The GLAS instrument on the NASA's ICESat mission has provided over a billion measurements of the Earth surface elevation and atmosphere backscattering at both 532 and 1064-nm wavelengths. The receiver performance has stayed nearly unchanged since ICESat launch in January 2003. The altimeter receiver has achieved a less than 3-cm ranging accuracy when excluding the effects of the laser beam pointing angle determination uncertainties. The receiver can also detect surface echoes through clouds of one-way transmission as low as 5%. The 532-nm atmosphere backscattering receiver can measure aerosol and clouds with cross section as low as 1e-7/m.sr with a 1 second integration time and molecular backscattering from upper atmosphere with a 60 second integration time. The 1064-nm atmosphere backscattering receiver can measure aerosol and clouds with a cross section as low as 4e-6/m.sr. This paper gives a detailed assessment of the GLAS receiver performance based on the in-orbit calibration tests

Abshire, James B.

Jester, Peggy L.

Lancaster, Redgie S.

McGarry, Jan

Palm, Stephen P.

Spinhirne, James D.

Sun, Xiaoli

Yi, Donghui

NASA Technical Reports Server

Source of Acquisition 
/NASA Goddard Space Flight Center 
ON ORBIT RECEIWK PERFORMAlrCE ASSESSMENT OF THE 
GEOSCIENCE LASER ALTIMETER SYSTEM (GLAS) ON ICESAT 
Xiaoli Sunc1), James B. Abshire@), James D. S~inhirne'~), Jan F. McGarry"', 
Peggy L. Jester(4), Donghui Yi('), Stephen P. Palm@), Redgie S. Lancaster(') 
(')NASA Gotldard Space Flight Center (GSFC), Code 694, Greenbelt, MD, USA, E-mail: XiaoliSun-l@nasa.gov 
(2)NASA GSFC, Code 690, Greenbelt, 1MD 20771, USA, E-mail: James.B.Abshire@asa.gov 
(?)NASA GSFC, Code 61 3, Greenbelt, 1MD 20771, USA, E-mail: James.D.Spinhirne@nasa.gov 
(4)SGT/lvASA GSFC, Code 614. W ,  Wallops Island, VA 2333 7, USA, E-mail: Peggy.L.Jester.I@gsfc.nasa.gov 
p)SGTnVASA GSFC, Code 614. I ,  Greenbe16 M D  20771 , USA, E-mail: Donghui. Yi. I@gs$c.nasa.gov 
(')SSmASA GSFC, Code 613, Greenbelt, nCa, 20771, USA, E-mail: Stephen.P.Palm. l@gsfc.nasa.gov 
UMBC/NAA GSFC, Code 61 3, Greenbelt, MD 20771, USA, E-mail: RedgieXLancaster. L@gsfc.nasa.gov 
ABSTRACT 
The GLAS instrument on the NASA's ICESat mission 
has provided over a billion measurements of the Earth 
surface elevation and atmosphere backscattering at 
both 532 and 1064-nm wavelengths. The receiver 
performance has stayed nearly unchanged since ICESat 
launch in January 2003. The altimeter receiver has 
achieved a less than 3-cm ranging accuracy when 
excluding the effects of the laser beam pointing angle 
determination uncertainties. The receiver can also 
detect surface echoes through clouds of one-way 
transmission as low as 5%. The 532-nm atmosphere 
backscattering receiver can measure aerosol and clouds 
with cross section as low as 1e-7/m.sr with a 1 second 
integration time and molecular backscattering from 
upper atmosphere with a 60 second integration time. 
The 1064-nm atmosphere backscattering receiver can 
measure aerosol and clouds with a cross section as low 
as 4e-6/m.sr. This paper gives a detailed assessment of 
the GLAS receiver performance based on the in-orbit 
calibration tests. 
1. INTRODUCTION 
The Geoscience Laser Altimeter System (GLAS) was 
developed at NASA Goddard Space Flight Center for 
the Ice, Cloud, and land Elevation Satellite (ICESat) 
missidn [l-21. It measures both the Earth surface 
topography and atmosphere backscattering profiles 
from a 600 km near polar circular orbit. It uses diode- 
pumped Q-switched NdYAG laser with 1064 and 532- 
IUTI dual wavelength outputs. The laser pulse energy at 
launch was about 75 mJ at 1064 nm and 35 mJ at 532 
nm. The laser pulse width is 5 to 6 ns and the pulse rate 
is 40 Hz. The laser footprint diameter on ground track 
is about 70 m. There are three receiver channels: (1) 
the surface elevation measurement channel at 1064-nm 
wavelength with a 1OO-Mtlz electrical bandwidth 
detector and 1-Gs/s pulse waveform sample rate; (2) 
the 1064-nm cloud backscattering measurement 
channel with a 2 Ms/s sample rate; (3) the 532-nm 
aerosol and cloud backscattering measurement channel 
with a set of single photon counting detectors and 75-m 
vertical range bins. GLAS also has a stellar reference 
system (SRS) that relates the laser pointing angle to the 
star field and hence the initial space. An on board GPS 
