ARM TR-019

> Micropulse Lidar (MPL) Handbook
>
> November 2004
>
> C. Flynn
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 1](#general-overview)

[2. Contacts 1](#contacts)

[3. Deployment Locations and History
1](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 2](#near-real-time-data-plots)

[5. Data Description and Examples 2](#data-description-and-examples)

[6. Data Quality 4](#data-quality)

[7. Instrument Details 5](#instrument-details)

# General Overview

The Micropulse Lidar (MPL) is a ground-based optical remote sensing
system designed primarily to determine the altitude of clouds overhead.
The physical principle is the same as for radar. Pulses of energy are
transmitted into the atmosphere; the energy scattered back to the
transceiver is collected and measured as a time-resolved signal. From
the time delay between each outgoing transmitted pulse and the
backscattered signal, the distance to the scatterer is infered. Besides
real-time detection of clouds, post-processing of the lidar return can
also characterize the extent and properties of aerosol or other
particle-laden regions.

# Contacts

## Mentor

Connor Flynn\
Pacific Northwest National Laboratory\
Phone: 509-375-2041

Fax: 509-375-4545\
<connor.flynn@pnl.gov>

## Instrument Developer

Vendor/Manufacturer\
Science and Engineering Services Inc. (SESI)\
Contact: I.H. Hwang\
Phone: 301-989-1896

# Deployment Locations and History 

**SGP-C1, Lamont**: The MPL at SGP is currently operating within
specifications.

**NSA-C1, Barrow**: A prototype MPL having polarization sensitivity has
been operating at Barrow since November 2003. As this capability is
still under development, only the standard MPL product is being
operationally produced from this prototype system.

**TWP-C1, Manus**: The MPL at Manus is currently operating within
specifications.

**TWP-C2, Nauru**: The MPL at Nauru is currently operating **below**
specifications. While still capable of detecting high cirrus, the
daytime sensitivity is much reduced. This system is anticipated to be
replaced in May 2004.

**TWP-C3, Darwin**: The MPL at Darwin has been returned to NASA GSFC for
renovation with return expected in early June 2004. This system is
anticipated to be replaced in Darwin in July 2004.

In response to unacceptable down-time and lengthy repairs, ARM is
purchasing additional MPL systems as spares. An additional three spares
have been contracted with the first of these to be received in April
2004. In addition, a new MPL will be procured for the ARM Mobile
Facility.

# Near-Real-Time Data Plots

See [SGP Quick
Looks](http://www.nsdl.arm.gov/Visualization/mpl/frame.htm).

# Data Description and Examples

See [MPL Quick looks](http://mplnet.gsfc.nasa.gov/) from NASA-Goddard
Space Flight center.

## Data File Contents

### Primary Variables and Expected Uncertainty

The Micropulse Lidar (MPL) has one measurement channel that records
backscatter signals up to 20+ kilometers. The primary quantity from this
signal is the lowest detected cloud base in meters.

Additional quantities possible through post-processing of the raw signal
return include a relative backscatter profile at 523 nm. From the
relative backscatter profile, other data products are possible including
multiple cloud decks, cloud and layer boundaries, as well as aerosol
extinction and backscatter profiles.

#### Definition of Uncertainty

The uncertainties in reported cloud base height have several sources.
There is an inherent calibration uncertainty of the timing electronics
of about 2%. This translates directly into an uncertainty of +/- 2% for
all reported distances.

Also, the measured lidar profiles are collected in discrete \"range
bins\" with finite width. Reported cloud heights are centered within the
range bin, so cloud base heights will have an uncertainty of +/- 1/2 the
range resolution. Early MPL systems deployed at SGP and TWP C1 (Manus)
had a range resolution of 300 meters. ARM MPL systems are currently
operated with 30 meter resolution including all MPL data ever collected
from NSA, TWP C2 (Nauru), and TWP C3 (Darwin).

