pulse and lifetimes in excess of 108 shots 

have been achieved, although a gas scrubbing system is required to inhibit impurity formation in the 

discharge to allow for space applications. Each excimer laser is directly tunable over a narrow bandwidth (1 to 2 nm),'and efficient frequency conversion with Raman shifting in gases has been 

reported. Wallplug efficiencies are in excess of 2 

percent. 

Performance Characteristics of Excimer Lasers - The rare-gas halide excimer lasers have 

undergone considerable development in the last 9 

years because they are the only lasers that provide 

high pulse energy and high average power output in 

the ultraviolet wavelength region. All the excimer 

lasers except XeF have continuous emission spectra 

that indicate an output tuning range of about 10 to 

20 A. Indeed, tunable, high-energy, narrow spectral 

output has been demonstrated for ArF, KrF, and 

XeCl in oscillator/amplifier systems. 

Pulsewidth, Energy, and Efficiency - The 

XeC1 laser at 4 percent has the highest demonstrated efficiency of any discharge excimer laser. To 

achieve this efficiency, a double discharge circuit is 

used. Standard discharge circuits typically produce 

1 to 2 percent efficiency. The overall wallplug efficiency for a double discharge XeC1 laser at high 

pulse energy and high average power in an oscillator/amplifier configuration can be engineered to be 

>1 percent with all ancillary equipment power inchided. The output pulsewidth of an èxcimer laser 

Table 34. The Number of Accumulated Laser Shots on Components for a

Continuously Operating Pulsed Laser System 

1 Hour 1 Day 1 Week 1 Month 1 Year 2 Years 

10 Hz 3.6 x 104 8.6 x 105 6.1 x 106 2.4 X 107 2.9 x 108 5.8 x 108 

100 Hz 3.6 x 10 8.6 x 106 6.1 x 107 2.4 x 108 2.9 x 109 5.8 x 10 

1,000 Hz 3.6 x 106 8.6 X 107 6.1 X 108 2.4 X 109 2.9 X 1010 5.8 X 1010 

10,000 Hz 3.6 x 107 8.6 x 108 6.1 X 108 2.4 x 1010 2.9 x 1011 5.8 x 1011

76 
Table 35. Pulsed Gas Discharge Laser Component Lifetime Issues 

Components Lifetime 

HV Capacitors 10 shots - depends on the total charge stored, current density, voltage reversal, 

inductance, rep-rate, capacitor construction, etc. 

HV Switches: 

Spark Gaps 10 to 10 shots - depends on charge switched, current density, rep-rate, etc. 

Thyratrons 106 to 1010 shots - depends on dildt, average current, type of tube, etc. 

Solid State Unlimited lifetime, but have low voltage and very slow current and voltage 

risetimes for most laser applications - used in the switched-mode power supply 

Magnetic Unlimited lifetime, but a primary switch is required 

Flashlamps 10 to 1W shots - depends on charge, current density, rep-rate, etc. 

Electrodes 108 shots - depends on gas mixture, discharge circuit, materials and 

preionization 

Gain Media 10 to 108 shots - depends on gas mixture, laser construction, catalysts, gas 

recirculation and clean-up, fault tolerance, etc. 

can be varied from less than a nanosecond to several 

hundred nanoseconds. However, for reliable and efficient operation of an excimer laser lidar system, a 

pulsewidth of 50 to 100 ns is normal. 

Spectral Bandwidth and Beam Divergence - 

For an untuned excimer laser, the bandwidth is 

about 1 A, and the divergence, which depends on 

the optical pulse duration, varies between 0.1 and 5 

mrad. Control of these parameters, as well as wavelength, is achieved by using an oscillator/amplifier 

system. For example, injection-locked excimer laser 

systems have produced 1 J pulses with a spectral 

bandwidth of <0.01 A with beam divergence of 100 

mrad. The XeCl laser has also been operated on a 

single longitudinal mode with a bandwidth of 102 

pm. Additionally, the excimer lasers can be contin- uously tuned with narrow spectral output over 10 to 

