NASA Technical Memorandum 104644

Design, Development, and Test-Flight of the

Rayleigh Scattering Attitude Sensor

E. Hilsenrath, E. C. Anderson, D. Flittner, D. Heath, S. Janz, J. Medeiros,

J. Cerullo, C. Thorpe, and T. Riley

September 1997
NASA Technical Memorandum 104644

Design, Development, and Test-Flight of the

Rayleigh Scattering Attitude Sensor

E. Hilsenrath

C. Thorpe

T. Riley

Goddard Space Flight Center

Greenbelt, MD

E. C. Anderson

New Millennium Astronautics

Charlottsville, VA

D. Flittner

University of Arizona

Tucson, AZ

D. Heath

Research and Support Industries

Boulder, CO

S. Janz

J. Medeiros

J. Cerullo

IDEA

Beltsville, MD

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland

1997
ACKNOWLEDGMENTS

The personnel involved in the development of RSAS were supported under NASA contracts NAS5-31729, NAS5-

31755, and NASA cooperative agreement NCC5-92. The authors wish to thank Mr. Thomas J. Kelly of Hughes STX

for his assistance in obtaining the PATH product, as well as his helpful suggestions. The efforts of the entire SSBUV

team should also be acknowledged in providing excellent support, without which the implementation of RSAS would

not have been possible. Finally, the authors wish to thank Mr. Patrick E. Frith, for his editing contributions and

overall assistance in the publishing of this manuscript.

NOTE

The list below shows all the authors who contributed to this document. Their current affiliation is shown and, in some

cases, their affiliation at the time they contributed to this document.

Affiliation at the time of their contribution/Present affiliation

E. Hilsenrath, Goddard Space Flight Center

E. C. Anderson, New Millennium Astronautics/Analytical Graphics

D. Flittner, University of Arizona

D. Heath,Research and Support Insdustries

S. Janz, IDEA/University of Maryland

J. Medeiros, IDEA/Allied Signal

J. Cerullo, IDEA/OMITRON

C. Thorpe, Goddard Space Flight Center

T. Riley, Goddard Space Flight Center

NASA Center for AeroSpace Information

800 Elkridge Landing Road

Linthicum Heights, MD 21090-2934

Price Code: A17

Available from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Price Code: A10
TABLE OF CONTENTS

LIST OF FIGURES ............................................................................................................................................................... v

LIST OF TABLES ................................................................................................................................................................. v

GLOSSARY OF TERMS ................................................................................................................................................... vii

ABSTRACT ........................................................................................................................................................................ ix

1. INTRODUCTION ............................................................................................................................................................. 1

2. DESIGN ............................................................................................................................................................................ 1

2.1 Requirements Overview ........................................................................................................................................... 1

2.1.1 RSAS Attitude Determination ........................................................................................................................ 1

2.1.2 Physical Description ...................................................................................................................................... 2

2.2 System Overview ...................................................................................................................................................... 3

2.3 Radiometric Characterization ................................................................................................................................... 3

2.4 Hardware Design ...................................................................................................................................................... 4

2.4.1 Mechanical Description ................................................................................................................................. 4

2.4.2 Electrical Description ..................................................................................................................................... 4

2.5 Software Design ....................................................................................................................................................... 6

2.5.1 RSAS Component Drivers ............................................................................................................................. 6

2.5.2 Grotmd Support Equipment Software ............................................................................................................ 6

2.5.3 RSAS Data Processing ................................................................................................................................... 7

2.6 Testing and Calibration ............................................................................................................................................ 7

2.6.1 Optical Calibration ......................................................................................................................................... 7

2.6.2 Structural/Mechanical Analysis and Testing .................................................................................................. 8

2.6.3 Thermal Environment Analysis ...................................................................................................................... 8

3. PERFORMANCE .............................................................................................................................................................. 8

3.1 STS-72 Payload Configuration ................................................................................................................................ 9

3.2 Analysis Methodology .............................................................................................................................................. 9

3.3 Results .................................................................................................................................................................... 10

4. SUMMARY ..................................................................................................................................................................... 11

REFERENCES .................................................................................................................................................................... 11

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor iii
LIST OF FIGURES

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Limb viewing geometry .............................................................................................................................. 2

Optics module .............................................................................................................................................. 2

RSAS system diagram ................................................................................................................................. 3

RSAS mounted in SSBUV GAS canister .................................................................................................... 5

RSAS electrical block diagram ................................................................................................................... 6

Variation in pre-flight to post-flight filter transmittance ............................................................................. 8

STS-72 payload orientation ........................................................................................................................ 9

Model radiance and radiance ratio under various lighting conditions ...................................................... 10

Normalized radiance vs. altitude ............................................................................................................... 10

Elevation angle measured by RSAS and PATH ........................................................................................ 11

Differences in elevation angle as measured by RSAS, PATH, and SSBUV Sun sensor ........................... 11

LIST OF TABLES

Table 1.

Table 2.

Table 3.