There are also several uncertainties that are more difficult to
quantify. The MPL is an eyesafe lidar, and as such transmits a very low
power laser beam, typically less than \~25 mW at 523 nm. Thus, it is
subject to signal to noise limitations in conjunction with solar
background noise. Moreover, the laser beam is attenuated or extinguished
as it passes through the atmosphere. These two effects combine to make
detection of high thin clouds more difficult during the day.
Furthermore, over time laser systems degrade and produce less powerful
pulses, so the sensitivity of the MPL will depend on the health of the
laser system in the MPL. In addition to these measurement limitations,
there are other uncertainties that are difficult to quantify. Exactly
\"what is a cloud\" is difficult to define. Algorithm differences can
yield biases in reported cloud base height. More significantly, one
algorithm may identify a particular atmospheric structure as being
\"cloud\" while another algorith may not, so algorithm sensitivity is
also a difficult uncertainty to quantify.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

Besides actual cloud-base heights, there are two sentinel values present
in the data.

A cloud-base height = 60 kilometers is reported for a \"blocked beam\"
condition. Literally, this condition means that insufficient backscatter
has been detected at even near-range bins. This situation can arise from
heavy fog or can be caused by water, ice, or debris accumulating on the
view port window.

A cloud-base height = 0 is reported when the sky overhead is determined
to be cloud-free.

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

This section is not applicable to this instrument.

## Frequently Asked Questions

**What MPL datastream should I use for clouds?**

Use ARSCL if it is available. If not, then use MPLnor. If neither, use
the a1-level file.

**What MPL datastream should I use for aerosol products?**

ARM MPL aerosol retrievals are currently in development but are not
operationally available. For limited periods, aerosol products from the
ARM MPL at SGP is available from NASA\'s
[MPLnet](http://mplnet.gsfc.nasa.gov/). For qualitative indications of
aerosol, the normalized backscatter profiles from MPLnor are excellent
indicators of aerosol layers and relative abundance. Use of a1-level MPL
datastreams for aerosol detection is not advised.

**What is the lowest cloud the MPL can detect?**

Early MPL systems had relatively coarse range resolution of 300 meters.
Also, these systems had high levels of detector \"afterpulse\", a
long-lived residual of the transmitted laser pulse evident in the
range-resolved profile, but not actually emanating from the atmosphere.
Additionally, all MPL systems require correction of near-range signal
with the magnitude of the correction increasing inversely with distance.
These three elements combined to limit initial MPL cloud detection to
about 300 meters. Improvements in all three areas now offer the
potential of reducing this to perhaps 60-90 bins, but current algorithms
still use the old lower limits.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

-   [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

-   [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
    plotting using.

The tables and graphs shown contain the techniques used by ARM\'s data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

QC frequency: Monthly basis

QC delay: Next week

QC type: Graphical plots

Inputs: Raw data

Outputs: Raw cloud base height (cbh) estimates compared with VCeil and
MPL cbh estimates; processed backscatter profiles

Reference: Routine data quality monitoring of the MPL at the SGP
consists mainly of cross comparison of raw MPL cloud base height (cbh)
estimates with those from the VCeil.

## Data Assessments by Site Scientist/Data Quality Office

All DQ Office and most Site Scientist techniques for checking have been
incorporated within [DQ HandS](http://dq.arm.gov/) and can be viewed
there.

## Value-Added Procedures and Quality Measurement Experiments

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into \"value-added\"
products or VAPs. Despite extensive instrumentation deployed at the ARM
CART sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not in order to fill unmet
measurement needs, but instead to improve the quality of existing
measurements. In addition, when more than one measurement is available,
ARM also produces \"best estimate\" VAPs. A special class of VAP called
a Quality Measurement Experiment (QME) does not output geophysical
parameters of scientific interest. Rather, a QME adds value to the input
datastreams by providing for continuous assessment of the quality of the
input data based on internal consistency checks, comparisons between
independent similar measurements, or comparisons between measurement
with modeled results, and so forth. For more information see [VAPs and
QMEs](http://www.arm.gov/data/vaps.stm).