20 A ranges. 

Repetition Rate and Gas Lifetime - Pulse repetition rates of 3 to 5 Hz are possible in a static gas 

system. Using laser gas recirculation and thermal 

heat exchangers, repetition rates up to the kilohertz 

range have been achieved without degradation of 

the pulse energy, of the laser efficiency, or of the 

beam quality, as long as the appropriate engineering 

criteria were adhered to. Maintenance-free operation of an XeC1 laser at 1 J/pulse at a repetition rate 

of 100 Hz for 108 shots has been reported in a Defense Advanced Research Projects Agency 

(DARPA) study. Both the static and operational gas

stability for an XeCI laser mixture are capable of 

lifetimes in excess of 108 shots if proper design features are maintained. However, the static and operational gas lifetimes for the excimer lasers containing fluorine are not as good, and presently 

require unacceptably large gas cleanup systems for 

long-term operation. Currently, for a space-based 

lidar, the XeC1 laser is the most appropriate of the 

excimer lasers from an efficiency and lifetime point 

of view. In addition, some of the proposed lidar 

missions for LASA have the potential to be accomplished with the energies and wavelengths derivable 

from a tuned XeCl laser with frequency shifting. 

The fundamental wavelength of the XeC1 laser is 

308 nm. 

Frequency Shifting - The high output power 

and high efficiency of excimer lasers allow the use 

of frequency shifting techniques to obtain an output 

in the visible part of the spectrum. The simplest of 

these techniques is stimulated Raman scattering. 

Because the wavelength shift is fixed in Raman scattering, tuning of the excimer laser also tunes the 

frequency shifted output. Table 36 lists the various 

wavelengths obtained from excimer lasers with several Raman-shifting gases. Multiple Stokes and antiStokes shifts are obtained from nonresonant stimulated Raman scattering. Conversion efficiencies to 

only the first Stokes shift can be as high as 60 percent. With a Raman oscillator/amplifier configuration that converts to the second and third Stokes 

shift, conversion efficiencies can be as high as 30 

77
POWER I POWER

PREIONIZER I MAIN 

SUPPLY I SUPPLY 

MAIN 

POWER 

c:;7;

SUPPLY 

UV ARCS CORONA 

• SPUTTERED MATERIAL • DIELECTRIC STRESS 

• NONUNIFORMITY • NONUNIFORMITY 

• SIMPLE & CHEAP

MAIN 

POWER 

SUPPLY MAIN 

POWER 

SUPPLY 

1,+j ELECTRON BEAM 

• THIN FOIL WINDOW 

• VERY HIGH PREIONIZATION 

H V • DENSITY 

POWER I 

SUPPLY

Figure 79. Preionization means.

H. V. 

POWER 

SUPPLY 

X-RAY 

• VERY UNIFORM 

• CLEAN 

percent. Since a gas Raman cell is a passive device 

(only beam optics and a gas cell are needed, no 

power is required) and in principle fault tolerant, it 

should be both a reliable and efficient method to 

generate a multiwavelength lidar for measuring atmospheric parameters. Other wavelength shifting 

methods include sum or frequency mixing with 

other sources, stimulated Brillouin scattering, and 

optical parametric oscillation. In particular, a urea 

OPO crystal would provide continuous shifting and 

tuning of the XeCI laser from 0.50 to 1.20 j.m, with 

efficiency near 20 percent. However, it remains to 

be determined if suitable crystal material can be 

obtained to achieve this conversion at the joule-perpulse energy level. 

Continuous Wave Low-Pressure Carbon Dioxide 

Lasers 

Continuous wave low-pressure carbon dioxide 

lasers can be used for DIAL measurements of water 

vapor. A significant effort in long-lifetime studies of 

sealed cw low-pressure carbon dioxide lasers has 

been reported (Willetts and Harris, 1985). Radio 

frequency-excited (RF), two-to-three watt sealed, 

120 torr carbon dioxide waveguide lasers have been 

operated for >10 hours (Javan, 1980; Hochuli, 

1984) as shown in Figure 81. The initial power reduction to steady-state is comparatively small, but 

after 4 x 10 hours, a drop to 70 percent power 

occurs. The RF excitation was produced with one

internal electrode (Pt/Cu) and an external capacitively coupled Au/In electrode (Figure 82). Demonstration of a direct current-excited (DC) sealed 