receiver provides the spacecd position and time 
information. 
The initial GLAS in-orbit measurement performance 
has been reported in [3]. A detailed description of the 
laser design and on-orbit performance can be found in 
[3-4]. The SRS design and performance has been 
reported in [5). This paper gives a detailed description 
of the ranging and atmosphere backscattering receiver 
performance based on the in-orbit calibration tests and 
comparison with other independent measurements. 
2. GLAS SURFACE ELEVATION 
MEASUaEMENT PERFORMANCE 
The GLAS surface elevation measurement channel 
consists of a 1-meter telescope, an aft optics assembly, 
a 0.75-nm bandpass filter, a Si avalanche photodiode 
(APD), post amplifiers, and an 8-bit 1Gds waveform 
digitizer. An on board software algorithm 
automatically detects the surface echo from the 
recorded waveform via a modified maximum 
likelihood detection technique, that uses a bank of six 
digital matched filters, a peak detector, and decision 
process to let swface echoes out weigh cloud echoes 
when both are present. The on board algorithm also 
autonomously adjusts the detection threshold and the 
post amplifier gain setting to maximize the receiver 
sensitivity and dynamic range. 
The performance of the GLAS surface elevation 
measurement receiver is calibrated periodically using 
the on board optical test source (OTS). The OTS 
consists of a laser diode that can simulate the 
transmitted and the echo laser pulse pairs with a 
https://ntrs.nasa.gov/search.jsp?R=20070010461 2019-08-30T00:36:35+00:00Z
preprogrammed delay and pulse amplitude. The GLAS 
measurements of these simulated transmitted and the 
echo pulse pairs demonstrated a e3  cm ranging 
standard deviation and -5% pulse amplitude 
measurement error. Fig. 1 shows an example of the 
OTS test data. The results fiom all other OTS tests 
since ICESat launch have been consistent without any 
measurable degradation. 
1.5 
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The on-orbit range measurement of Lake Vostok in 
Antarctic accuracy, a flat and relatively smooth surface 
area, showed a ranging precision of 2-3 cm, which 
agreed with the above OTS test results. The on-orbit 
range measurement at another calibration site, salar de 
Uyuni in Bolivia, also showed a 2-3 cm ranging 
precision when compared to the independent survey 
results using the Global Position System (GPS) 
receivers [63. 
The time stamp accuracy of each laser pulse has been 
verified to be within 3 ps using an array of 
photodetectots and GPS based timers at the White 
Sand Space Harbor in Texas E7]. The surface echo 
detection algorithm performed well despite of the fast 
varying swfhce reflectance, slope, and atmosphere 
conditions. The algorithm reliably detects weak surface 
echoes in the mist of cloud echoes under most of 
circumstances. The false detection rate has been kept to 
below 1% in a11 the measurementi. The automatic gain 
control algorithm keeps the echo pulse amplitude to 
within 50% to 80% of the full scale of the waveform 
digitizer regardless of the surface reflectance and slope. 
The detector gain control loop response time is about 
0.1 s (5 laser shots) with little overshoot in response to 
a sudden change in the received signal pulse amplitude. 
The photodetector responsifvity is monitored through 
the detector shot noise level in response to the 
background light from the sunlit Earth. The detector 
dark noise is also monitored when ICESat is on the 
dark side of the Earth. Fig. 2 shows a typical plot of the 
detector noise over an entire ICESat orbit. The detedor 
responsivity and dark noise have been stable since 
launch and no measurable degradation has been 
observed except for a known temperature and Sun 
angle effect. 
2.5 
9 
Detector gain Daytime 1 fixedat128125$ 
17:00!00 17:30!00 18:00!00 1830!00 
Time (UTC, 5/8/2006) 
Fig. 2. Detector noise standard deviation (stdev) 
calculated fiom the digitized waveform over m 
orbit. 
It was also found that the received signals from ice and 
snow surfaced over Antarctic continent are several 
times stronger than we had expected. The atmosphere 
transmission appeared to be nearly 90% most of the 
time when clouds were not present. The ice and snow 
surface appeared to have a nearly 100% Lamberth 
equivalent reflectance due to the opposition effect and 