Two VAPS currently use the raw MPL datastream. Whenever possible, the
following value-added products should be used in preference to the raw
or a1-level MPL datastream.

-   **MPLnor**: \"MPLnor\" stands for *MPL normalized*. It produces
    \"normalized\" backscatter profiles (in arbitrary units) with all
    known instrument artifacts removed. To improve signal to noise,
    MPLnor applies further temporal and spatial averaging. It also
    reports up to three layers of clouds along with cloud base and cloud
    top when possible. Both a \"sensitive\" and \"robust\" cloud mask
    are provided where the \"robust\" cloud mask is simply the
    \"sensitive\" mask with some filters applied to remove false
    positives.

-   **ARSCL**: \"ARSCL\" stands for *Active Remotely-Sensed Cloud
    Locations* . It represents a composite product combining
    measurements from ceilometers, lidar, and radar. Lidar and radar
    measurements are complementary in that lidar are more sensitive to
    smaller particles often found in cirrus or low water vapor clouds.
    However, radar is able to penetrate multiple cloud decks that are
    impossible for lidar to penetrate. Thus, this composite product
    provides the best of both instruments and is currently ARM\'s last
    word on cloud detection.

In addition, several other VAPs involving MPL measurements are under
development including:

-   Thin-cloud optical depth retrieval

-   Aerosol properties retrievals

-   Depolarization ratios

-   Slant-path optical depth retrievals.

# Instrument Details

## Detailed Description

### List of Components

The MPL consists of four main components: 1) a computer, 2) a dedicated
data acquisition and lidar control system, 3) a diode-pumped Nd-YLF
laser system, and 4) a co-axial transceiver for transmitting the laser
pulses and detecting the collected photons. Following is a description
of each component:

1.  **The Computer**: Currently, IBM laptops are used with all ARM MPL
    systems. A migration to an alternative small form factor PC is
    planned in 2004.

2.  **Lidar Control System**: The lidar control system, custom produced
    by Science and Engineering Services Inc. (SESI), provides
    conditioned power to the photon detector and laser energy monitor.
    It contains an integrated A/D converter for reporting of vital
    system parameters to the instrument PC. It also contains the
    range-selectable multi-channel scalar which accumulates the
    range-resolved backscatter profiles.

> At present, all ARM MPL systems use this lidar control system by SESI,
> but a migration to lidar control system by ASRC is planned in 2004.

3.  **Laser-Diode Pumped Nd-Ylf Laser System**: A rack-mount laser
    system provides CW laser diode infrared pump radiation via fiber
    optic to the Nd-YLF laser head within the transceiver. The
    rack-mount laser system also controls the pulse repetition rate of
    the Nd-YLF laser head incorporated into the MPL transceiver
    (described below). Originally, all MPL systems used Spectra Physics
    lasers (model 7300 or \"R-Series\"), but as these lasers were
    discontinued some MPL systems now use laser systems with similar
    specifications from other third party vendors.

4.  **Co-Axial Transceiver**: The \"transceiver\" serves as both
    transmitter of the outgoing laser pulses and receiver of
    backscattered light. Approximately 1.0 watt of infrared CW pump
    radiation is converted to about 25 mW pulses of green laser light
    (523 nm) at 2500 Hz by the Nd-YLF laser head with non-linear optical
    frequency doubler. The pulses of green light are passed through a
    linear polarizing beam splitter, a depolarizing wedge, and expanded
    to fill an 8\" Celestron telescope.

> The detection optics begins with the same 8\" Celestron telescope.
> Returning photons incident on the telescope are collected and pass
> through the depolarizing wedge. About half of the collected photons
> pass through the polarizing beamsplitter cube and half are reflected.
> Light passing through the beamsplitter is collimated and passed
> through two narrow-band interference filters (0.27 nm fwhm) in order
> to reject most of the ambient light, and is ultimately focused onto a
> photon counting APD module from Perkin Elmer model SPCM-AQR-121-FC.