carbon dioxide waveguide laser with 4.2 X 104 hour 

operation at 80 percent initial power (Figure 83) was 

described by Hochuli (1981). In this laser, special 

precautions were taken to shield the laser mirrors 

from sputtering. However, in RF-excited lasers the 

sputtering should be much less severe. A compact 

waveguide laser with external RF excitation using 

axially displaced steel or Pt electrodes is being developed (Figure 84). Lasers with this design have 

operated for >20,000 hours at power outputs sufficient for local oscillator or injection oscillator applications. A large number of cw sealed carbon 

dioxide lasers are available under commercial or 

contractor development. Some of these lasers demonstrate long lifetime (including shelf life), however, 

with considerable reduction in steady-state power. 

This may be acceptable for some applications, but 

not for LASA. 

LASER TECHNOLOGY ASSESSMENT

FOR SURFACE-RANGING

MEASUREMENTS 

The technology for LASA experiments that require nontunable laser energy is currently available. 

Nd:YAG is the most advanced of all solid-state laser 

78
uv 

>1J 

= 2.5% 

Figure 80. Functional diagram of a tunable injection-locked gas laser system. 

materials, and is capable of running either in a 

pulsed or continuous mode. The capabilities of 

Nd:YAG have been discussed extensively in the previous section. Energy outputs in the range of 1 J/ 

pulse at 1.06 p.m are routinely achieved in laboratory systems. Frequency doubling to 0.532 p.m has 

been accomplished with efficiencies greater than 50 

percent, and 35 percent efficiencies are routinely 

obtained. This material also lases near 1.3 p.m, although at lower output energies. Overall wailpiug 

efficiency for pulsed systems at 1.06 p.m is about 0.5 

percent to 2 percent when pumped by flashlamps. 

High-quality material is available from a variety of 

commercial sources. 

Nd:glass is also a well-advanced solid-state laser 

technology. Glass configured as a laser rod cannot 

handle the energy density that the YAG crystal can. 

However, a new slab configuration has yielded glass 

lasers of much higher power and repetition rates 

than YAG crystals. Neodymium laser emission in 

glass, as contrasted to the crystal form, is tunable 

over a narrow range (approximately 5 nm), thus, 

allowing its use as a source for parametric mixing to 

obtain wavelength tunability. 

Table 36. Wavelengths Obtained from

XeC1 Excimer Laser Using RamanShifting Gases

Raman-Shifting 

Gas

Stokes-Shift 

Wavelength (nm) 

1st 2nd 3rd 

H2 353 414 499 

339 377 426 

CH, 338 375 422 

N2 337 359 392

3

- 

cc 

w 

0

I I 

102 103 104 10

TIME (hr) 

Figure 81. Lifetime study of RF-cw carbon dioxide

waveguide lasers. 

COMPARISON OF LASA OBJECTIVES

WITH LASER CAPABILITIES 

As seen from the previous sections, there are 

several laser sources which may be used to satisfy 

LASA scientific objectives. Each of these laser 

sources has attributes and performance characteristics which can apply to more than one of the many 

different LASA experiments, and, in many cases, 

can be applied to several scientific objectives. An 

analysis was conducted as to the applicability of current and future (emerging) laser technology for each 

LASA objective. 

Table 37 shows of the applicability of current 

laser sources to the various LASA missions. As 

seen, depending on pulse energy requirements, a 

large fraction of the objectives can be satisfied with 

proper engineering of current laser technology. 

Some of the most stressing engineering issues (listed 

at the bottom of the table) still need to be 

79 
L 

Pt/Cu ELECTROI 

THIN BeO LEAF 

Au/In ELECTRODE 

Figure 82. Channel structure of RE-excited carbon

dioxide waveguide laser.

ELECTRODE 

7 

BeO

7:

ELECTRODE 

£ RESERVOIR 

Figure 84. Integrated structure of RE-excited carbon

dioxide waveguide laser. 

