gravimeter, magnetometer, a laser altimeter and an ice-
penetrating radar. Positional information is provided by
differential Global Positioning System (GPS), supplemented by
Inertial Navigation System (INS), and precision pressure
altimetry data. During most of the surveys performed by SOAR
all sensors are collecting data allowing to map the ice sheet
surface, the internal layering of the ice and the bedrock
geometry and composition simultaneously. However, only the
laser system, the ice-penetrating radar and auxiliary equipments
were used during our laser altimetry missions. The altimetric
measurements are made while the aircraft flies over a pre-
determined flight path guided by real-time GPS.
3.1 Base Station Positioning
Two Ashtech dual frequency receivers located at different base
camps are used as static base stations. The origin of each flight
determines which base station is used. One of these base
stations is located on a very slow moving (0.34 m/a) portion of
the ice-sheet known as Siple Dome (Fig. 1). A survey of this
nearly static base station was performed on the first and last day
of flying. Positions with an overall RMS of 0.02 m were
acquired using the Automated GIPSY system developed by Jet
Propulsion Laboratory. Ice motions at this site are small and
uncomplicated, so time-linear interpolation is used to determine
the position of the base station for ihe intermediate days.
Another base station is located at the Down-B camp located
near the middle of Ice Stream B (Fig. 1). The ice there moves at
a rate of over 500 m/a. The position of the Down-B receiver
relative to the Siple Dome receiver is computed using
GPSurvey processing software (Trimble Inc.). A position is
determined for each flight originating at Down-B. The
calculated horizontal position of this station changes in a linear
fashion as expected (Fig. 2a). The calculated vertical position
of the station changes in an irregular manner (Fig. 2b). This
magnitude of elevation change is not characteristic of an ice
sheet over such short distances and is most likely an error in the
GPS positioning due to the long baseline between receivers
(280 km) and the horizontal movement of the Down-B receiver
during each survey. The elevation of this base station is
estimated to be the mean from five out of six surveys with an
RMS of 0.065 m. The calculated position of the sixth survey is
considered an outlier because it deviates from the mean by more
than four times the reported RMS and is therefore not used (Fig.
23.
3.2 Aircraft Positioning
Once the base station position is established, the relative
position of the Ashtech or TurboRogue receiver on the aircraft
is calculated using GPSurvey. Shi and Cannon (1995) showed
that the accuracy of GPS positioning on a moving aircraft can
be at the 0.10 m level if tropospheric, ionospheric, precise
satellite orbits (ephemerides), and multipath corrections are
used during differential carrier phase post-processing (Shi and
Cannon, 1995). GPSurvey software is used for processing and
found to be reliable when used on a computer with a large
amount of memory and a fast processor. For 11 out of twelve
flights the maximum RMS reported by GPSurvey is 0.10 m.
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
Y (km)
170
The 12™ flight was not used, because phase ambiguities were
not resolved for a large portion of the flight.
To assess the performance of the GPSurvey processing, one
survey was processed with GUITAR (GPS Inferred Trajectories
for Aircrafts and Rockets, courtesy of John Sonntag, EG & G).
The two solutions agree well, having a maximum difference of
1 cm in the horizontal and 8 cm in the vertical.
-575.975
-575.977 ——
-575.979 — u
outlier
-575.981 |
-282.356 -282.352 -282.348
X (km)
106.2 outlier
à (b)
— 1506.
z 4
5 / 17a
+ 105.8 se / 17b
3 |... 44a 14b P1
uu Te
105.4 — |
0 4 8
Distance (m)
Fig. 2. (a) Measured changes in horizontal position of the Down-B
receiver. The points are labeled according to the day of the year that
the survey took place. Two surveys, a and b, were conducted on days 14
and 17. Black line is a linear best fit through the data calculated
without the point marked “outlier”. The X and Y coordinates are polar
stereographic. (b) Elevation of each Down-B position plotted versus
horizontal movement. Black line is the mean, elevation calculated
without the point marked “outlier”. Error bars are based on a 1-sigma
RMS about the mean elevation.
3.3 Laser Ranging
The Azimuth LRY 500 is a diode pumped Nd:YAG pulsed
laser transceiver, operating in the near infrared domain (1064
nm). Pulsed lasers measure the travel time of a laser pulse from
the laser firing point to the surface and back to the receiver. To
measure the time between the transmitted and the received
pulses the Azimuth LRY 500 rangefinder uses 50% constant
fraction discrimination. The timer starts at some consistent
point on each transmitted pulse. Each timing event ends when
the return pulse strength reaches half of its maximum
amplitude. Thus the need of “range walk” correction is
Internatior
eliminated. The ma
of 0.1m for distance:
Hz and 64 pulses ar
divergence was set fc
meter diameter fron
results in one averagi
the aircraft flies at 30
3.4 Aircraft attitude
The attitude of the a
angles determine the
angles are measured
Litton Aero Products
accuracy of 0.05? in
which translates to le
elevations when the
terrain clearance and
15°. These angles ar
the measured attitude
budget.
3.5 System timing
Each data collecting
synchrony with the ot
is used as the standa
The laser and INS m
that is corrected to |
GPS time code gener
for the: individual ati
records one angle at
these time lags are as
pitch and 50 ms in rc
It is possible to chec
pitch and roll maneu
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nadir angles are larg
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been recorded. Interp
are recorded at 2 Hz.
every at 500/64 Hz, ar
4. COMPUTA’
The laser range record
surface. To compute
global, geographic co
position, and aircraft
scheme described in
(1996), and Ridgway
procedure, which
transformations startin
the laser firing point (
WGS-84 Cartesian re