### System Configuration and Measurement Methods

The MPL is configured to operate autonomously in an unattended manner 24
hours a day. Standard ARM deployments have the MPL oriented vertically
(or slightly off vertical).

### Specifications

Wavelength of laser pulse: 523.5 nm

Length of laser pulse: \~10 ns = 3 m

Range resolution (height interval): 30 m

Maximum range for cloud base height: 18 km

Typical averaging: 30 sec (60 seconds for some locations)

## Theory of Operation

The principle is straightforward. A short pulse of laser light is
transmitted from the telescope. As the pulse travels along, part of it
is scattered by molecules, water droplets, or other objects in the
atmosphere. The greater the number of scatterers, the greater the part
scattered. A small portion of the scattered light is scattered back,
collected by the telescope, and detected.

The detected signal is stored in bins according to how long it has been
since the pulse was transmitted, which is directly related to how far
away the backscatter occurred.

The collection of bins for each pulse is called a profile. A cloud would
be evident as an increase or spike in the back-scattered signal profile,
since the water droplets that make up the cloud will produce a lot of
backscatter.

## Calibration

### Theory

Little calibration is necessary for cloud-base height determination. To
fix the distance scale, it is necessary to use a calibrated-pulse
generator capable of producing a trigger pulse and a second delayed
pulse with an accurately known time lag. The two pulses are used to
mimic a transmitted laser pulse and detected backscatter pulse with time
delay relating to a simulated distance.

Absolute calibration of the magnitude of the lidar signal is much more
difficult. The following instrument-level corrections are required:

1.  \"Dead-time\" correction to account for detector non-linear response

2.  Detector \"afterpulse\" subtraction

3.  Background subtraction

4.  Range-squared correction

5.  Near-field detector overlap correction

6.  Energy-monitor normalization.

Even after these various corrections are applied, the overall system
transmittance is only coarsely known. Determination of this overall
system calibration is typically obtained by comparison against other
external measurements, modeled results, or both.

### Procedures

This section is not applicable to this instrument.

### History

As MPL systems are returned to PNNL for repair, timing calibration
checks of the multichannel scalar are performed using HP precision pulse
generators with sub-nanosecond accuracy. We have never yet found a
multichannel scalar out of specification. Each of the calibrations
described in the above section are applied by the MPLnor value-added
product. The detector deadtime corrections are from vendor-supplied data
sheets unique to each detector. Each of the other corrections is
determined either through measurements conducted during installation, or
from the real-time data.

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

Little maintenance is required other than routine cleaning of the
viewport window and gentle cleaning of dust from the telescope.
Occasionally, the software or computer may lock up so visual
confirmation that the program is operating, that the clock is updating,
and that the displayed measurement agrees with reality are also
required.

### Software Documentation

This section is not applicable to this instrument.

### Additional Documentation

[Preventative Maintenance Procedure Summaries for the
MPL](http://www.db.arm.gov/emp-bin/MDS/ListPM.pl?instrument=MPL) at the
Southern Great Plains (SGP) Site.

## Glossary

See the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

LIDAR: Light Detection and Ranging\
\
Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Campbell, J.R., D.L. Hlavka, E.J. Welton, C.J. Flynn, D.D. Turner, J.D.
Spinhirne, and V.S. Scott. 2002. Full-time Eye-Safe Cloud and Aerosol
Lidar Observation at Atmospheric Radiation Measurement Program Sites:
Instruments and Data Processing. *J. Atmos. Oceanic Technol*., **19**,
431-442.

Spinhirne, J.D. 1993. Micro pulse lidar. *IEEE Trans. Geosci. Remote
Sens*., **31**, 48-55.

Spinhirne, J.D., J.A.R. Rall, and V.S. Scott. 1995. Compact eye safe
lidar systems. *Rev. Laser Eng*., **23**, 112-118.