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Gravity Probe B

Testing Einstein's Universe



On mission day #269, the spacecraft is in excellent health, with all subsystems performing well. The GP-B spacecraft is flying drag-free around gyro #3, maintaining a constant roll rate of 0.7742 rpm (77.5 seconds per revolution.) All four gyros are digitally suspended in science mode. The temperature inside the Dewar is holding steady at 1.82 kelvin. We have been collecting science data for 20 weeks, just under halfway through the science phase of the mission. The data collection process continues to proceed smoothly, and the quality of the data remains excellent.

In this week’s highlights, we describe the systems and process that we use to collect both status and scientific data from the spacecraft. In an upcoming highlight, we will describe how we use safemodes to enable the spacecraft to automatically protect itself when anomalies occur on-board, and in another upcoming highlight, we will provide a general description of our data reduction and analysis process. These highlights will help to provide an understanding of why events like proton strikes from solar radiation have not significantly affected our experimental results. The information about telemetry and data collection in this week’s highlights was provided by GP-B Data Processing Lead and Webmaster, Jennifer Spencer.


Our GP-B spacecraft autonomously collects data—in real time--from over 9,000 sensors (aka monitors). On-board the spacecraft there is memory bank, called a solid-state recorder (SSR), which has the capacity to hold about 15 hours of spacecraft data—both system status data and science data. The spacecraft does not communicate directly with the GP-B Mission Operations Center (MOC) here at Stanford. Rather, it communicates with a network of NASA telemetry satellites, called TDRSS (Tracking and Data Relay Satellite System), and with NASA ground tracking stations.

Many spacecraft share these NASA telemetry facilities, so GP-B must schedule time to communicate with them. These scheduled spacecraft communication sessions are called “passes,” and the GP-B spacecraft typically completes 6-10 TDRSS passes and 4 ground station passes each day. During communications passes, commands are relayed to the spacecraft from the MOC, and data is relayed back via the satellites, ground stations, and NASA data processing facilities. The TDRSS links have a relatively slow data rate, so we can only collect spacecraft status data and send commands during TDRSS passes. We collect science data during the ground passes. That’s the “big picture.” Following is a more detailed look at the various data collection and communication systems described above.


An SSR is basically a bank of Random Access Memory (RAM) boards, used on-board spacecraft to collect and store data. It is typically a stand-alone “black box,” containing multiple memory boards and controlling electronics that provides management of data, fault tolerance, and error detection and correction. The SSR on-board the GP-B spacecraft has approximately 185 MB of memory—enough to hold about 15.33 hours of spacecraft data. This is not enough memory to hold all of the data generated by the various monitors, so the GP-B Mission Operations staff controls what data is collected at any given time, through commands sent to the spacecraft.

Many instruments on-board the spacecraft have their own memory banks. Data rates from these instruments vary—most send data every 0.1 second, but some are faster and others slower. The data from all of these instruments is 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 itself is categorized into five subtypes: 

  1. Sensor programmable telemetry--High data rate of 0.1 seconds, greater than 9000 monitors, mostly used 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 the earth and when we emerged)
  3. Database readouts--Used to confirm that the on-board database is the same as the ground folks think it is – 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 & multibit errors occur (this is not collected data, but only programmable processes—i.e., the spacecraft’s Operating System). If we find errors in the MROs, we can re-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 some on-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 telemetry bandwidth, to relay the high-rate data back to the MOC all the time. (Perhaps we should upgrade to DSL…) However, like all numerical analysis methodology, an FFT can become “lost” because an FFT is not always performed from the same starting abscissa (x-axis) value. The Snapshot allows us to see some of the original data sets being used for the FFTs and confirm that they are not lost--or if they are, we can fix them by making a programmed adjustment on-board. Other systems--the Telescope Readout (TRE) and Gyro Suspension System (GSS)--use their snapshots for similar instrumentation and data reduction validity checks.

