3.5 Pioneer telemetry data

Telemetry received from the Pioneer 10 and 11 spacecraft was supplied to the Pioneer project by the DSN in the form of Master Data Records (MDRs). MDRs contained all information sent by the spacecraft to the ground, and some information about the DSN station that received the data. The information in the MDRs that was sent by the spacecraft included engineering telemetry as well as scientific observations.

The Pioneer project used engineering data extracted from the MDRs to monitor and control the spacecraft, while scientific data, also extracted from the MDRs, was converted into formats specific to each experiment and supplied to the experimenter groups.

Far beyond the original expectations17, telemetry is now seen to be of value for the investigation of the Pioneer anomaly, as the MDRs, specifically the telemetry data contained therein, are helpful in the construction of an accurate model of the spacecraft during their decades long journey, including a precise thermal profile, the time history of propulsion system activation and usage, and many other potential sources of on-board disturbances. After recent recovery efforts [397Jump To The Next Citation Point], this data is available for investigation.

3.5.1 MDR data integrity and completeness

The total amount of data stored in these MDR files is approximately 40 GB [397Jump To The Next Citation Point]. According to the original log sheets that record the transcription from tape to magneto-optical media, only a few days worth of data is missing, some due to magnetic tape damage. One notable exception is the Jupiter encounter period of Pioneer 10. According to the transcription log sheets, DOY 332–341 from 1973 were not available at the time when the magnetic tapes on which the MDRs were originally stored were transcribed to more durable magneto-optical media.

Other significant periods of missing data are listed in Table 3.5. It is not known why these records are not present, except that we know that very few days are missing due to unreadable media (i.e., the cause is missing, not damaged, tapes.)



Table 3.5: Pioneer 10 and 11 missing MDRs (periods of missing data shorter than 1 – 2 days not shown.)
Spacecraft Year Days of the Year (DOY)
Pioneer-10 1972 133–149
  1973 004–008, 060–067, 332–341
  1974 034–054
  1979 025–032, 125–128, 137–157, 171–200
  1980 173–182, 187–199, 248–257
  1983 329–348
  1984 346–359
Pioneer-11 1973 056–064, 067–080, 082–086, 088–094
  1980 309–330, 337–365
  1982 318–365
  1983 001–050
  1984 343–357
  1990 081–096

So, the record is fairly complete. But how good is the data? Over forty billion bytes were received by the DSN, processed, copied to tape, copied from tape to magneto-optical disks, then again copied over a network connection to a personal computer. It is not inconceivable that the occasional byte was corrupted by a transmission or storage error. There are records that contain what is apparently bogus data, especially from the later years of operation. This, plus the fact that the record structure (e.g., headers, synchronization sequences) is intact suggests reception errors as the spacecraft’s signal got weaker due to increasing distance, and not copying and/or storage errors.

The MDRs contain no error detection or error correction code, so it is not possible to estimate the error rate. However, it is likely to be reasonably low, since the equipment used for storage and copying is generally considered very reliable. Furthermore, any errors would likely show up as random noise, and not as a systemic bias. In this regard, the data should generally be viewed to be of good quality insofar as the goal of constructing an engineering profile of the spacecraft is considered.

3.5.2 Interpreting the data

MDRs are a useless collection of bits unless information is available about their structure and content. Fortunately, this is the case in the case of the Pioneer 10 and Pioneer 11 MDRs.

The structure of an MDR is shown in Appendix C (see also [402Jump To The Next Citation Point]). The frame at the beginning and at the end of each 1344-bit record contained information about the DSN station that received the data, and included a timestamp, data quality and error indicators, and the strength of the received signal. The middle of the record was occupied by as many as four consecutive data frames received by the spacecraft.

The MDR header is followed by four data frames (not all four frames may be used, but they are all present) of 192 bits each. Lastly, an additional 8 words of DSN information completes the record. The total length of an MDR is thus 42 words of 32 bits each.

The 192-bit data frames are usually interpreted as 64 3-bit words or, alternatively, as 32 6-bit words. The Pioneer project used many different data frame formats during the course of the mission. Some formats were dedicated to engineering telemetry (accelerated formats). Other formats are science data formats, but still contain engineering telemetry in the form of a subcommutator: a different engineering telemetry value is transmitted in each frame, and eventually, all telemetry values are cycled through.

The Pioneer spacecraft had a total of 128 6-bit words reserved for engineering telemetry. Almost all these values are, in fact, used. A complete specification of the engineering telemetry values can be found in Section 3.5 (“Data Handling Subsystem”) of [292Jump To The Next Citation Point]. When engineering telemetry was accelerated to the main frame rate, four different record formats (C-1 through C-4) were used to transmit telemetry information. When the science data formats were in use, an area of the record was reserved for a subcommutator identifier and value.

The formats are further complicated by the fact that some engineering telemetry values appear only in subcommutators, whereas others only appear at the accelerated (main frame) rate.

In the various documentation packages, engineering data words are identified either by mnemonic, by the letter ‘C’ followed by a three-digit number that runs from 1 through 128, or most commonly, by the letter ‘C’ followed by a digit indicating which ‘C’ record (C-1 through C-4) the value appears in, and a two-digit number between 1 and 32: for instance, C-201 means the first engineering word in the C-2 record.