Limb Radiance for Minimum and Maximum BSDF at Three Altitudes ..................................................... 4

Optical Characteristics ................................................................................................................................. 4

Expected Stress Levels ................................................................................................................................ 8

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor v
GLOSSARY OF TERMS

A/D

APACS

BSDF

EMI

FIFO

FPGA

FWHM

GAS

GSFC

GSE

FOV

IR

LORE

PATH

PCB

RSAS

S/N

S/P

SOLSE

SSBUV

SZA

TOMS

UV

- analog to digital

- Atmospheric Pollution, Aerosol and Chemistry Satellite

- bi-directional scattering distribution function

- electromagnetic interference

- first-in, first-out

- field programmable gate array

- full width at half maximum

- Get Away Special

- Goddard Space Flight Center

- ground support equipment

- field of view

- infrared

- Limb Ozone Retrieval Experiment

- post-flight attitude and trajectory history

- printed circuit board

- Rayleigh Scattering Attitude Sensor

- signal-to-noise

- signal per pixel

- Shuttle Ozone Limb Sounding Experiment

- Shuttle Solar Backscatter Ultraviolet

- solar zenith angle

- Total Ozone Mapping Spectrometer

- ultraviolet

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor vii
ABSTRACT

The Rayleigh Scattering Attitude Sensor (RSAS), a proof-of-concept device, was designed, developed, and tested at the

NASA Goddard Space Flight Center. The prototype instrument was flown aboard the Space Shuttle Endeavor (STS-72) in

January 1996. RSAS uses a 512-element linear photodiode array and an interference filter to measure incident solar radiation

that is Rayleigh-scattered from the Earth's limb at a wavelength of 355 nm. System requirements and a system overview are

presented. Design and development of mechanical, electrical, and optical subsystems of RSAS are also documented, including the software interface structure and ground support equipment (GSE). Pre-flight and post-flight testing procedures,

calibration, and optical characterization results are discussed. Limb scattering radiances observed by RSAS during the testflight show good agreement with predictions by the GSFC ultraviolet radiative transfer model. The attitude determination

algorithm applied to the limb scattered signal is presented. The attitude measured by RSAS is shown to be in excellent

agreement with concurrent Shuttle pointing data (+ 0.05°). These data indicate that RSAS-type instruments could yield

significantly better performance (-0.01 ° resolution) than state-of-the-art horizon sensors (-0.1 ° resolution), at a considerably

lower cost.

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor ix
1.0 INTRODUCTION

Recent theoretical and technological advancements have

motivated the design, development, and test-fight of the

Rayleigh Scattering Attitude Sensor (RSAS), which uses a

linear photodiode array and an interference filter to measure

incident solar radiation that is Rayleigh-scattered from the

Earth's limb at a wavelength of 355 nm. Currently, the

Goddard Space Flight Center (GSFC) ultraviolet radiative

transfer code accurately models atmospheric ultraviolet

(UV) scattering at this wavelength. This algorithm has been

employed for over two decades in the analysis of satellite

ozone data from atmospheric UV remote sensors such as

the Total Ozone Mapping Spectrometer (TOMS) and the

Shuttle Solar Backscatter Ultraviolet (SSBUV) a instrument. More recently the model has been used to compute

radiance in the Earth's limb from Rayleigh scattered

sunlight. By comparing measurements made by RSAS with

this computational model of UV radiance versus altitude,

the spacecraft attitude can be accurately determined.

The development of RSAS received funding through the

GSFC Director's Discretionary Fund. Instrument development (including its optical, electronic, mechanical, and

support subsystems) progressed through the phase-A

conceptual design, critical design, fabrication, testing,

calibration, and payload integration stages in only 10

months. During its initial test flight aboard STS-72

(launched January 11, 1996), RSAS shared the SSBUV

instrument's mechanical platforms and used its existing

power and data systems.

Current commercial spacecraft attitude sensors range in cost

from $150,000 to greater than $1,000,000 with quoted

accuracies of a few hundredths to a few thousandths of a

degree. Some of the larger, more expensive types of

attitude sensors, such as star trakkers which detect the

positions of known star fields. Infrared (IR) horizon

sensors, commonly used on Earth-orbiting satellites,

monitor the position of the Earth's horizon by measuring

the altitude gradient of infrared radiation in the atmosphere.

Whereas current IR horizon sensors provide tracking

accuracy of N0.05-0.1 ° RSAS could easily resolve to 0.01 °.

The GSFC ultraviolet radiative transfer code predicts that

the 355 nm limb radiance decreases by 15% per km at

altitudes above 20 km 2. By way of comparison, IR flux

varies by only 2% per km along a vertical column of

atmosphere, and can fluctuate with season and weather. The

large UV gradient allows a more accurate measurement of

the position of the horizon and thus spacecraft attitude than

standard IR techniques. RSAS could thus provide up to an

order of magnitude performance gain over current horizon

sensors.

RSAS further demonstrates other applications of viewing

the Earth's limb radiances in the ultraviolet. Limb remote

sensing has the potential for measuring accurate, high

altitude resolution profiles of ozone, aerosol, and possibly

water vapor and temperature. The Limb Ozone Retrieval

Experiment (LORE) and the Shuttle Ozone Limb Sounding

Experiment (SOLSE) will employ this technique to measure

high vertical resolution ozone profiles. This technique could

be useful for future limb imaging instruments that require

highly accurate knowledge of the viewing location for

atmospheric modeling, and has already been proposed for

the Atmospheric Pollution, Aerosol, and Chemistry Satellite

(APACS).

In addition to testing the limb-scattering detection process,

RSAS was also a technology demonstration platform for its

components. For instance, the ultra-stable, narrow

bandpass UV filter used by RSAS has become available

only very recently and was flight-proven by RSAS. This

355 nm UV filter, whose coatings were deposited using

"hardened" techniques 1RSAS, provides the 10 nm

bandpass range that RSAS requires. Additionally, RSAS

data are used to validate recent modifications and improvements to the GSFC ultraviolet radiative transfer model that

allow accurate prediction of the UV flux under such

variations as changing solar zenith angle (SZA) and

azimuth angle, and the non-uniform reflectivity of the

Earth's surface 2.

