The results are presented of a capability study of Earth orbiting lidar systems, at various wavelengths from 1.06 to 10.6 microns, for the measurement of wind velocity and aerosol backscatter, and for the detection of clouds. Both coherent and incoherent lidar systems were modeled and compared for the aerosol backscatter and cloud detection applications

Huffaker, R. M.

Kavaya, M. J.

English

NASA Technical Reports Server

N87-10361
NAVELENBTH DEPENDENCE OF COHERENT AND INCOHERENT
SATELLITE-BASED LIDAR MEASUREMENTS OF
WIND VELOCITY AND AEROSOL BRCKSCRTTER
of poor
Michael J. Kavaya
R. Milton Huffiker
Coherent Technologiesv Inc.
P.O. Box 7488
Boulder, Colorado 80306-0403
USA
INTRODUCTION
81obal profiling of Earth's wind from a satellite would
provide numerous civilian, commercial, military, and
scientific benefits. In addition, satellite-based
measurements of aerosol and cloud backscmtter are of great
interest. Studies sponsored by the Defense Meteorological
Satellite Program (DMBP) have shown the feasibility of
measuring the global wind field using a satellite-based
pulsed coherent C02 laser radar [1]. RCA Corp. has recently
completed a study for an Earth-orbiting lidar to measure
aerosol backscattmr and cloud top heights using a NdmYA8 and
doubled NdmYA8 laser operating in an incoherent mode. The
NdmYA8 wavelengths at 1.Oh and 0.53 microns are in a
wavelength region where strong consideration must be given to
eye safety. Only coherent C02 laser radars, which operate in
the 9-11 micron wavelength region, have demonstrated accurate
long-range remote wind velocity measurements to date.
However, solid-state laser technology has progressed to the
point where it should also be considered. Stanford
University researchers have built a solid-state NdmYA8 lidar
and have recently demonstrated coherent return signals from
clouds and aerosols. Solid-state lamer systems, compared to
C02 systems, will be smaller, lighter, may require lima
power, may not require cooled detectors, and have the
potential of several years lifetime in space. A technology
llSlSSmlrlt hal shown the feasibility of fabricating a ground-
based NdmYA8 solid-state coherent lidar system at this time
for wind velocity and aerosol backscatter measurements t2].
Research is progressing at Stanford University and elsewhere
on eyesore solid-state sources suitable for coherent lidar
measurements. The "modular" nature of solid-state systems
would allow for the eventual upgrade of NdmYA8 systems to an
myesafe wavelength as the technology permits.
This paper presents the results of a capability study
of Earth-orbiting lidar systems, at various wavelengths from
1.0a to 10.a microns, for thm measurement of wind velocity
and aerosol backmcattmr, and for the detection of clouds [3].
Both coherent and incoherent lidar systems were modeled and
269
https://ntrs.nasa.gov/search.jsp?R=19870000928 2020-03-23T14:45:43+00:00Z
_ L
compared for
appl icati one.
the aerosol backmcatter and
COMPUTER SIMULATION
cloud detection
The Coherent Technologies, Inc. lidar computer
simulation was used to estimate the performance of a
satellite-based laser radar system [1]. A set of satellite
base parameters was selected which was used in an earlier
analysis by NOAA to determine global wind velocity monitoring
feasibility at 9.11-micron wavelength. The wavelength
dependence of several of the parameters was reviewed and
incorporated in the simulation. The wavelength and height
variation of the atmospheric extinction was modeled through
the use of the 1982 AFBL atmospheric absorption line
parameters compilation. A simplified wavelength**(-2.5)
dependence of the aerosol backscatter coefficient was adopted
after reviewing recent modeling results and measurements. A
factor of 2 was used at 9.11 microns (vs 10.6 microns),
instead of 1.5, due to the predicted and measured enhancement
at that wavelength. Atmospheric refractive turbulence is
modeled in the computer simulation but was found to have a
negligible impact even at 1.0a microns. A pulse duration of
6.67 microseconds was used at 10.6 and 9.11 microns yielding
a 1 km range resolution and a 8aussian pules Fourier
tranmform-limited radial wind velocity resolution of