partid specular reflection. As a result, a significant 
amount of the received pulse waveforms were 
saturated. To cove& for the range walk due to the 
saturation, a series of laboratow experiments were 
conducted using a flight spare detector assembly. It 
was found that although the pulse shape was severely 
distorted under saturation, the pulse area under the 
waveform still follows a definitive relationship to the 
actual laser echo pulse energy. One au ld  then infer the 
input pulse energy from the detected pulse arm. For 
flat areas with slope < O S  degrees, the received pulse 
width can be assumed to be the same as the transmitted 
ones. Therefore one could estimate the pulse centroid 
time from the leading edge time and the puke area. The 
saturation correction algorithm has been shown to 
reduce the range error to less than 5 cm in range bias 
and -3 cm in standard deviation [6,8] over flat areas. 
The minimum detectable surface echo signal from 
Antarctic is about 0.05 f3/pulse, which is about 1/400 
the signal level under clear sky conditions. Therefore, 
GLAS can range through clouds of opacity as high as 3 
(5% one-way transmission) over Antarctic, including 
all high altitude clouds and some mid to low level 
clouds along the coastal regions. 
3. GLAS 1064-pIIM CLOUD BACKSCATTERING 
MJ3ASUREMENT PERFORMANCE 
The 1064-nm cloud backscattering measurement 
channel shares the same Si APD detector with the 
surface elevation measurement channel. However, it 
uses a different set of detector post amplifiers, an anti- 
aliasing lowpass fdter, and a separate waveform 
digitizer with 0.5 ps sampling interval (75-m vertical 
range bin). The overall receiver noise level is about 3 
times as those shown in Fig. 2 considering the 
differences in the amplifier gain and bandwidth. The 
nighttime noise standard deviation is about 4 in the 
digitizer output unit, which is equivalent to the 
backscattered signal pulse amplitude from clouds of 
cross section of 5e-6/m-sr. The maximum input signal 
before saturation corresponds to a cloud cross section 
of 4.Oe-4/m-sr. The entire a receiver linear dynamic 
range covers from planetary boundary layer aerosol to 
all but dense cumulus clouds. Signals from these dense 
clouds have similar pulse waveforms as s u r f b e  echoes 
and they are often recorded by the surface elevation 
measurement channel. All of these have been verified 
by the airborne lidar measurements under flying 
ICESat [9]. 
The AC couplmg between the photodetector and the 
ampliers mandated by the surface elevation 
measurement channel causes a droop in the received 
waveform after a strong cloud echo and the effects can 
last as long as 100 ps. A novel digital filter is devised 
to compensate for the pulse waveform distortion in the 
ground data processing. The filter consists of a cascade 
of two boxcar integrators with their scaling factors 
matching the t h e  constants of the AC coupling 
chuits. 
4 GLAS 532-NM CLOUD BACKSCATTERING 
MEASlJREMENT PERFORMANCE 
The 532-nm cloud and aerosol backscattering 
measurement channel uses a set of Si single photon 
counting models (SPCM) as the detectors. It measures 
the atmosphere backscalterhg profiles up to 40-km 
altitude at a 75-m vertical resolution. The background 
photons are sampled before and after each 40-km 
backscattering proflle. The bandpass filter assembly in 
fiont of the detectors consists of a 0.28-A bandwidth 
temperature-tuned Etalon and a 3.51% blocking filter. 
The center waveieflgth of the Etalon is automatically 
tuned to the average laser wavelength via a software 
feed back control loop. A pointing mechanism is used 
to center the 0.17 mrad receiver field of view onto the 
Iaser footprint. An on board test source, a green light 
emitting diode (LED), is used to verify the detector 
performance and the receiver fimctionality. 
The SPCM photon counter performance is monitored 
from dark count rate and the responses to the sunlit 
Eartfi, Fig. 3 shows an exampie of the SPCM house 
keeping data, consisting of counts from each individual 
SPCMs over a 10 ps time interval after the laser pulse 
emission but before it reached Earth atmosphere. The 
peak value gives an indication of the SPCM photon 
counting efficiency illuminated by the sunlit Earth, 
which can be considered as constant light source when 