2.0 DESIGN

2.1 Requirements Overview

2.1.1. RSAS Attitude Determination

The technique employed to deduce attitude makes use of

the steep gradient in the Rayleigh scattering function for

altitudes above 20 km. Viewed at a wavelength of 355 nm,

where ozone absorption is negligible and above aerosol

scattering, the limb radiance falls off at a rate of about 15%

per kilometer. This gradient provides an easy to detect

a SSBUV, also developed at GSFC, consists of two Get Away

Special (GAS) canisters. It measures atmospheric ozone concen

trations by comparing direct solar ultraviolet radiation with

radiation backscattered from the Earth's atnlosptlere. SSBUV

compares tile obselvations of several ozone measuring instruments

aboard tile National Oceanic and Atnlosptlefic Adnfinistration

NOAA 9, NOAA11, and NOAA 14 satellites, the Russian

Meteor 3/TOMS satellite and GSFC's Upper Atmosphere Research

Satellite (UARS). SSBUV data are used to calibrate these

instruments and ensure the most accm'ate readings possible for the

detection of atmospheric ozone trends. RSAS was flown on one of

the SSBUV canisters during STS_2.

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor 1
\\\\

Solar Radiation

Spacecraft \ \ \ \

Local

Horizon A_ Atmos_ _View

Earth

"R

-.

R .

Figure 1. Limb viewing geometry.

signature, where changes in radiance at the instrument

boresight location indicate changes in the tangent height (or

a vertical altitude) of the scene in view. Once the change

in tangent height is determined, the change in spacecraft

pointing angle (and thus spacecraft attitude) can be calculated.

As shown in figure 1, the optical axis of RSAS was inclined

at an angle (90 ° - (9) from the local horizon in order to

center its field of view (FOV) at the correct altitude on the

limb. From this viewing geometry the attitude angle (9 can

be derived from

R + TH (1) sin O -

R+A

where R is the radius of the Earth, TH is the tangent height,

LD is the line of sight distance to the limb, and A is the

spacecraft altitude. For an Orbiter altitude of 306 km and

the desired FOV centerline limb tangent height of 30 km,

the limb distance was found to be 1899 km and (9 was

determined to be 73.4 °.

2.1.2 Physical Description

An isometric diagram of Rayleigh-scattered solar radiation

passing into RSAS is shown in figure 2. Incident radiation

first passes through the filter, provided by Optical Corporation of America (OCA), where 355 nm radiation with a

bandpass of 10 nm is allowed through the telescope lens.

The UV light is then reflected by the transfer mirror onto

the pixels of the detector array, where it induces a charge

build-up on the photodiodes.

TELESCO

MIRROR

FILTERI _ _/>

_CTOR

Figure 2. Optics module.

2 Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor
2.2 System Overview

The first of the system's two mechanical components, the

optics module, includes the high quantum efficiency, solidstate, linear (512-pixel) photodiode detector array and

preamplification electronics, a lens and filter arrangement,

and a transfer mirror. The photodiode array elements are 25

pm x 2.5 mm in size and are spaced 25 pm center to center.

The camera's focal length of 12 cm and full FOV of 6 °

correspond to a pixel FOV of N0.01 °. From an altitude of

306 km, this provides a per pixel resolution of 0.35 km at

the Earth's limb. The high stability, advanced interference

filter provides an out-of-band to in-band response ratio on

the order of 10 7. The optics module is about four inches

long and two inches wide, and was designed to fit into a

small volume (usually used by one of the SSBUV

instrument's four IR horizon sensors) on the top deck of the

SSBUV payload. The angle of elevation of the optical axis

of RSAS from the SSBUV mounting plate is equal to the

pointing angle (9. (See figure 1).

The second component of the RSAS system, the controller

module, houses both the power subsystem and the digital

control and avionics electronics, enables command and

control of the RSAS system (including power ON/OFF and

regulation of detector integration times), and was mounted

just below the top deck of the SSBUV payload during STS72. The entire RSAS system is pictured in figure 3.

2.3 Radiometric Characterization

The radiometric performance parameters for RSAS were

determined in an analysis by Research and Support Instruments, Inc. (RSI). Using the GSFC ultraviolet radiative

transfer code, the limb radiance for _, = 355 nm was

modeled as a function of altitude, SZA, and surface

reflectivity. It was determined from the model that the

maximum and minimum values for the bi-directional

scattering distribution function (BDSF) of 355 nm UV flux

at an altitude of 30 km were 0.1 sr 1and 0.02 sr 1,respectively. The minimum BDSF at 40 km was expected to be

LENS & FILTER PREAMP BOARD #1--_.

.O'DER-\ \ f ® ®

SECTION A-A

MIRROR _

_OPTICS MODULE

BASEPLATE

OPTICS MODULE__ 0 /NONES #2PREAMP BOARD

FEED THRU_ /CONTROL ELECTRONICS

......

DATA_

Figure 3. RSAS system diagram.

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor 3
0.006sr1.Thenominalfuxdensityofsolaradiationat355

nm(E{355nm})isaconstant 1.1x 104W/(cm2nm).The

limbradiance(L)atthiswavelengthandlimbtangent

heightwasthendeterminedaccordingtoequation2:

L{355nm, TH} = BDSF{355nm, TH} xE{355nm} (2)

The values of L over the range of possible altitudes at

constant E are given in Table 1.

Table 1. Limb Ra [iance for Min/Max BSDF at Three Altitudes

L{355 nm} (W/(cm 2nm sr)

Limb Altitude Minimum Maximum

20km 1.0x 10 5 2.0x 105

30km 3.3 X 10 6 1.1 × 10 5

40km 6.7 x 107 2.0x 106

The theoretical signal per pixel (S/P) is given by equation 3:

S/P = (L)(A2)(U2L)(AmxeL)(TL)(TF)(Rz), (3)

where A_ is the ban@ass of the UV filter, and g2L= 3.6 x 102 sr

is the solid angle subtended over the instrument's FOV. The

value of AWXEL,the surface area of each of the 512 pixels in

the linear array, is 6.25×10 4 cmL The filter and lens

transmittance, TF and TL, were determined experimentally by

RSI to be 0.6 and 0.8, respectively. The photodiode pixel

response R_ at 355 nm is 0.1 A/W. These parameters yield

an S/P of 97 pA at the maximum value for limb radiance L.

