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The Geostationary Operational Environmental Satellite (GOES) 13 is the first in a new series of weather satellites. Compared to previous GOES satellites, GOES-13 has improved Image Navigation and Registration (INR) accuracy and can continue imaging during eclipse periods. Post launch testing showed that GOES-13 met all INR requirements, but systematic patterns appeared in differences between actual landmark and range measurements and 24-hour measurement predictions based on estimated orbit and attitude parameters. Presence of these residual patterns implied that the models used in Orbit and Attitude Determination (OAD) did not accurately match reality. Thorough investigations were conducted to determine the modeling error source. It was concluded that an angular bias between star and landmark observations was the most likely modeling error, although small uncompensated range biases could also be present. Further analysis of on-orbit and test data eliminated numerous potential instrument, spacecraft and ground processing error sources. Although the cause of the landmark-star bias is still unknown, the bias causes no operational problems because OAD solution of a range bias compensates for the error. Even without range bias adjustment, GOES-13 met all INR requirements with margin.

sThe Spaceborne Laser Ranging System is a proposed short pulse laser on board an orbiting spacecraft (1, 2, 3, 4). It measures "he distances between the spacecraft and many laser retroreflectors (targets) deployed on the earth's surface. The precision of these range measurements is assumed to be about ±2 cm (5). These measurements are then used together with the orbital dynamics of the spacecraft, to derive the intersite vector betwt;,-n the laser ground targets. The errors associated with this vector are on the order of 1 to 2 cm. The baseline distances to be determined range from 25 km to 1200 km. By repeating the measurements of the intersite vector, strain and strain rate errors are estimated. These quantities are essential for crustal dynamic studies which include determination and monitoring of strain near seismic zones, land subsidence, and edifice building preceding volcanic eruptions. The realizable precision for intersite distance determination is estimated to be on the order of 0.5 cm at 300 km and about 1.5 cm at 1200 km. The corresponding inaccuracies for the intersite distances are larger, that is 1 cm and 3.5 cm respectively. The corresponding precision in the vertical direction is 1 cm and 3 cm. The accuracies in the vertical direction which can be achieved are 3 cm and 10 cm. These values were obtained for a six day observing period with 507o cloud cover.

The Geostationary Operational Environmental Satellite (GOES) 13 is the first in a new series of weather satellites. Compared to previous GOES satellites, GOES-13 has improved Image Navigation and Registration (INR) accuracy, can continue imaging during eclipse periods, periodically yaw-flips to provide better instrument cooling, and uses daily momentum dumping maneuvers to offset solar torque generated by the single solar array. Post launch testing showed that GOES-13 met all INR requirements, but post-maneuver orbit predictions, particularly longitude drift, were not nearly as accurate as desired. Ground software computes the maneuver velocity change ( v) for the predictions using firing data (telemetry if available, or predicted firing if not) and a propulsion model for the twelve 9.25 N bi-propellant thrusters. Investigations verified that the propulsion model met flowdown requirements on maneuver v accuracy, but several problems were uncovered. In particular, the effect of plume impingement on the solar array was not modeled, the propulsion model was not accurate for pulsed firing with short off-times, and prelaunch calibrations of jet thrust and direction were slightly in error. A plume impingement model was developed using detailed models of plume momentum flux and plate reflection. Corrections to calibrations of jet thrust and direction were computed using a thruster calibration algorithm that fits maneuver v estimates (obtained from orbit determination), angular momentum change ( h) telemetry data, and prior information. Thruster mass and energy flow was analyzed to understand factors affecting pulsed firing. Problems in modeling short off-time pulsed firing were eventually traced to the effects of liquid propellant remaining between the jet valves and thrust chamber after valve closure. A new propulsion model was developed and validated using data for hundreds of test firings. With these changes, the ground system can accurately predict GOES-13 maneuver performance for all maneuver types.