Fundamental Technologies

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

Spin Sector

Motor Position

0123 456
Aa1 a2a4 a5a6 a16
Ba7a8 a9
Ca3 a10a11a12 a17
Da13a14 a15

TABLE 27. Rate Channel Mapping for 7-Bin "Low Resolution" Channels

Spin Sector

Motor Position

0123 456
Ab1

b2

b3

b7

Bb4
Cb5
Db6

TABLE 28. Rate Channel Mapping for 2-Bin "Omni-res" Channels

Spin Sector

Motor Position

0123 456
Ac1

c2

B
C
D

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Updated 7/27/04, T. Hunt-Ward
tizby@ftecs.com