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

Figure 13-35. GPS Antennae Field of View13.4.2 Hardware Modifications for GP-BDespite the equipment’s rugged heritage, pre-launch testing revealed hardware modifications were required toincrease robustness. Non-stainless steel screws were replaced with stainless steel equivalents. Resistor andcapacitor components were replaced with space compatible versions, as were connectors. Copper heat sinkswere added to better facilitate heat transfer in orbit. Thicker aluminum housing covers replaced the originalcovers to provide increased radiation shielding and more stiffness. As is typical in space applications, allelectronic boards were conformal coated.The receivers were also retrofit with Pulse Per Second (PPS) circuitry, which outputs a 5V pulse every second,on the GPS second. The PPS signal is used to reconcile vehicle time with UTC time.The majority of modifications involved changes to the receiver’s software. The receiver’s search algorithm wasmodified for space. To find a GPS satellite, the receiver must compare the incoming signal with the matchingsatellite PRN code. A terrestrial receiver looks through a Doppler shift range of ~10 kHz. Because theenvironment of GP-B is more dynamic, the Doppler shift window was increased to ~80 kHz. This increasedsearch capability allows the receiver to do a “cold” search in space. While bootstrap acquisition is a definite plus,Ephemeris Aiding allows the receiver to acquire the constellation typically within ten minutes. After power up,or receiver reset, the receiver is initialized with Keplerian orbital elements, as well as a complete set GPSephemerides.Other modifications enhance communication with the GP-B flight computer. For example, while the flightcomputer interface operates at 10 Hz, a complete set of GPS telemetry occurs only every 10 seconds. Theintroduction of a Solution Quality Index (SQI) provides information about the receiver’s acquisition statusbetween successive sets. SQI has proved invaluable for understanding GPS telemetry, and is particularly usefulas a filter during post processing.13.4.3 Pre-Launch Testing and ResultsThe modifications necessitated strenuous qualification testing. Environmental testing focused on hardwaremodifications, and included the normal regimen of space qualification tests: EMI, thermal vacuum cycling,random vibration, etc. All tests were completed and data thoroughly reviewed prior to the equipment’sinstallation on the vehicle in September 2002. Selected highlights are given in Table 13-5. Integration testingwas successfully completed by August 2003. Prior to launch, the GPS subsystem was routinely used toaccurately time tag ground testing data.380 March 2007 Chapter 13 — Other Payload Subsystems Analyses