Galileo Spacecraft Pages
New EPD Format and Implementation
Source: S. Jaskulek, May 11, 1993
Implications of LGA Mission for EPD
In its normal mode, EPD uses 912 bps to transmit 64 rate channels, with time resolutions of 1/3, 2/3, or 4/3 seconds. The S/C spin period is ~20 seconds, and the EPD motor moves to a new polar angle every spin. Ground processing converts this time-based data stream into displays with 7 polar angles and 64, 32, or 16 azimuthal sectors.
In the LGA mission EPD was allocated bit rates of 5, 10, 15, 20, 30, or 40 bps, depending on S/C TM format. The lowest bit rates will be the most common.
At these low rates the EPD measurements must be sectored on board the S/C, and accumulated for extended periods. The challenge was to design a realizable combination of channels, time and angular resolution that fits the available bit rate and maximizes science return.
EPD LGA Science Mission Design
Low bit rates forced emphasis on magnetospheric survey; microphysics can best be addressed in the tape-recorded LRS periods.
Worst-case time resolution chosen to be ~12 minutes (at 5 bps). This should be reasonable--much of the Voyager analysis was done with 15 minute averages. Higher resolution will be available at the higher bit rates. Angular resolution chosen to cover 4p with 1, 6, or 16 measurements (and cal).
Ion angular distributions can be very anisotropic (convection flows dominated by high-Z ions, boundaries, etc.). Emphasis on providing reasonable angular resolution to the lower-energy LEMMS ion (and electron) channels, and to CMS TOF channels (lower efficiency, but known composition). High energy CMS DE x E channels dropped, except for LRS periods.
PHA events are needed for TOF channel analysis and monitoring, but they are transmitted at low rates (worst case 1/minute). Rate channels are compressed to 9 bits--5 bit exponent, 4 bit mantissa (resolution varies from 3% to 6%).
EPD Sectoring Formats
TABLE 26. Rate Channel Mapping for 17-Bin "High Resolution" Channels
TABLE 27. Rate Channel Mapping for 7-Bin "Low Resolution" Channels
TABLE 28. Rate Channel Mapping for 2-Bin "Omni-res" Channels
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Updated 7/27/04, T. Hunt-Ward