Charge saturation, or "full well," for a Reticon SC linear

array pixel is 1 x 10 11C. Therefore, the integration time

was calculated to be no longer than about 0.1 s. For a dark

current (DC) of 0.2 pA, estimates for full well signal and

amplifier noise were made. Signal-to-noise (S/N) for full

well was calculated to be 2.3 x 104 for the theoretical

maximum signal at 30 km. For the theoretical minimum

signal occurring at 40 km, predicted S/N dropped to 1.4 x 103.

The theoretical altitude resolution (TAR) shown below was

calculated by dividing the line of sight distance to the limb

(LD = 1899 km) by the focal length, and then multiplying

by the height of one pixel (h = 25 pm) as shown in equation 4:

(4)

According to the above relation, the TAR was found to be

0.39 km/pixel. RSAS's optical parameters, are summarized

in Table 2.

Table 2. Optical Characteristics

Limb Distance (nominal) LD = 1899 km

Focal Length F = 12.0 cm

Lens Diameter D= 3.5 cm

Focal Distance F = 12 cm

Integration Time T = 0.1 s

Signal per Pixel S/P = 97 pA

Dark Current DC = 0.2 pA

Filter Bandpass A_, = 10 nm

FOV Solid Angle _2L= 3.6×10-2 sr

Pixel Height

Pixel Width

hp---- 25 _tm

Wp= 2.5 mm

Pixel Area ApIXE L = 6.25× 10 4 cm 2

Filter Transmittance TF = 0.6

Lens Transmittance

355 nm Linear Diode Response

T_,= 0.8

R)_ = 0.1 A/W

Theoretical Altitude Resolution TAR = 0.35 km/pixel

2.4 Hardware Design

2.4.1 Mechanical Description

The RSAS system was designed to share the existing

SSBUV mechanical platform, and to use SSBUV's power

and data output systems. The mechancial and optical

design was constrained by the space available on the top

deck of the SSBUV canister. Mechanically, RSAS is

comprised of two components: The optics and the controller

modules. The optics module replaces one of the SSBUV

payload's four horizon sensors on the top deck of the

canister, as shown in figure 4.

The control module is situated just beneath the top deck of

the SSBUV canister. It is connected to the SSBUV power

bus, and also to the RSAS optics module via a hole in the

canister and the mounting plate. The optics module and

controller module weigh 1.43 lb. and 1.10 lb., respectively,

and are made from Aluminum 6061-T6. The optics module

is coated with silver Teflon and the controller module has

an irridite finish. Together, both units consume a total

system power of 4.2 W.

2.4.2 Electrical Description

2.4.2.1 Signal Processing

The incoming radiation, passing through the filter and

telescope, strikes the 512-pixel linear photodiode array,

where a charge proportional to the incident light intensity is

generated on each pixel. The current induced by the charge

buildup is amplified by the RSAS preamplifier circuitry,

conditioned, and transferred to the controller module as a

voltage.

4 Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor
Figure4. RSAS mounted in

The analog diode charge output voltage is converted to a

digital signal using a 14-bit analog-to-digital (A/D) converter. The controller module circuitry was also responsible

for configuring the RSAS system for data acquisition by

translating RSAS commands. Ancillary electronics were

used to control, buffer, and transfer the flight data through

the controller to the SSBUV avionics electronics. Figure 5

illustrates the electronic components of RSAS.

2.4.2.2 Optics Module Electronics

There are three main electrical components in the optics

module. The Reticon EG&G (RL0512SCQ-011) photodiode array was chosen for its radiometric performance,

small size, and serial output. This array and the ancillary

discrete components reside on one printed circuit board

(PCB). The dual preamplifier design is based on the first

stage differential recharge mode video amplifier from the

Reticon EG&G Evaluation Circuit (RC1032LNN-011).

The preamplifier consists of two PCB mounted are the

optics module as shown in figure 2. This design was

necessary to make the best use of the small space available.

The final component of the optics module is the set of

command line drivers.

SSBUV GAS canister.

The active and dummy (dark current) video outputs of the

sensor array are fed to a preamplifier PCB where they are

differentially amplified by the first amplifier stage for noise

reduction. The output is essentially a train of voltage pulses

corresponding to the charges detected on each photodiode

as they are read out in sequence. The second amplifier

stage subtracts a positive DC voltage level video line bias

from the signal, while a third stage integrates the video

signal. The second preamplifier board generates the

necessary voltage references used by the first preamplifier

PCB, as well as conditioning other necessary sensor chip

control signals. The video output of this preamplifier is

then sent to the controller module.

2.4.2.3 Controller Module Electronics

The major components of the controller module are the A/D

converter, the first-in first-out (FIFO) data buffer, three

field programmable gate arrays (FPGA) for controller logic

and a local clock, an additional preamplifier stage, a power

supply module (a DC/DC converter that converts Orbitersupplied power) and an EMI filter. The FPGAs, manufactured by Actel, Inc. (A101B) contain 1200 gates. These

FPGAs not only generates all clocks needed by the photoDesign, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor 5
RSAS Optics Box RSAS Electronics Box

Photons

Line D____vel_

Differential Recharge Parallel _ _ _]| Signal: I A/D I Data _gl _ I

_e° kanplffiel" _ i__ : : Enable

_. ;; ;; " Clock 6.144 KHz

CY

Rm

Data 1024

' I I _'_ IliT Rtn 8 bit bytes ,d

Reset I __ __l A _ _ IH. ¢_ I I ' %1]=11_: Integrate

----_--_] I )) )) iq ControlLogic /£to II 02 - .. ' I ......... ¢-- Smial +-_ N I _-" +5 Volt

0II "_'_ "_'_ '-] 8 bit "_ p ....... HI_ p°werRenma

Field Programmable Gate Array

(AIOIOB)

Figure 5. RSAS electrical block diagram.

diode array, the A/D converter, and the FIFO, but also have a

parallel to serial converter for use in data transfer. The AID

converter is a 14-bit CS5014 unit whose output is in two 8-

bit words. The IDT7205 FIFO has 8 kilobytes of storage.