approximately 1 m/s. The pulse duration at other wavelengthm
was made proportional to wavelength in order to preserve a
constant velocity resolution.
WIND VELOCITY PERFORMANCE
An example of the satellite-based coherent lidar
performance in measuring the horizontal wind velocity
component along the track of the satellite is shown in Figure
1 for a wavelength of 2.10 microns. The pulse duration and
lO.a-micron backscatter profile multiplying factors are shown
in the figure. The corresponding along-track RM8 error at
9.11 microns (not shown) varied between 0.8 and 2 m/s for
altitudes between ground level and 20 km. The C02 isotopic
laser wavelength at 9.11 microns is a strong candidate due to
the lower atmospheric C02 absorption and due to the enhanced
aerosol backscatter coefficient. The eyesafs wavelength at
2.10 microns also has relatively low atmospheric absorption
and is a promising solid-state laser wavelength [3].
Using the backscatter multiplier of 60 at 2.10 microns_
we mee in Figure 1 that the wind velocity error is much
greater than at 9.11 microns. Adding a further backscatter
enhancement of 10 for a total enhancement of 600 results in
better performancl at 2.10 microns than at 9.11 microns and
much better than is needed. The aerosol model assumed in the
270
ORIGINAL P._L:::.; _,"
OF POOR
ORIGINAL F_ Fi)
OF POOR _AUi'Y
computer simulation ia the most uncertain parameter In the
performance estimate. Aerosol backscatter measurements at
several eyesa(e wavelengths in addition to the CO2 laser
wavelengths are needed. Ths performance curves in Figure i
do not include any range-gate averaging. The (actor of 5
shorter pulse duration at 2.10 microns would allow the
averaging of 5 range gates while preserving range and wind
velocity resolutions equal to those at 9.11 microns. This
would reduce the wind errors shown in Figure 1.
BACKBCATTER PERFORMANCE
An example of satellite-baled coherent and incoherent
lidar cloud detection performance at l.Oa microns is shown in
Figure 2. The lO.6-micron aerosol backscatter profile was
multiplied by a +actor of 316 above 5 km altitude. To
mimulate a cloud, the backscatter coefficient below 5 km was
met to a value of 1.0E-5 llm llsr. The correct BNR
expression +or incoherent detection depends on the type of
detector in use, how it is umed, the measurement scenario,
and whether detection is in the pulse-counting or current-
measurement mode. No BNR expression covers all cases.
Realistic device parameters Here inserted into an equation
+or non-avalanche photodiodes and the remults are shown in
Figure 2 as curve A. A state-of-the-art BNR expression,
which allowm variation of wavelength and detection bandwidth,
is shown as curve B. The eSfect of the cloud on the 8NR is
clear +or all 3 curves. Detection should be posmible with
either coherent or incoherent systems.
REFERENCES
. R.M. Huffaker, T.R. Lawrence, M.3. Post, 3.T. Priestly,
F.F. Hall, 3r., R.A. Richter, and R.3. Keeler,
"Feasibility studies f.r a global wind measuring
satellitm system (Hindmat)m analymim of simulated
per+ormancs," Applied Optics 23, 2523 (1984).
. R.M. Huffaker, M.3. Kavaya, and D.M. 8eiger_ "Specify
and Design a Baled-State Pulsed Coherent Laser Radar,"
Technical Report CTI-TR-Ba01, Coherent Technologies,
Inc. (3anuary 1786).
R.M. Huffakmr_ M.3. Kavaya, and D.M. Beiger, "Coherent-
Incoherent Lidar 8ymtsm Capability Study," Technical
Report CTI-TR-8602, Coherent Technologies, Inc.
(3anuary 1986).
271
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io
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O
!
I
I
-4
I I I i I i I I i
/
1OxBeta 2.10 Lm
!
i
I I I I
8
SA_LLITE
BASEPAR/V_-]I_
X= 2.10 uM
PULSE []UIbITION = 1._ US
BETAB_ " 60 x BETA10.6UM
I I I t t
12 16 20
Wind Error (m/s)
FIGURE 1
0
ur_
o
u_
O
INCOHERENT - B
I
I
I
I I I
SATELLITE
BASE PARAI-IETER5
= 1.06 Mkccons
Pulse DuraLk0n = 0.67 _s
|Beta = 316 x Beta 10.6 Microns
Cloud at 5 km
\ -
\
\
COHERENT IINCOHERENT
I I
-20 0
I I
20 40
SNR (db)
FIGURE 2
272


Wavelength dependence of coherent and incoherent satellite-based lidar measurements of wind velocity and aerosol backscatter

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