3.5.3 Available telemetry information



Table 3.6: Available parameter set that may be useful for the Pioneer anomaly investigation.



Parameters Subsystem Telemetry words






TEMPERATURES



RTG fin root temps Thermal C 201, C202, C203, C204
RTG hot junction temps Thermal C220, C219, C218, C217
TWT temperatures Communications C205, C206, C207, C228, C223, C221
Receiver temperatures Communications C222, C227
Platform temperatures Thermal C301, C302, C304, C318, C319, C320
PSA temperatures Thermal C225, C226
Thruster cluster temps Propulsion C309, C326, C310, C311, C312, C328, C325
SRA/SSA temperatures ACS C303, C317
Battery temperature Power C115
Propellant temperature Propulsion C327
N2 tank temperature Propulsion C130
Science instr temps Science E101, E102, E109, E110, E117, E118, E125, E128,
    E201, E209, E213, E221



VOLTAGES



Calibration voltages Data handling C 101, C102, C103
RTG voltages Power C110, C125, C131, C113
Battery/Bus voltages Power C106, C107, C117, C118, C119
TWT voltages Communications C224, C230
Science instr voltages Science E119, E129, E210, E211, E217, E220



CURRENTS



RTG currents Power C 127, C105, C114, C123
Battery/Bus currents Power C109, C126, C129
Shunt current Power C122, C209
TWT currents Communications C208, C211, C215, C216
Science instr currents Science E111, E112, E113



PRESSURE



Propellant pressure Propulsion C 210



OTHER ANALOG



TWT power readings Communications C231, C214
Receiver readings Communications C111, C212, C232, C121, C229, C213



BINARY/BIT FIELDS



Conscan Communications C313, C314, C315, C316
Stored commands Electrical C305, C306, C307
Thruster pulse counts Propulsion C329, C321, C322, C330
Status bits Data handling C104
  Power C128
  Electrical C120, C132, C324, C332
  Communications C308
Power switches Electrical C108, C124
Roll attitude Data handling C112, C116
Precession ACS C403, C411, C412, C415, C416, C422, C423, C424,
    C425, C428, C429, C430
Spin/roll Data handling C405, C406, C407, C408, C417
Delta v ACS C413, C414, C426
ACS status Propulsion C409
  ACS C410, C427, C431, C432
Star sensor ACS C404, C419, C420, C421



SCIENCE INSTRUMENTS



Status/housekeeping Science E108, E124, E130, E202, E224, E131, E132, E208
JPL/HVM readings Science E103, E104, E105, E106, E107, E203, E204, E205
UC/CPI readings Science E114, E115, E116, E206, E212, E214, E215, E216
GE/AMD readings Science E122, E123, E222, E223
GSFC/CRT readings Science E126, E127
LaRC/MD readings Science E207




Pioneer 10 and 11 telemetry data is very relevant for a study of on-board systematics. The initial studies of the Pioneer anomaly [24Jump To The Next Citation Point27Jump To The Next Citation Point194390Jump To The Next Citation Point] and several subsequent papers [28391Jump To The Next Citation Point392Jump To The Next Citation Point393Jump To The Next Citation Point] had emphasized the need for a very detailed investigation of the on-board systematics. Other researchers also focused their work on the study of several on-board generated mechanisms that could contribute to an anomalous acceleration of the spacecraft [164245327]. Most of these investigations of on-board systematics were not very precise. This was due to a set of several reasons, one of them is insufficient amount of actual telemetry data from the vehicles. In 2005, this picture changed dramatically when this critical information became available.

Table 3.6 summarizes all available telemetry values in the C (engineering) and E (science) telemetry formats. The MDRs also contain a complete set of science readings that were telemetered in the A, B, and D formats.

As this table demonstrates, telemetry readings can be broadly categorized as temperature, voltage, current readings; other analog readings; various binary counters, values, and bit fields; and readings from science instruments. Temperature and electrical readings are of the greatest use, as they help to establish a detailed thermal profile of the spacecrafts’ major components. Some binary readings are useful; for instance, thruster pulse count readings help to understand maneuvers and their impact on the spacecrafts’ trajectories. It is important to note, however, that some readings may not be available and others may not be trusted. For example, thruster pulse count readings are only telemetered when the spacecraft is commanded to send readings in accelerated engineering formats; since these formats were rarely used late in the mission, we may not have pulse count readings for many maneuvers. Regarding reliability, we know from mission status reports about the failure of the sun and star sensors; these failures invalidate many readings from that point onward. Thus it is important to view telemetry readings in context before utilizing them as source data for our investigation.

On-board telemetry not only gives a detailed picture of the spacecraft and its subsystems, but this picture is redundant: electrical, thermal, logic state and other readings provide means to examine the same event from a multitude of perspectives.

In addition to telemetry, there exists an entire archive of the Pioneer Project documents for the period from 1966 to 2003. This archive contains all Pioneer 10 and 11 project documents discussing the spacecraft and mission design, fabrication of various components, results of various tests performed during fabrication, assembly, pre-launch, as well as calibrations performed on the vehicles; and also administrative documents including quarterly reports, memoranda, etc. Information on most of the maneuver records, spin rate data, significant events of the craft, etc. is also available.


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