The preamplifier stages are derived from the EG&G

Evaluation Circuit, but do not include a sample-and-hold

stage. The 5 V quiet DC power supply module actually is

an externally mounted, triple output DC/DC converter that

transforms the 28 V unregulated DC power coming from

the Orbiter via the removed nadir sensor's connector. An

EMI filter provides well-conditioned power to both the

controller module and the optics module at the appropriate

voltage levels.

2.5 Software Design

The software written for RSAS serves several independent

functions. Certain existing programs were modified to

accommodate RSAS on the SSBUV payload. Initially, the

original SSBUV avionics software needed to be modified to

allow for control and handling of the incoming RSAS data

stream. Software was written in FORTH (the operating

system of the S SBUV computer) to utilize S SBUV ground

support equipment for testing and evaluating the RSAS

system at several stages of system development. Because

of the fast-paced development schedule, this software was

needed especially to identify system problems so that they

could be corrected as soon as possible. Since the development of both the optics module and the controller module

occurred in parallel, a method of testing the sensor module

was needed early in the development of the system hardware. As the hardware development progressed, several

software and test harness packages were fabricated to

provide quality assurance testing at each stage of development. Detector integration time was under software control

and commandable via the ground support equipment (GSE).

2.5.1 RSAS Component Drivers

The controller module FPGA logic was programmed using

the VHDL design language to implement RSAS system

control features. Once programmed, these FPGAs provided

a method of synthesizing logic that resulted in a significant

savings in the number and size of the discrete electronic

components that would have otherwise been necessary to

implement the control features required by the RSAS

system specifications. The end result was an electronics

control capability that easily fit the volumetric, weight, and

power restrictions required by the placement of RSAS on

the SSBUV platform.

2.5.2 Ground Support Equipment Software

Changes to the SSBUV avionics code (again in the FORTH

language) had to be made in order to accommodate command and control of the RSAS avionics system. The

SSBUV avionics system was well-established and flightproven. Since it was the vehicle for sending commands to

and receiving telemetry from the RSAS system, great care

had to be taken to provide software capability for all the

RSAS system functions without affecting any of the flightcritical SSBUV software or exceeding the confines of the

SSBUV avionics. Specifically, RSAS GSE software for the

STS-72 flight consisted of a 80386 Ur/FORTH program

that was capable of sending the RSAS integration time,

6 Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor
flushing the RSAS data stream, reading and storing blocks

of data, and displaying the data as either a graph or a table.

Other ground system segment software was modified not

only to enable the sending of RSAS system commands, but

also to allow receipt and display of the RSAS telemetry and

science data. These programs were written in Turbo Pascal

and ran on PC-based platforms. During the Shuttle flight,

several multipurpose commands were available using

SSBUV workstations to control the RSAS from the ground.

These commands used a single parameter which allowed

either power and response control (integration time was

adjustable within the range 0.052 s-15 s), depending on the

parameter entered.

2.5.3 RSAS Data Processing

The RSAS data was interleaved into the SSBUV data

stream without affecting it, as only unused SSBUV spare

data words in the SSBUV telemetry stream were replaced

with RSAS data. Each time a snapshot of the horizon was

taken (every 80 seconds), RSAS raw data was organized

into a single scan of size 1035 words (2070 bytes). This

scan included 1,024 words--512 words for background and

512 words for 355 nm light intensity data on each pixel in

the 512 pixel array. The remaining words were used either

to record the integration time, the ambient temperature, or

as buffer data bytes. Programs written in C++ and the

Interactive Data Language (IDL) were used to extract the

data from the SSBUV telemetry and convert pixel counts to

calibrated engineering units.

2.6 Testing and Calibration

2.6.1 Optical Calibration

2.6.1.1 Pre-Flight Optical Axis Alignment

The optical axis of RSAS was aligned using a Davidson D

275 autocollimator optical alignment telescope, and an

Oriel slit assembly. In order to isolate outside effects such

as scattered light and optical cross-talk, the predicted signal

distribution over a minimum number of pixels was compared with the actual measurement. The lens was then

adjusted so that a minimum number of pixels were illuminated at an autocollimator beam incidence angle of 0 °.

With the RSAS optics module mounted on a goniometric

fixture, the number of photodiode pixels illuminated was

measured as a function of autocollimator beam incidence

angle in steps of 0.1 ° over the 6.0 ° FOV of the instrument.

Also, the azimuthal response of the detector was measured

as a function of elevation angle at each whole degree from

-3.0 ° to 3.0 ° . After these measurements, fine adjustments

of the lens barrel were made to obtain the peak response.

The slit center was thus aligned with respect to the mechanical axis of the lens assembly.

The true optical focus was determined in the visible by

using the autocollimator alignment telescope, and was

measured at 12.0 cm. With the autocollimator focused on

infinity, RSAS' lens was moved until the image on the

linear array was focused onto the image plane of the

autocollimator. By finding the position of minimum image

width of an illuminated slit, RSAS' focal length was finely

adjusted. To adjust the system for 355 nm radiation the

instrument was illuminated with a precision slit with a

width of 0.25 mm at a distance of 2.58 m. The focal length

found using this method was 12.58 cm, which verified the

theoretical calculation. Finally, the infinity and vacuum

values for the focal length were calculated for the final lens

positioning.