VOLTAGE - 

CC 

LU 

0 

0. 

i0 

a- i-0 

D 

00

10 102 103 104 io

HOURS 

Figure 83. Lifetime study of DC-cw carbon dioxide waveguide laser. Characteristics of the laser: Laser G2A (refill), insulated 214.5 Ag 5 Cu cathode, 0.8 mm Pt PIN 

anode, 5.6 mm x 250 mm bore, 6 ma D.C., 50 cm3, 

He:CO2 :CO:Xe:H2 , 15:7:7:1.5:0.2; 20 ton (Hochuli, 

1981). 

addressed, in particular laser lifetime. In this regard, laser lifetime is ultimately limited to less than

a year by fiashlamp technology for continuous operation at 10 Hz. Attainment of long lifetime is also 

an issue for electrical discharge lasers. The high 

pulse energy (>1 J/pulse) solid-state laser (Alexandrite) would consume high input power (of the 

order of 7 kW/laser). 

Table 38 shows similar information for emerging 

laser technology. As seen, several new technologies 

may offer significant advantages for new laser 

sources, but issues involving energy conversion efficiency, tunability, and lifetime will still have to be 

addressed. 

In summary, from an overall systems viewpoint, 

the laser transmitter should operate at the maximum overall efficiency for the longest possible duration. Tunability should be accomplished by efficient nonlinear conversion of the pump laser, 

followed by the pumping of the tunable source. As 

seen in Tables 37 and 38, several candidate lasers 

exist which should be seriously considered in any 

tradeoff study for a wide range of LASA objectives. 

-OUTPUT POWER

LU 

0 

5 KV 

4 KV

LU 3 KV

F-

(continued) 

80
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82
VI. Eos COMMAND AND DATA MANAGEMENT

SYSTEMS FOR LASA 

This section deals with the command and data 

management systems required to support LASA 

and GLRS operation within the Eos mission. The 

baseline lidar system configuration includes capabilities for making measurements of stratospheric 

and tropospheric aerosols, cloud measurements, 

and selected DIAL features as described earlier in 

this document. For altimetry and ranging measurements, the same LASA laser transmitter and receiver instrument components will be used in the 

altimetry mode. Primary emphasis in altimetry is on 

time-of-flight measurements of the laser pulse and 

precise knowledge of laser pointing angles with respect to instrument nadir and precise knowledge of 

spacecraft orbital position. A suitable lidar might 

include one fixed-frequency laser with fundamental, 

second, and third harmonic outputs, and one or two 

tunable lasers for making the on-line and off-line 

DIAL measurements. The altimeter mode of operation would employ only one of these lasers, but 

would require additional data sets on laser beam 

pointing angle and spacecraft position. The GLRS 

instrument for geodynamic ranging is separate from 

the LASA instrument, but will have similar data 

management requirements to the LASA altimeter. 

No assumptions have been made about the 

character of the onboard control and data management services that will be provided by the Eos platform to its resident instruments. Therefore, this 

analysis presents a worst-case approach to defining 

the data and control system requirements for the 

LASA systems.

DATA RATES 

The estimated bit rate for science data generated by the system is based on the following 

assumptions: 

• The highest estimated bit rate for science data 

generated by the system is desirable. Therefore, a five-channel receiver is indicated as 

well as a five-channel digitizer. 

• The DIAL measurement capability requirement dictates an equivalent 12 or more bit 

dynamic range. While current technology 

(1985) makes 12 bits the practical limit, advances in high-speed analog-to-digital converter technology by the late 1980s could 

make converters with resolution in excess of 

12 bits technically feasible. The altimetry 

mode of operation requires a digital timer 

with at least 26-bit time resolution, but a very 

low data rate - one digital timer word supplemented by waveform digitization over a short 

time interval centered on the surface return 

pulse. Total data rate requirement for altimetry is less than 500 kbit/sec including 

pointing angle, GPS, and housekeeping data. 

• The altitude range of interest for Eos lidar 

systems extends from approximately 50 km 

down to the ground. This translates to a datagathering period of about 400 ms. 

• The minimum desired range-bin resolution is 

15 m for all lidar (non-altimetric) measurements. This translates to a digitizer sampling 

rate of 10 million samples per second. 

