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International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
the time line that when easily identified were accounted for and
fixed. Duplicate time tags that resulted from a buffering
problem during data recording were also identified, and
eliminated before geolocation. For several observations (5, 6,
14 and 14a) time-tag inconsistencies have not been resolved,
and the data have therefore not been geolocated.
2.2. Shuttle Orbit Determination
In support of SLA-02, meter level Root-Mean Square (RMS)
Shuttle radial orbit accuracy has been achieved from Tracking
and Data Relay Satellite System (TDRSS) Doppler
observations. — Traditionally, the Tracking and Data Relay
Satellite (TDRS) orbits themselves have been the dominant
source of error in Shuttle orbit determination during quiescent
attitude periods. The technique utilizing TOPEX/Poseidon's
(T/P) precise orbit knowledge, plus the TDRSS-T/P Doppler
tracking in conjunction with Biliteration Ranging Transponder
System (BRTS) and Telemetry, Tracking and Command
(TT&C) range data were used to precisely position the TDRS
(Luthcke et al., 1997). Furthermore, a special T/P-TDRSS
tracking scenario was devised and implemented in support of
the STS-85 mission. This tracking scenario, optimizing the
sampling of the TDRS orbits with the best possible tracking
data, was not employed for STS-72. The significant
improvement in TDRS-4 orbit precision gained from this
tracking scenario can be seen in Table 1, when compared to the
TDRS orbit precisions obtained in support of STS-72. The
TDRS-1 orbit precision is significantly worse than the other
TDRS due to that fact that T/P was not tracked by this TDRS.
However, nearly all of the STS-85 tracking data was acquired
with TDRS-4 and -5.
Mission TDRS-1 TDRS-4 TDRS-5
Supporting (m) (m) (m)
STS-72 4.08 0.82
STS-85 3.57 0.80 0.92
Table 1. TDRS RMS Orbit Overlap Differences; Total
Position
Table 2 presents a comparison of model fits to Shuttle- TDRS 2-
way range rate data expressed as residual RMS averaged over
all orbit arcs during SLA operation. The data shows an
improved fit for the STS-85 case. This was mainly due to more
relaxed constraints employed for STS-85 and significantly
shorter arcs on average. However, it should be noted that the
improved fitting of the tracking data does not necessarily
indicate improved orbit accuracy.
Shuttle- TDRS
2-way range-rate
Residual RMS (mm/s)
Mission Supporting
STS-72 2.37
| STS-85 1.41
Table2. Residual RMS (average over all arcs).
In support of SLA-01, an extensive STS-72 orbit precision and
accuracy study was performed (Rowlands et al., 1997). "This
study showed the shuttle orbits to be accurate to within 1.5 m
radial RMS and 8 m total position RMS. From the STS-72
study results, the TDRS orbits precision and shuttle tracking
data presented above, and some limited orbit accuracy analysis,
the STS-85 orbits are considered to be accurate within 10 m
total position RMS and a few meters radial RMS. Ocean
comparisons for the first 4 observation periods showed —2
meter radial orbit accuracy for the well-fit middle of the arcs.
The STS-85 orbit accuracies are considered not to be as good as
those that were obtained for STS-72 due to shorter arc lengths
and significantly more attitude and orbit maneuvers.
2.3. — Altimetry Geolocation
Once precise Shuttle orbits are obtained, SLA range data
(corrected for a constant range bias and tropospheric effects)
are combined with Shuttle attitude data to solve for the laser
bounce point location using GEODYN (Rowlands et al., 1993).
GEODYN is a state-of-the-art precision orbit determination and
geodetic parameter estimation software suite developed at
Goddard Space Flight Center. This software suite has been
extensively modified to include a rigorous laser altimeter range
measurement model and new dynamic cross-over analysis
algorithms. The laser bounce point is geolocated using using
T/P consistent reference frames, precise shuttle orbits described
above, a SLA optical center to Shuttle center-of-gravity offset
correction, a -5.6 meters altimeter range bias, and the Marini
Murray tropospheric refraction correction. The range used in
the geolocation process is the range to the first backscatter
signal above the detection threshold. The resulting elevations
thus correspond to the highest detected surface within the 100
meters diameter laser footprint. For cloud-free paths to land
targets this could be the upper-most canopy where vegetation is
present, the tops of buildings or structures, or the highest
ground where vegetation, buildings and structures are absent.
2.4. Extracting Pointing Biases
With the excellent shuttle orbit accuracies achieved from the
above described precision orbit determination (POD) analysis,
the remaining significant factor driving vertical and horizontal
geolocation accuracy is the laser pointing knowledge,
significantly affected by laser and spacecraft systematic body
misalignments. These can be due to mounting offsets, Inertial
Measurement Unit (IMU) misalignment, and Shuttle body
flexure. An attempt was therefore made to extract pointing
biases from the data. The shuttle orientation is maintained
during SLA observations by a ‘dead band’ attitude control
system, resulting in SLA pointing controlled to be within either
1 degree or 0.1 degree of nadir. Errors in the a priori Shuttle
body attitude, established by an IMU periodically calibrated in-
flight by star-camera observations, contribute to the resulting
SLA elevation errors, which are significantly larger during 1
degree dead-band modes than during 0.1 degree modes.
However, it is considerably easier to both observe and separate
the roll and pitch errors during 1 degree dead-band than during
0.1 degree dead-band, even though the increase in attitude hold
thrusting required impacts the orbit determination process by