The machining tolerance of the elevation of the RSAS

housing (1 arc minute) was verified with the an optical flat

and the autocollimator alignment telescope. Resolution

across the full FOV of the instrument was verified by

mounting RSAS on a rotary table and measuring the image

width of the illuminated slit as a function of scan angle.

This was done along two orthogonal axes, one with the slit

image parallel, and one with the image perpendicular to the

2.5 mm pixel length. Before flight, the RSAS optics

module was tested for angle alignment and finely tuned.

2.6.1.2 Pre-Flight and Post-Flight Radiometric

Calibration

RSAS was calibrated for image quality and absolute

sensitivity both prior to and after the Space Shuttle flight.

The filter bandpass and center wavelength were also

measured pre-flight and post-flight. Prior to flight, the lens

barrel was repositioned for imaging the limb at a range of

1900 km. Out-of-band filter transmittance was characterized using RSI's double monochromator system, and was

found to be less than 10 6.

The image quality and FOV mapping consisted of scanning

the instrument in elevation and azimuth while viewing a slit

and recording the resultant image with the photo diode

array. The FOV measurements agreed with the calculated

values of 6.1 ° x 1.2 ° with an out-of-field response less than

10 6. The full width at half maximum (FWHM) of the slit

image ranged from 25pm at center to 70pm at + 2.8 °. The

post-flight measurement of these quantities showed little or

no change.

The absolute calibration was performed using a NASA 20"

internally-illuminated, large-aperture spherical integrator.

The calibration technique consists of illuminating the

instrument with a uniform, large-area source of known

radiance. The radiometric stability of RSAS' optical

system was demonstrated by measuring its response to the

spectral radiance of the aperture in 10 minute intervals over

a 2 hour period. The device's polarization sensitivity was

also measured using a 2" diameter UV polarizer, and

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor 7
intensitylossesduetopolarizationwererecordedin5°

incrementsofpolarizerrotationangle.Differencesin

sphereradianceoverthe3.8°FOVwereaccountedforby

varyingRSAS'distancefromtheintegrator.Thecalculated

infinityandvacuumfocalengthsforRSASwerealso

verified.Theexactcalibrationproceduresaredescribedin

moredetailelsewhere3,4,5.

Theresultsofthepost-flightcalibrationshowedadropin

instrument sensitivityof about 3%. This has been traced to

a shift in the center bandpass of the interference filter by

about 0.2 nm, as shown in Figure 6, below. This shift to

shorter wavelengths reduces the apparent radiance of the

source. The reason for the change in the filter behavior is

not known and is currently being investigated. This shift in

band pass has no effect on the performance of RSAS with

respect to attitude sensing.

2.6 for material yield and ultimate strength, respectively.

The smallest margin of safety was 9.7 for the fail-safe

analysis of the fasteners holding the mounting plate to the

housing. The margins of safety are shown in Table 3.

Table 3. Expected Stress Levels

Description Bolts Margin of Safety

Mounting Plate to #6-32 11.8

SSBUV Plate

Mounting Plate to #4--40 9.7

Housing

Optics Barrel Axial __ 275

Stress

Optics Barrel Hoop

-- 17.3 Stress

Controller Box #6-32 16.2

060

0.50

• 040

.o

@

co

E 0.30

o

020

0.10

345

RSAS Filter- Trensmission

, , Preflight--

Posfflight

350 355 360 365

Wavelength [nrn]

Figure 6. Variations in pre-flight to post-flight filter

transmittance.

2.6.2 Structural/Mechanical Analysis and Testing

2.6.2.1 Vibration Test

The RSAS system underwent a vibration test to verify

stability, structural soundness, and workmanship. This test

provided no evidence of potentially dangerous resonant

frequencies for the system. All loads experienced by the

system during the vibration did not place undue stress on

any of the components. It was determined that RSAS was

not in danger of being damaged by any vibration that could

have occurred during the launch of STS-72.

2.6.2.2 Stress and Fracture Analysis

The anticipated load levels used in the stress analysis of

RSAS were taken from Get Away Special (GAS) documents detailing load levels undergone by the SSBUV

instrument on previous STS flights. These levels are

+ 11 g's in the Orbiter's x, y, and z axes. The margins of

safety were determined using factors of safety of 2.0 and

2.6.3 Thermal Environment Analysis

Preliminary thermal environment computational predictions

were performed by Swales & Associates, Inc. Using solar

incidence angles based on data from SSBUV's previous

mission (STS-66), it was concluded that during its use the

temperature of the uncoated (white) RSAS optics module

would fluctuate by no more than + 3° about a mean value of

17 °C. It was predicted that RSAS' optics module would

always be slightly hotter than the top deck of the SSBUV

canister, whose median temperature would be approximately 15 °C.

Later analyses, based on modeled solar and Earth reflectance

incidence angles for STS-72, predicted that the temperature

of the optics module would vary between 17 °C and 23 °C,

slightly higher than the original model had estimated. In

either case, however, the temperatures were not expected to

leave the operational range of 0 °C to 40 °C at any time

during the mission. Although both analyses were performed

under the assumption that the RSAS optics module would

fly uncoated, it was covered with silver Teflon before

STS-72. The silver Teflon tape was applied to act as an

insulator, to reduce heat transfer to and from RSAS, and to

keep the temperature variations down.

3.0 PERFORMANCE

While other instruments have used the Rayleigh scattered

limb signal to deduce viewing altitude 6,7,RSAS was designed solely to test this concept and was flown alongside

traditional infrared horizon detectors on the Space Shuttle.

Along with the Shuttle attitude system, which consists of

star trackers and inertial devices, ground-based radar altitude

8 Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor
measurements providedanaccurate(0.05°) baselinefor

evaluatingtheperformanceof RSAS. In the following

sections, both the algorithm used to convert limb radiance

data to pointing information and results from the first flight

of this instrument are presented.

