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

Figure 4-6. Eta and Eta-Average Histories (Degrees) Associated with GP-B4.4.3 Post-Launch Orbit EventsOn April 20, 2004, the Delta II 7920-10 launch vehicle injected the Gravity Probe-B satellite into an excellentorbit. The altitude, eccentricity, argument of perigee and ascending node were well within specifications and,most importantly, the inclination was almost perfect—within 5x10 -5 degrees of the target (a cross track error ofabout 6 meters!) That was well within the ±0.00020 degree inclination trimming goal, so it was decided that notrimming would be used during IOC.However, the target values for inclination had been based on a 2 month IOC span, so as the need to extend theIOC period beyond 2 months arose, estimates of η-average errors for the current GP-B orbit for longer IOCspans were required. Figure 4-7 illustrates GP-B’s computed eta-average histories for 60-, 90- 120- and 150-dayIOC periods as a function of science mission duration. The red lines delineate the ±0.025 deg limits onη-average imposed by science. It can be seen there that even with the actual 141-day IOC duration (forgyroscopes 1-3) and the 158-day IOC duration (for gyroscope 4), the GP-B orbit would satisfy the eta-averagerequirement for any science mission lifetimes lasting longer than two months. For the actual expected sciencemission lifetimes of 10 or 11 months the eta-average errors will be less than 0.020 degrees, well within the etaaveragerequirement.Gravity Probe B — Post Flight Analysis • Final Report March 2007 113