Data is stored to the SSR in a “First in, First out” queue. Once the memory is 15.33 hours full, the new data begins to overwrite the oldest data collected. That’s why it’s a good idea to dump the SSR to the ground at least every 12 –15 hours (for safety!)…which is exactly what we do. 


So, how do we talk to the SV and get our data? NASA communicates with its spacecraft in several ways. Its highest availability is through TDRSS, two communications satellites in orbit just waiting for communications. These two satellites can’t handle much data at a time, and we only transmit to them at a 1K or 2K (kilobit per second) rate. For GP-B, this rate is only enough to exchange status information and commands, not for science.

NASA also uses ground-based stations. There are several around the world, but each ground station network used is determined by satellite type and orbit. Because GP-B is in a polar orbit communicating primarily at 32K (32kilobits per second), we use the NASA Goddard “Ground Network”. This network includes stations in Poker Flats, Alaska; Wallops, Virginia; Svalbard, Norway; and McMurdo station, Antarctica. We haven’t used the South Pole station yet, but someday we may need to.


Usually, we schedule a ground pass at one of the four ground stations every six hours or so. We talk to TDRSS about six times per day. Communications must be scheduled and arranged, and like all international calls, these communications passes are not cheap! Interspatial (satellite-to-satellite) calls are most expensive, and ground-to-space calls are slightly less costly. We could talk to TDRSS and the ground stations more often, but it costs money, so we follow our regular schedule unless there’s an emergency. During safemodes or other anomalies, we schedule extra TDRSS and ground passes as needed. We are not in contact with the spacecraft at all times.

While streaming 32K SSR data to the ground, we cannot record to the SSR. Our data collected by the sensors during the transmission is therefore enfolded into the transmission, bypassing the SSR. We have to change antennas (switching from forward to aft antenna) midway through each ground pass. During this antenna change, we lose about 30 seconds of the real-time data because it’s being “beamed into space”. Our data capture on the mission thus far is 99.01%, and our spec from NASA was 90%, so we are doing just fine in terms of data capture, despite the antenna switches. It takes about 12 minutes to collect the entire content of the SSR.


When the spacecraft data goes through TDRSS, it is transmitted directly to the Stanford University GP-B MOC through our data link with NASA, in real time. We record it on our computers in a format similar to the ground station data, and then send it through our data processing center.

However, data relayed though ground stations goes through an intermediate step, before it is sent to our Mission Operations Center. When data arrives at a ground station, 32 byte headers are put on each data packet to identify it. The identifiers include our spacecraft ID, the ground receipt time, whether or not Reed-Solomon encoding was successfully navigated, the ground station ID, and several error correction checks. The data is stored at the local station and a copy is sent to a central NASA station. After it passes transmission error checks at NASA, it comes to us at Stanford University. The average 15-hour file takes between 1.5 and 4 hours to arrive here. (Click here to download a PDF document about Reed-Solomon encoding.)


Once the data is here at Stanford, a laborious process begins. Spacecraft data, by its very nature, must be highly compressed so that as much data as possible can be stored. While we have over 9,000 monitors on-board, we cannot sample and store more than about 5,500 of them at a time. That’s why our telemetry is “programmable”--we can choose what data we want to beam down. However, in order to get as much data as possible, we compress it highly. 

The data is stored in binary format, and the format includes several complexities and codes to indicate the states of more complex monitors. For example, we might encode the following logic in the data: “if bit A=0, then interpret 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 data is replete with this kind of logic. In order to decompress and decode all of this logic, we use a complex map. Our software first separates the data into its five types (described above). Then, type one undergoes “decommutation.” Once all data is translated into standard text format and decommutated, it is stored in our vast database (over one terabyte). This is the data in its most useful, but still “raw” form; we call this “Level 1 data.” It takes about an hour to process 12 hours of spacecraft data. It is the Data Processing team’s job to monitor this process, making sure files arrive intact, and unraveling any data snarls that may come from ground pass issues. 