3.1 STS-72 Payload Configuration

During the test flight, the SSBUV GAS canister was

situated on the starboard side of the Orbiter, as illustrated in

figure 7, below. RSAS was mounted on the top deck of the

SSBUV canister, and its lens was inclined at the pointing

angle of approximately 16.5 ° with respect to the local

horizon. The azimuth pointing orientation was 32.1 ° with

respect to the Orbiter's y-axis.

RSAS collected data when the Orbiter's payload bay and the

SSBUV GAS canister lid were open. Also, since the RSAS

optics module was fixed in orientation with respect to the

Orbiter, a constant bay-to-Earth attitude had to be maintained to obtain useful data. All attempts were made to

ensure that RSAS had a clear FOV.

3.2 Analysis Methodology

The limb radiance measured by RSAS is a function of SZA

and azimuth angles (lighting conditions), altitude and

pointing, and the reflectivity of the Earth and atmosphere

below the viewing altitude. To simplify the calculation of

altitude and pointing changes, the lighting conditions and

reflectivity dependence can be virtually eliminated using a

ratio technique. This technique makes use of the observation that the shape of the limb radiance signal is nearly

independent of these varying factors. The left chart in

figure 8 shows the limb radiance signal as a function of

altitude. To the right, the ratio of the nominal boresight

signal at an altitude of 30 km to the same signal at 20 km is

OAST-FLYER

EVA

$FU-RETR

\

/

/

/

\

\

\

\

Figure 7. STS-72 payload orientation.

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor 9
IL

7E

c

E

0_

o

c

.2

rY

0.1000

0.01 O0

0.0010

0.0001

Model Limb Radionce

__,_ : ISol'ar Zeniih _ O' '

! Azimuth = 0

Nominol boresight Iocotion X

20 40 60

Tangent Height [km]

E

0

o4

E

._o

8

E

._0

0.410

0.400

0.390

0.380

0.370

Nominol Boresight Ratio

Azimuth angles 0,90,180

Reflectivity 0,.5,.8

I , , , I , , , I , , , I

0 2o 40 6o

Solar Zenith Angle [degrees]

i i

8O

Figure 8.Model radiance and radiance ratio under various lighting conditions.

shown as a function of SZA for a wide range of lighting

conditions and surface reflectivity values. The chart on the

right shows that this ratio changes very little over these

conditions, even though the radiance may change substantially.

The variations shown in the figure do imply potential

modeling errors on the order of 0.01 ° . Additionally, errors

on the order of 0.05 ° can exist for SZAs greater than 80 °.

Therefore, the data analysis is restricted to SZAs less than

80 °. During the mission, RSAS images were taken during

46 orbits at a rate of one every 80 seconds. Because the

limb filled roughly half the RSAS FOV, pixels pointed

above the atmosphere could be used to correct for dark

current and stray light.

3.3 Results

The GSFC ultraviolet radiative transfer code was used to

model the expected 355 nm radiance during various periods

of the STS-72 flight, and a theoretical altitude gradient for

UV radiance was generated. To best compare the actual

data with the predicted data, a series of normalized radiances as a function of tangent height (TH) was created.

This series is shown in figure 9, along with the same ratio

calculated from an RSAS image during the actual Shuttle

flight. The similarity in the shape of both curves can easily

be discerned. Although each set of data has been normalized, the variation in the measured UV flux with altitude

matches the model very closely. The limb ratio measured at

the boresight pixel was then converted to a tangent height

by comparing the normalized radiance with the model

calculations using equation 1.

RSAS and Model Rod[once Ratios

- i

o 08 _x _ _

_ Model

:_ RSAS scan

_o 0.6

_ 02

20 25 30 35 40

Tongent Height [km]

Figure 9. Normalized radiance vs. altitude.

The RSAS calculated pointing changes were then compared

to the shuttle measured values using the post-flight attitude

and trajectory history (PATH) data 8 . The PATH data has a

quoted accuracy of 1 km in altitude and 0.05°/axis in attitude

per standard deviation. Figure 10 compares the elevation

angle of the RSAS boresight with respect to the Shuttle's

x-y plane to the Shuttle's known attitude measurements,

using both the RSAS limb data and the PATH product during

one daylight pass. From figure 11, it is evident that RSAS

indicates a consistent offset with respect to the PATH data.

Figure 11 shows the difference between the two measurements over a series of orbits. Also plotted in figure 11 is the

SSBUV Sun sensor measurement of the same offset during a

subsequent orbit. It can be seen that the Sun sensor agrees

well with the RSAS measured offset. The measured

elevation difference between the SSBUV top deck and the

shuttle x-y plane was 0.08 ° _+0.05 °.

10 Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor
Elevation Angle of Boresight to Shuttle x y plane

PATH data

FRSAS data

9250 9240 9250 9260

Mission Elapsed Time [min]

Figure 10. Elevation angle measured by RSAS and PATH.

Difference in PATH and RSAS Measured Boresight Elevation

T

e

2

o.2b

<

o.4_ [

0.61 ,

900xlO 3

Solar Aspect Sensor DifferenceJ

i , , , i .... i ....

9.50x/0 ' 1,00xlO 4 1 05xlO 4 / 10x/04

Mission Elapsed Time [min]

Figure 11. Differences in elevation angle as measured

by RSAS, PATH, and SSBUV Sun sensor.

4.0 SUMMARY

The Rayleigh Scattering Attitude Sensor was designed,

built, tested, and flown on the Space Shuttle by personnel at

the NASA GSFC. Motivation for the design stemmed from

the possibility of developing a very accurate spacecraft

horizon sensor at low cost, while at the same time providing

a test bed for new technologies and improvements to

current radiative transfer models. Several aspects of the

overall design have been documented, including initial

requirements, hardware specifications, and explanations of

software support systems. Additionally, system testing

procedures and results have been explained, and pre-flight

and post-flight calibration differences have been summarized. Finally, the test-flight configuration, data retrieval

procedures, and initial performance analysis have been

presented.

