GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

After some initial experimentation a decision was made to solve for 9 parameters during each fit. Initially thosewere the 6 state variables (position and velocity) plus a drag coefficient and two sinusoidal in-track accelerationsat orbital period to account for mis-modeled drag and eccentricity vector component rates. Because slowlyvarying constant and sinusoidal axial thrusting components persisted throughout the science mission, GP-Borbit determination continued to solve for those 9 parameters after IOC. The unconstrained drag coefficientabsorbed the dominant in-track effects of the slow semi-major axis variations due to the sinusoidal axialthrusting term, and the in-track sinusoidal parameters modeled the eccentricity vector rate errors due to theconstant axial thrusting term.Figure 4-8. Usable GPS Points per Day (2005)Daily residual plots (Figure 4-9) are generated to assess how well the ephemeris fits the data. Standarddeviations for cross-track residuals are typically less than 5 meters, in-track residuals less than 10 meters, andradial residuals around 15 meters. It can be seen from Figure 4-6 that, for this solution, RMS values areprimarily due to data noise – the ephemeris errors are far smaller. The corresponding overlap comparisonsbetween successive daily 30-hr solutions are a better indication of actual orbit errors. Those daily orbitcomparisons typically yield in-track and radial variations of just a few meters, and cross-track variations areoften less than a meter.There are exceptions to this general pattern of operations. On several days (for example, day 66 in Figure 4-5)there were gaps in the GPS data that prevented the use of a normal 30-hour fit span. When such gaps occurred,2 or 3 days of GPS data were used to create an ephemeris segment spanning the gap. Also, on some days whenthe sinusoidal axial thrusting interacted with the orbital motion so as to generate in-track perturbations with anearly daily period, the in-track effects could not be well approximated by fitting a drag coefficient alone. Forthose ephemeris segments, it is possible to reprocess the data with shorter fit spans and fit additional parametersas needed.Gravity Probe B — Post Flight Analysis • Final Report March 2007 115