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

a relatively slow data rate, so we could only collect spacecraft status data and send commands during TDRSSpasses. We were able to collect science data only during the ground passes. That’s the “big picture.” Following isa more detailed look at the various data collection and communication systems described above.14.1.1.1 The Solid State RecorderAn SSR is basically a bank of Random Access Memory (RAM) boards, used on-board spacecraft to collect andstore data. It is typically a stand-alone “black box,” containing multiple memory boards and controllingelectronics that provides management of data, fault tolerance, and error detection and correction. The SSRonboard the GP-B spacecraft has approximately 185 MB of memory—enough to hold about 15.33 hours ofspacecraft data. This is not enough memory to hold all of the data generated by the various monitors, so theGP-B Mission Operations staff controlled what data were collected at any given time, through commands sentto the spacecraft.Figure 14-2. A typical solid state recorder (SSR) manufactured by SEAKR EngineeringMany instruments on-board the spacecraft have their own memory banks. Data rates from these instrumentsvary—most send data every 0.1 second, but some are faster and others slower. The data from all of theseinstruments are collected by the primary data bus (communication path) and sent to the central computer,called the CCCA. The CCCA then sends the data to the SSR.The data were categorized into five subtypes:1. Sensor programmable telemetry—High data rate of 0.1 seconds, greater than 9000 monitors, mostlyused for science & engineering)—this is GP-B’s “primary” useful data, including most science data.2. Event data—For example, whether the vehicle is in eclipse (tells when we entered eclipse behind theearth and when we emerged)3. Database readouts—Used to confirm that the on-board database is the same as the ground folks think itis – use this to verify that say, filter setting commands, were received and enacted.4. Memory readout (MRO)—Used to ensure that the binary memory on-board is the same (error free)memory we think it is—this is where single & multi-bit errors occur (this is not collected data, but onlyprogrammable processes—i.e., the spacecraft’s Operating System). If we find errors in the MROs, we canre-load the memory. Solar wind (proton hits), for example, can cause errors here.5. Snapshot data—This is extremely high-speed data (1/200th of a second) from the SQUID (Super-Conducting Quantum Interference Device), Telescope and Gyro readout systems. The CCCA does someon-board data reduction, performing Fast Fourier Transforms (FFT) on some of the incoming SRE data.This on-board reduction is necessary, because we do not have room in our SSR, nor the telemetrybandwidth, to relay the high-rate data back to the MOC all the time. (Perhaps we should upgrade toDSL…) However, like all numerical analysis methodology, an FFT can become “lost” because an FFT is400 March 2007 Chapter 14 — Data Collection, Processing & Analysis