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

Figure 14-5. The GP-B building and Mission Operations Center (MOC) at Stanford UniversityHowever, data relayed though ground stations went through an intermediate step, before being sent to ourMission Operations Center. When data arrives at a ground station, 32 byte headers are put on each data packetto identify it. The identifiers include our spacecraft ID, the ground receipt time, whether or not Reed-Solomonencoding was successfully navigated, the ground station ID, and several error correction checks. The data werestored at the local station and a copy was sent to a central NASA station. After it passed transmission errorchecks at NASA, it came to us at Stanford. The average 15-hour file took between 1.5 and 4 hours to arrive here.14.1.1.4 Uncompressing and Formatting the DataOnce the data were received here at Stanford, a laborious process began. Spacecraft data, by its very nature, mustbe highly compressed so that as much data as possible can be stored. While we have over 9,000 monitors onboard,we cannot sample and store more than about 5,500 of them at a time. That’s why our telemetry is“programmable”—we could choose what data we wanted to beam down. However, in order to get as much dataas possible, we compressed it highly.The data were stored in binary format, and the format included several complexities and codes to indicate thestates of more complex monitors. For example, we might encode the following logic in the data: “if bit A=0, theninterpret bits B and C in a certain way; but if bit A=1, then use a very different filter with bits B and C.” The datawere replete with this kind of logic. In order to decompress and decode all of this logic, we used a complex map.Our software first separated the data into its five types (described above). Then, type one underwent“decommutation.” Once all of the data in a set was translated into standard text format and decommutated, thatset was stored in our vast database (over one terabyte). This was the data in its most useful, but still “raw” form;we called this “Level 1 data.” It took about an hour to process 12 hours of spacecraft data. It was the DataProcessing team’s job to monitor this process, making sure files arrived intact, and unraveling any data snarlsthat may have come from ground pass issues.Our science team took the Level 1 data sets, filtered them, and factored in ephemeris information and otherinteresting daily information (solar activity, etc). The science team also performed several important “preprocessing”steps on the data sets. Once that initial science process was complete, a data set was stored in the“Level 2” database. From there, more sophisticated analysis could be performed.14.1.2 Detecting and Correcting Computer Memory Errors in OrbitThe computer and electronics systems on-board every spacecraft must undergo special “ruggedization”preparations to ensure that they will function properly in the harsh environment of outer space. GP-B's onboardcomputers and other electronics systems are no exception. For example, the components must beradiation-hardened and housed in heavy-duty aluminum or equivalent cases. Furthermore, the firmware(built-in hardware-level programming) must include error detection and correction processes that enable theGravity Probe B — Post Flight Analysis • Final Report March 2007 403