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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