• The laser pulse repetition rate is 10 pulses/sec 

for all atmospheric studies and 20 pulses/sec 

for altimetry mode operation (surface topography studies). The altimetry pulse rate is 

twice that of the lidar, but it will occur only 

in a burst mode for a small percentage of each 

orbit. Even at 20 Hz, the altimetry data rate 

is less than that for lidar because no high data 

rate digitization is required. Based on these 

assumptions, the average data bit rate of the 

lidar system is given by the equation: Bit Rate 

= B R Tdata N S where B = number of bits 

per word (12), R = digitizer sampling rate 

(1 x 10 sec'), Tdata = data period (4 x 

10 see), N = number of receiver channels 

(5), and S = number of laser shots/sec (10). 

For the values shown, the bit rate is approximately 2.4 Mb/sec. Data compression schemes 

could be implemented that could decrease the volume of redundant data generated by the lidar systems and thereby reduce the downlink data rate by 

orders of magnitiude. A good example is the 1 x 

300 km resolution required for stratospheric aerosol 

measurements. 

COMMAND UPLINKING AND

CONTROL 

Considerable study should be given to the design of the onboard computer that will control the 

lidar system on Eos. Suitable onboard data evaluation strategies can minimize the uplink command 

and control activity. In addition, the system should 

have onboard capability to evaluate system performance and modify system operating parameters to 

compensate for long-term shifts in component characteristics (i.e., an adaptive system is called for). 

The system should have a default or "safe-state" 

mode in which it operates in the absence of any 

83
uplink command or after a platform power interruption. Other options might include the capability to 

command a particular mode in real time or to load 

sets of modes to be executed at particular times or 

at particular geophysical locations in orbit. A system to satisfy the above requirements would include 

five digitizer channels in addition to the onboard 

control computer. 

SYSTEM REDUNDANCY 

In view of the 2-year period between revisits to 

the platforms for maintenance and repair, some degree of redundancy in the system would be prudent. 

For redundancy, three additional digitizer channels 

are recommended. A detector output switching matrix with cross-strap redundancy would allow the 

connection of any receiver channel to any digitizer

channel. And finally, a backup control computer 

would be desirable. 

GROUND COMPUTER FACILITIES 

A ground computer system will be required to 

process, display, and store the data received from 

LASA, and to generate and format commands to be 

transmitted to LASA. Preliminary analysis capability should be included so that the quality of the 

incoming data can be evaluated as it is received. In 

addition to the backscattered and background data 

stored onboard during the previous orbit, the downlinked data will include real-time status and housekeeping information, which must be formatted and 

displayed. The ground command data set will include all routine operating command sequences and 

special commands for trouble-shooting and calibration procedures. 

84
VII. CONCLUSIONS AND RECOMMENDATIONS 

Lidar and cw laser systems, operating in the 

single wavelength or tunable mode can make unique 

contributions to the overall capabilities of Eos. 

These contributions will be in the areas of atmospheric composition and structure measurements and 

a set of surface-ranging measurements. Operation 

in the ultraviolet, visible, and infrared regions of 

the electromagnetic spectrum makes several unique 

measurement features available. 

• The transmitter wavelengths are comparable 

to the sizes of atmospheric aerosols and cloud 

droplets, thereby providing the means to obtain information on aerosol profiles, cloud-top 

heights, FBL heights, and the geometric 

thickness and optical depths of thin clouds. 

• The high level of monochromaticity of lidar 

can be used to uniquely identify molecular 

species and measure their concentrations 

against interfering background spectra. With 

tunable laser sources, water vapor and ozone 

can be measured through the DIAL technique with high vertical resolution. 

• Monochromaticity and tunability of lidar, especially in the absorption bands of uniformly 

mixed gases such as 0 2 and CO, make possible the direct measurement of atmospheric 

temperature and pressure with high vertical 

resolution. 

• Heights of ground surfaces can be measured 

with unprecedented resolution and accuracy. 

Laser altimetry with height resolution better than 10 cm is achievable. Ice and 

terrain roughness measurements at centimeter levels offer higher accuracy than any 

other technique. 

• Centimeter-level crustal movements can be 

detected through retroranging for improved 

understanding of earthquakes and tectonic 

plate motions. High-precision geodetic control networks can be established on a regional 

and global basis. 

