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

solutions. Accurate orbit solutions was needed both to determine the extent of orbit trim required during theinitialization phase of the mission, and for precise vehicle position and velocity knowledge during the sciencephase of the mission.13.4.1 GPS HardwareThe GPS equipment is divided into two fully redundant sets. Each set is composed of a Trimble TANS VectorIII GPS receiver and four matching Trimble antennas. Originally built for battlefield GPS attitude applications,a modified TANS Vector III flew successfully on STS-77 in 1996, as part of the GPS Attitude and NavigationExperiment (GANE).The Trimble TANS Vector III is a six channel L1 receiver which calculates position, velocity, and time (PVT)every 1.7 seconds. The receivers are mounted on the spacecraft superstructure. Each receiver has inputs for fourantennae, two of which are mounted each fore and aft, as shown in Figure 13-34, and are placed as to maximizeantenna baseline and optimize sky view, as delineated in Table 13-4. The forward antennae’s field of view areobstructed slightly by the telescope sun shade, as depicted in Figure 13-35, but otherwise the antennae enjoy afull view of the sky.Table 13-4. Installed Position of GPS AntennaeAzimuth angleElevation angleAntenna 1 90 o 45 oAntenna 2 270 o 45 oAntenna 3 0 o -45 oAntenna 4 180 o -45 oFigure 13-34. Location of GPS equipment on GP-BGravity Probe B — Post Flight Analysis • Final Report March 2007 379