RSAS has succeeded in proving the concept of using a UV

limb detector for spacecraft attitude determination purposes.

This novel attitude sensor that detects Rayleigh-scattered

solar radiation has successfully completed its tests on the

Space Shuttle Endeavor during the STS-72 flight. RSAS

performed as good or better than expected, by producing

measurements of its boresight pointing angle accurate to at

least 0.05 ° . In principle, this accuracy can be improved by

carefully considering optical system aberrations, better

modeling the varying factors that alter the nominal UV

radiance profile, and improving the component precision of

the RSAS system.

REFERENCES

1 M. Scoby and P. Stupik. "Stable ultra-narrow band pass filter," Presented at SPIE Annual Conference, July 1994.

2 B. Herman, D. E. Flittner, R. D. McPeters, and P. K. Bhartia,

"Monitoring atmospheric ozone from space limb scatter measurements," Proc. SPIE Vol. 2583, p. 88, September 1995.

s j. H. Walker, C. L. Cromer, and J. T. McLean, "A Technique

for Improving the Calibration of Large-Area Sphere

Sources," Proc. SPIE, Vol. 1493, pp. 224-230, April 1991.

4 D. E Heath, W. Zhongying, W. K. Fowler, and V. W. Nelson,

"Comparison of spectral radiance calibrations of SBUV-2

satellite ozone monitoring instruments using integrating

sphere and flat-plate diffuser techniques ", Metrologia, Vol.

30, pp. 259-264, October 1993.

s S. J. Janz, E. Hilsenrath, J. Butler, D. F. Heath, and R. P.

Cebula, "Uncertainties in radiance calibrations of backscatter ultraviolet (BUV) instruments," Metrologia, Vol. 32, pp.

637-641, May 1996.

6 H.J. Wang, D. M. Cunnold, and X. Bao, "A critical analysis

of stratospheric aerosol and gas experiment ozone trends,"

J. Geophys. Res., Vol. 101, pp. 12,495-12,514, May 1996.

7 D.W. Rusch, G. H. Mount, C. A. Barth, R. J. Thomas, and

M. T. Callan, "Solar mesosphere explorer ultraviolet spectrometer: measurements of ozone in the 1.0-0.1 mbar region," J. Geophys. Res., Vol. 89, pp. 11,677-11,687, December 1984.

s A. S. King, "Final report for the STS-72 PATH product,"

Johnson Space Flight Center Tech. Rep. 660-NAV-720-96-

025, April 1996.

Design, Development, and Test-Flight of the Rayleigh Scattering Attitude Sensor 11
Form Approved

REPORT DOCUMENTATION PAGE OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

September 1997 Technical Memorandum

4. TITLE AND SUBTITLE

Design, Development, and Test-Flight of the Rayleigh Scattering

Attitude Sensor

6. AUTHOR(S)

E. Hilsenrath, Anderson, E. C., D. Flittner, D. Heath, S. Janz,

J. Medeiros, J. Cerullo, C. Thorpe, T. Riley

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS (ES)

Laboratory for Atmospheres

Goddard Space Flight Center

Greenbelt, Maryland 20771

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS (ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

NAS5 -31729

NAS5- 31755

(Code 916)

8. PEFORMING ORGANIZATION

REPORT NUMBER

97B00044

10. SPONSORING / MONITORING

AGENCY REPORT NUMBER

TM-104644

11. SUPPLEMENTARYNOTES E. C. Anderson, New Millennium Astronautics, Charlottsville, Virginia: D. Flittner,

University of Arizona, Tucson, Arizona; D. Heath, Research Support Instruments, Boulder, Colorado; S. Janz, J.

Medeiros, & J. Cerullo, The Aerospace Engineering Group of IDEA, Inc., Beltsville, Maryland

12a. DISTRIBUTION / AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 19

Report available from the NASA Center for AeroSpace Information,

800 Elkridge Landing Road, Linthicum Heights, MD 21090; (301) 621-0390.

12b. DISTRIBUTION CODE

13.ABSTRACT(Maximum200 words) The Rayleigh Scattering Attitude Sensor (RSAS), a proof-of-concept

device, was designed, developed, and tested, at the NASA Goddard Space Flight Center. The prototype instrument was flown aboard the Space Shuttle Endeavor (STS-72) in January 1996. RSAS uses

a 512 element linear photodiode array and an interference filter to measure incident solar radiation that

is Rayleigh-scattered from the Earth's limb at a wavelength of 355 nm. System requirements and a

system overview are presented. Design and development of mechanical, electrical, and optical subsystems of RSAS are also documented, including the software interface structure and ground support

equipment (GSE). Pre-flight and post-flight testing procedures, calibration and optical characterization results are discussed. Limb scattering radiances observed by RSAS during the test flight show

good agreement with predictions by the GSFC ultraviolet radiative transfer model. The attitude

determination algorithm applied to the limb scattered signal is presented. The attitude measured by

RSAS is shown to be in excellent agreement with concurrent Shuttle pointing data (+ 0.05°). These

data indicate that RSAS type instruments could yield significantly better performance (-0.01 o resolution) than state-of-the-art horizon sensors (-0.1 o resolution), at a considerably lower cost.

Attitude Sensing, Ultraviolet, Rayleigh Scattering, Limb Scattering, Optics, Solar Radiation.

14..SUBJECT TERMS

Attitude Sensing, Ultraviolet, Rayleigh Scattering, Limb

Scattering, Optics, Solar Radiation

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

18

16. PRICE CODE

20. LIMITATION OF ABSTRACT

UL

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39.18

298-102