• Lidar is an active probing tool allowing for 

direct recovery of the measurement variable 

• without requiring inversion techniques, thus 

reducing overall uncertainties. Active probing 

with lidar also allows height resolution of variables from the Earth's surface upward. When 

range information is not essential, cw lasers 

offer a considerable advantage over current 

pulsed systems in that their expected lifetimes 

are considerably longer. 

In this report, the experiments have been divided into two basic categories, which reflect the 

degree of instrument readiness. Experiments requiring only currently available and demonstrated 

technology include: 

• Precision altimetry and retroranging (GLRS) 

• Water vapor column content 

• Cloud-top heights 

• PBL heights 

• Optical depths of thin clouds 

The above measurements are, in principle, candidates for early implementation on Eos (IOC). 

Experiments that have not yet been performed 

from aircraft and/or that require additional technology development are: 

• aerosol profiling and total aerosol optical 

thickness, 

• tropospheric and stratospheric water vapor 

profiles, 

• surface pressure and vertical profiles of temperature and pressure, and 

• ozone total column amount and vertical 

profiles. 

Some of the above experiments may be performed 

from aircraft platforms prior to final selection of an 

initial LASA configuration. 

An integral strategy of Eos is to strive for technological advances in critical areas so that the measurement basis may be expanded. Toward this end, 

the LASA Panel Committee recommends: 

• System-level studies should be initiated early 

in Eos to address the question of trade-offs 

among the following criteria: high wailpiug 

efficiency, fault tolerance, long lifetime, and 

ability to operate at high pulse energies in a 

narrow spectral band. 

• Continuous technological development of iidar subsystems, including detectors, flashlamps, laser diode pumping arrays, lightweight telescopes, and laser wavemeter-spectrometers as calibration instruments. 

• An interdisciplinary solid-state material's research program is needed to shorten the lead 

time in laser transmitter development. 

• Development of efficient harmonic conversion 

techniques for nonlinear crystals. 

• Development of X-ray sources for pre-ionization to achieve long, unattended operation 

of discharge gas lasers from space. 

• Continued and more detailed simulation studies, verified through aircraft and/or Shuttle 

and possibly space station experiments, as a 

natural progression from concept to Eos application. 

85
• Implementation of methods allowing for 

LASA scanning capabilities on future Eos investigations. 

The LASA experiments open up an exciting 

new opportunity to measure geophysical parameters

with vertical and horizontal resolutions never before 

obtainable on a global scale. The resulting data will 

enable scientists to understand the Earth's atmosphere and surface processes at a level previously 

unattainable. 

86
APPENDIX A 

EYE SAFETY ISSUE 

The laser eye safety issue is characterized by the 

transmitted laser pulse energy and intensity, and the 

footprint illuminated on the ground. Such a consideration is related to the FOV of the LASA telescope 

since the laser beam is adjusted to be equal to the 

telescope FOV. Under these circumstances, the appropriate FOV of the telescope is approximately 

1 mr. This value may be incorporated with the allowable maximum permitted exposure (MPE) for 

laser excitation for safe eye exposure as presented

in Table A.1 (Sliney and Wolbarscht, 1982) and tabulated in Table A.2. 

As can be seen in Table A.3, lasers with energies of 1 J/pulse are eye safe from Eos altitudes for 

all spectral wavelengths for the unaided eye. It 

should be noted, however, that higher safety factors 

may be appropriate for viewing by the optically 

aided eye (binoculars or telescope), and where scintillation aperture averaging is considered. While little experimental data exists for these latter considerations, further work will be required for LASA 

and other downlooking laser-based systems. 

Table A.1. Laser Energy, Field-of-View, and Eye Safety 

- Wavelength Maximum Permitted Exposures (J cm') 

300 to 400 nm 0.56 to-" [t = pulse length (sec)] 

400 to 700 nm 5 x 10-7 

700 to 1,050 nm 5 x 10 x 10[0.002(k - 700 

1,050 nm 5 x 10 

Allowable Energy = MPE x Area of Footprint/Safety Factor

Area of Laser Footprint on Ground = (FOV x Orbit Height) 2 x ir/4 

Data on MPE and the formulas used to calculate the maximum allowable laser energy per pulse 

from an orbital Eos altitude of 700 km (Sliney and Wolbarscht, 1982). 