averaged over a large area and over a long time. 
2 5 0 1 , ,  , 1 ,  ~ ~ I , ,  ! ,  , , 1 
I . SPCM2 
L SPCMB 
21:OO:OO 21 :45:00 22:30:00 23:15:00 24:OO:OO 
Date & Time (UTC, 2/20/2006) 
Fig. 3. Sample instantaneous SPCM output count 
rate over two ICESat orbits. 
All the SPCM house keeping data are similar to those 
shown in Fig. 3 and there has been no measurable 
degradation in photon counting efficiency. 
Fig. 4 shows the average SPCM dark count rate over 
the past three years since ICESat launch. The dark 
count rate has been increasing steadily at about 60 
count& per day per SPCM due to space radiation 
damage. Although the SPCM dark count rates have 
risen significantly since launch, they are still much 
lower than the signal backscattered from clouds and 
aerosol. More details on the SPCMs can be found in 
[lo]. To date, these SPCMs have been in orbit for more 
than three years. The accumulated operating time has 
reached 290 days (7000 hours). 
Fig. 4. SPCM dark count rate and radiation 
damage since ICESat launch (Jan. 12,2006). 
The Etalon bandpass filter also functioned well in orbit, 
which enables GLAS to measure atmosphere 
backscattering against the brightest background light at 
daytime. The Etalon temperature control loop keeps 
the etalon center wavelength to the laser center 
wavelength. The control loop has a time constant of 
about 7 minutes, which is fast enough to track out the 
orbit variation (90 minutes period) but long enough to 
average out the laser wavelength fluctuation. The Iaser 
is mostly single mode on each shot but can hop among 
2 to 3 longitudinal modes, 0.02-0.03 apart, from shot 
to shot. Occasionally, the laser can have 2 to 3 modes 
simultaneously on a single shot. A software algorithm 
is used to estimate the laser center wavelength and to 
reject outliers before feeding back to the control loop. 
The Etalon filter and the control loop not only serve to 
minimize the background light onto the detectors, but 
also provide a direct measurement of the laser spectral 
characteristics. 
Comparison between the GLAS measurements and the 
near real time airborne lidar measurements shows that 
the GLAS 532-nm receiver can detect signals from 
atmosphere with cross section as low as le-7/m-sr with 
an 1 s integration time. The maximum dynamic range 
is about 5e-Wm-sr. The receiver can also detect 
molecular scamring at 30 km altitude with a 60 s 
integration time 191. 
5. CONCLUSION 
AI1 thee G U S  receiver channels performed well in 
orbit and meet and exceed the +science measurement 
requirements. The surface elevation measurement 
channel achieved 2-3 cm range measurement accuracy. 
The 1064-nm cloud backscattering measurement 
chamel is capable of detecting aerosol with cross 
section of 5e6/m-sr. The 532-nm aerosol and cloud 
backscattering measurement channel is about 50 times 
more sensitive and capable of detecting molecular 
backscattering in upper atmosphere. 
REFERENCES 
1. Zwally, H. J. et al., ICESat's laser measurements of 
polar ice, atmosphere, ocean, and land, .I Geodyn, Vol. 
34,405-445,2002. 
2. Schutz, B. E. et al., Overview of the ICESat 
Mission, Geophys, Res. Lett., Vol. 32, L21S01 , 2005. 
3. J. B. Abshire et al., Geoscience Laser Altimeter 
System (GLAS) on the ICESat mission: on-orbit 
measurement performance, Geophys. Res. Lett., Vol. 
32, L21S02,2005. 
4. Afial, R. S. et al., Space qualification of the 
Geoscience Laser Altimeter System (GLAS) laser 
transmitters, Technical Digest CLEO 2002, Opt. SOC. 
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5.  Sirota, J. M. et aZ., The transmitter pointing 
determination in the Geoscience Laser Altimeter 
System, Geophys. Res. Lett., Vol. 32, L22S11,2005. 
6. Fricker, H. A. et al., Assessment of ICEsat 
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7. Magruder, L. et al., In situ verification of the ICESat 
altimeter pointing and timing using a ground based 
system, Geophys. Res. Lett., Vol. 32, L21S04,2005. 
8. Sun, X., Abshire, J.B., Yi, D., and Fricker, H. A., 
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On Orbit Receiver Performance Assessment of the Geoscience Laser Altimeter System (GLAS) on ICESAT

On Orbit Receiver Performance Assessment of the Geoscience Laser Altimeter System (GLAS) on ICESAT

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