Our science team takes the Level 1 data, filters it, factors in ephemeris information, and other interesting daily information (solar activity, etc). The science team also performs several important “pre-processing” steps on the data that will be described in a future highlight. Once that initial science process is complete, that data is stored in the “Level 2” database. From there, more sophisticated analysis can be performed. 


Just before the holidays, Swiss amateur astronomer and physics teacher, Stefano Sposetti sent us two new photos of the GP-B spacecraft in orbit, one of which is shown to the right. In this photo, taken on 6 December 2004 (during the spacecraft’s full-sun season), Mr. Sposetti used a 20mm f2.8 photo lens, coupled with a CCD camera on a fixed tripod, with a 60-second exposure to capture a beautiful time exposure of the GP-B spacecraft, rising over a rooftop. You can view other astronomical photos taken by Mr. Sposetti, including photos of the GP-B spacecraft on Mr. Sposetti’s Web page in the Astronomical Image Data Archive (AIDA). As always, we are most grateful to Mr. Sposetti for sending us this extraordinary photo.



If you're going to be in Los Angeles anytime before 30 May 2005, and if you’re interested in Einstein’s life and work, the Einstein Exhibition at the Skirball Cultural Center (just north of the Getty Museum on Interstate 405) is the most comprehensive presentation ever mounted on the life and theories of Albert Einstein (1879-1955). It explores his legacy not only as a scientific genius who re-configured our concepts of space and time, but also as a complex man engaged in the social and political issues of his era. It examines the phenomenon of his fame and his enduring status as a global icon whose likeness has become virtually synonymous with genius.

In this exhibit, you can examine Einstein's report card, inspect his FBI file, and enjoy his family photographs, love letters, and diary entries. Exhibition highlights include scientific manuscripts and original correspondence—including original handwritten pages from the 1912 manuscripts of the special theory of relativity and his 1939 letter to President Roosevelt about nuclear power—and a wealth of other documents from the Albert Einstein Archives at the Hebrew University of Jerusalem.

In addition to these displays of Einstein memorabilia, the exhibit also features a number of interactive components that help provide an understanding of Einstein's revolutionary theories. Furthermore, several “explainers,” identified by their red aprons, are on hand to discuss various aspects of the exhibit and to explain and demonstrate difficult concepts, such as time dilation and warped spacetime. At the end of the exhibit, you’ll find one of GP-B’s gyro rotors on display.

The Einstein exhibition was jointly organized by the American Museum of Natural History (AMNH), the Hebrew University of Jerusalem, and the Skirball Cultural Center. It was designed by the AMNH under the supervision of Dr. Michael Shara, curator of the exhibit and chairman of the museum’s Astrophysics Department. It opened in November 2002 at the AMNH in New York and then traveled to Chicago and Boston, spending about 8 months in each location. It will remain at its final U.S. stop at the Skirball Center in Los Angeles through 29 May 2005, after which time it will move permanently to the Hebrew University in Jerusalem.

Information about the Einstein exhibition is available on the Skirball Center Web site. If you can’t make it to Los Angeles, you can visit the AMNH’s virtual Einstein exhibit on the Web.

Photos and Drawing: GP-B Public Affiars Coordinator, Bob Kahn, created the composite photo of the GP-B spacecraft in orbit and the drawing of the spacecraft in Earth's gravity well using a scale model of the spacecraft, a NASA photo of the Earth, and Adobe Photoshop, The photo of a solid state recorder was taken from the Web site of SEAKR Engineering Inc.The photo of the TDRSS satellite and the photos of the ground tracking stations at Svalbard, Wallops, and Poker Flats are courtesty of NASA. The photos from the Einstein Exhibit are courtesy of the Skirball Cultural Center. Click on the thumbnails to view these images at full size.

Please Note: Until further notice, we intend to continue posting these GP-B highlights on a weekly basis. Also, from time to time, we may post special reports and special updates, as warranted by mission events.

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