Table A.2. American National Standards Institute Maximum Permissible Energy for

Differing Fields-of-View (Unaided Eye) 

Wavelength Range . 300 to 400 nm, 

1.4 Rm to 10 m 400 to 700 nm 760 nm 1,060 nm 

ANSI MPE (J/cm2) 1.0 x 10 5.0 x 10 6.59 x 10 5.0 x 10_6 

Table A.3. Maximum Permissible Laser Pulse Energy for Differing Fields-of-View 

FOV Ground Footprint 

mr 

'

/

(cm') Maximum Permissible Laser Pulse Energy (J) 

from 700 km Orbit - 

0.05 9.6 x 106 9.6 x 103 4.8 5.3 48.1 

0.10 3.8 x 107 3.8 x 104 19.2 25.4 192.0 

0.20 1.5 x 108 1.5 x 105 77.0 101.0 770.0 

0.50 9.6 x 108 9.6 x 105 481.0 634.0 4,810.0 

1.00 3.8 x 109 3.8 x 106 1,920.0 2,540.0 1.9 x 10 

2.00 1.5 x 1010 1.5 x 107 7,700.0 1.0 x 104 7.7 x 10 

5.00 9.6 x 1010 9.6 x 10 4.8 x 104 6.3 x 104 4.8 x 10 

10.00 3.8 x 1011 3.8 x 108 1.9 x 105 2.5 x 105 1.9 x 106

The maximum permissible laser pulse energy from a 700 km orbit based on the American National Standards Institute (ANSI) MPE 

levels (as shown in Table A.2) for wavelengths from the ultraviolet to the near infrared. These maximum permissible laser pulse 

energies are given for various FOVs and are for the unaided eye with no safety factors included. Attenuation losses of the laser beam 

as it propagates through the atmosphere are not included.

87 
REFERENCES 

Abshire, J. B., A comparative study of optimum 

and sub-optimum direct-detection laser-ranging receivers, NASA TP 1315, GSFC, Greenbelt, MD, 

September 1978. 

Abshire, J. B., and J.E. Kalshoven, Jr., Multicolor 

laser altimeter for barometric measurements over 

the ocean: Experimental, App!. Opt., 22, 2578, 

1983. 

Abshire, J.B., J.F. McGarry, R.S. Chabot, and 

H.E. Rowe, Airborne measurements of atmospheric pressure with a two-color streak camerabased laser altimeter, Paper PD6, Conference on 

Lasers and Electro-Optics, Baltimore, MD, May 

21-24, 1985. 

Abshire, J.B., and C. S. Gardner, Atmospheric refractivity corrections in satellite laser ranging, 

IEEE Trans. Geosci. Remote Sens., RE-23, 414, 

1985. 

Alley, C.O., Proper time experiments in gravitational fields with atomic clocks, aircraft, and laser 

light pulses, in Quantum Optics, Experimental 

Gravity, and Measurement Theory, edited by P. 

Meystre and M.O. Scully, p. 363, Plenum Publishing Corp., NY, 1983. 

Bandeen, WR., and R.S. Fraser, (Eds.), Radiative 

effects of the El Chichon volcanic eruption: Preliminary results concerning remote sensing, NASA TM 

84959, 1982. 

Brooks, R.L., WJ. Campbell, R.O. Ramseier, 

H.R. Stanley, and H.J. Zwally, Ice-sheet topography by satellite altimetry, Nature, 274, 539, 1978. 

Browell, EX, A.F. Carter, S.T Shipley, R.J. Allen, 

C.F Butler, M.N. Mayo, J.H. Siviter, Jr., and WM. 

Hall, NASA multipurpose airborne DIAL system 

and measurements of ozone and aerosol profiles, 

App!. Opt., 22, 522, 1983. 

Browell, EX, A.K. Goroch, T.D. Wilkerson, and 

S. Ismail, Airborne DIAL water vapor and aerosol 

measurements over the Gulf Stream wall (abstract), 

12th International Laser Radar Conference, p. 151, 

Aix en Provence, France, August 13-17, 1984. 

Browell, E.V., S.T. Shipley, C.E Butler, and S. Ismail, Airborne lidar measurements of aerosols, 

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