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global climate
the global sea
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altimetry data.
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in 2002.
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ichieved only
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s wind gener-
ons.
>n carried out
| topographic
brief descrip-
' of raw data
follows. The
in the Jakob-
] its compari-
2. LASER ALTIMETRY FROM SPACECRAFT
Satellite laser altimeters have been developed at NASA, to
study the Earth, the Moon, and the Mars (Bufton, 1989). Verti-
cal resolution of a few centimeters can be achieved from space-
craft by employing gain switched solid state lasers with a pulse
length of 1-10 nsec. The footprint size on the surface is in the
order of 50-300 m (Gardner, 1992). Because of the orbital alti-
tude of several 100 km the precise knowledge of the pointing
direction and high stability is required. For example for the
GLAS system the position will obtained from GPS, and the
attitude determination will be based on stellar cameras and INS.
2.1 Analysis of Laser Altimetry Waveform
The range between the spacecraft and the surface is determined
from the round-trip propagation time of short laser pulses. To
estimate the timing performance of the laser altimeters and to
derive other useful surface parameters, such as surface rough-
ness and albedo, the knowledge of the received waveform is
essential. Several factors, such as terrain variations within the
laser footprint, nadir angle effects, and the curvature of the laser
beam contribute to the spreading of the reflected pulse. The
detected signal is also contaminated by optical and electronic
noise.
In special cases, such as flat or uniformly sloped terrain, the
laser altimeter's detection statistics and timing performance can
expressed in closed form (Gardner, 1992). As a complementary
approach to the analytical calculations, the Goddard Laser Al-
timetry Simulator (Abshire at al., 1994) is suitable for evaluat-
a) Estimated and Actual Terrain
|. SEA ICE
^ SURFACE
TTTTTTTTTTTY
60 80 100 120 140 160
Along track distance (meters)
Receiver Waveform
0.1.0: he dei dt ctecducde À sd 1
0.081 -
2 0064 -
© J
> ]
0.04 -
0.024 3
0.00 bor e Sp
0 100 200 300 400 500
Time (100ps)
ing the laser altimeter performance over a wide range of condi-
tions.
The simulator uses a simplified 2-D measurement geometry
(height vs. along track distance). The terrain surface is assumed
to be a Lambertian reflector, and its reflectivity and height can
be specified for every centimeter of the along-track distance.
The surface parameters can be randomized using first and sec-
ond order Markov processes. The simulator computes the
waveform as it is propagated to and from the terrain surface
and, after detection through the altimeter receiver. The surface
elevation is estimated in the following fashion. First the coarse
range is calculated as the time interval between the laser firing
and the receiver's first threshold time. Then a fine range cor-
rection is computed from the waveform. Finally a correction is
applied to remove the effect of the low-pass filtering in the
receiver. An example of the computed waveforms is given in
Figure 1. The estimated surface elevation is marked with a tri-
angle in Figure 1.a.
The shape of the received waveform is closely related to the
height distribution within the laser footprint (Bufton, 1989). For
many applications, the determination of surface slope or rough-
ness is of considerable interest. In case of horizontal, random,
rough, Lambertian surfaces the surface roughness within the
laser footprint can be computed from the pulse spreading. An
algorithm based on the analytic solution described in (Gardner,
1992) is recommended for the estimation of sea ice surface
roughness in (Csatho and Thomas, 1995b).
b) Spacetime Waveform
100 er ET ee lt
80-
601 r
Photons
40 4 :
201
ODOL d
0 100
TT TT TT
200 300 400 500
Time(100ps)
d) Digitizer Waveform
1004 EE
801 -
60- -
40] -
A/D Counts
207 L
‘200 300 400
Time (100ps)
Tr
097.7300
Figure 1: Simulated laser altimetry waveforms for GLAS system computed the Goddard Laser Altimetry Simulator. Surface was
created from sea ice elevations measured by airborne laser altimetry.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
