An essential component of properly modelling the nonconservative forces with a macromodel is to define the spacecraft attitude at all times. Since the solar arrays account for such a large surface area (16 m2) on MGS, modelling their orientation as a function of time is also essential.
In February 1999, after entry into GCO, the spacecraft was maintained in Array Normal Spin (ANS). In this attitude, the spacecraft +X axis (containing the stowed High Gain Antenna) was pointed at the Earth, and the spacecraft rotated once every 100 minutes about this axis. The spacecraft solar arrays alternated between two fixed orientations, as depicted by the solar array quaternions in Figure 3 for the Y+ solar array.
During fixed HGA mapping in March 1999, the attitude regimes were more complicated. The spacecraft alternated between the collection of science data 18 to 20 hours per day (+Z axis pointed nadir), and collection of tracking data four to six hours per day (+X axis pointed at the Earth). During nadir pointing (mapping mode) the solar arrays articulated continuously, whereas during the Earth point regimes, the orientation of the solar arrays was held fixed. After deployment of the High Gain Antenna, the spacecraft remained nearly exclusively in nadir pointing (mapping) mode, with the exception of the period of the HGA anomaly (April 16 to April 28), and the second period of fixed High Gain Antenna mapping (April 28 to May 5, 1999).
Gaps in the telemetered spacecraft and solar array quaternions, caused some difficulties in the modelling of the spacecraft attitude. Our data processing with GEODYN required a complete time history of the quaternions over each orbital arc. The quaternions had to be preprocessed and prefiltered to fill in the gaps. Gaps of a few minutes could be easily filled with linear interpolation, providing sufficient accuracy for the macromodel and even the measurement model calculations. However, gaps of several revolutions had to be filled in by manufacturing the appropriate spacecraft or solar array quaternions using some knowledge of the spacecraft attitude control laws. During fixed HGA mapping in March 1999, and again from April 28 to May 5 1999, the time of transition between the Earth point and nadir pointing regimes was not always obvious because of the gaps in the spacecraft quaternions. These transition times had to be inferred from other sources, such as the start and stop times of actual tracking, the start and stop times of altimeter data collection, and information in the mission sequence of events (SOE) files.
