Fundamental Technologies

Galileo Spacecraft Pages

Energetic Particle Detector Experiment Flight Software System Documentation

Source: H. Malcom, July 30, 1982

Table of Contents

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This document describes the onboard flight software for the Galileo Energetic Particle Detector (EPD) data system. The software system acquires and processes data that are reported in the EPD low rate science frames. Each frame contains six categories of data:

  1. Engineering Measurements
  2. EPD Sensor Data
  3. Multiplexed Singles and Background Data
  5. CMS PHA
  6. Reference Measurements (Housekeeping/Status)

The processing required for each category will be described in subsequent sections.

General Definitions

1. One low rate science (LRS) frame occurs every 0.667 seconds.

2. Ninety-one LRS frames comprise one major frame which occurs every 60.667 seconds.

3. Two LRS frames complete one EPD logical record.

4. Each LRS frame provides a 76-byte group of data called a "packet."

5. The two data packets that comprise one EPD logical record are referred to as the even and odd packets.

6. The 0.667 second duration of an LRS frame is called a "packet time."

7. The EPD consists of two distinct solid state detector configurations--Low Energy Magnetospheric Measurement System (LEMMS) and Composition Measurement System (CMS).

Eight detectors are housed in the LEMMS telescope, namely A, B, C, D, and the detector pairs E1, E2 and F1, F2.

Nine detectors are housed in the CMS telescope: Ja, Jb, Jc, K, L, J'a, J'c, K'a, and K'c.

8. One EPD packet includes:


a. Nine bytes of subcommutated housekeeping and status data.
b. Five bytes of LEMMS PHA data.
c. Twelve bytes of CMS PHA data.
d. Fifty bytes of LEMMS and CMS sensor data (40 rate channel readouts consisting of 10 bits each).

9. Thirteen times throughout one major frame (when the MOD91 counter registers 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, and 84) the 12 bytes of CMS PHA data in a packet are replaced by LEMMS PHA data. These 12 bytes together with the 5 bytes of LEMMS PHA data already stored in each of the 7 packets of EPD telemetry form one complete 47-byte LEMMS PHA spectrum.

The MOD 13 counter in byte 9, bits 5-8*, of the EPD packet is the reference counter for the LEMMS PHA spectrum cycle; the counter increments by 1 every 7 LRS frames.

* The bytes within an EPD packet are numbered 1 through 76.  The eight bits within a byte are numbered 1 through 8 starting  with the MSB on the left and ending with the LSB on the right.

The MOD 7 counter in byte 9, bits 2-4, of the EPD packet increments by one for each LRS frame.  When this counter is zero, a LEMMS PHA spectrum will be accumulated for a 2/3 second interval and the CMS PHA data will be replaced with LEMMS PHA data.

10. One LRS frame provides 40 rate channels of LEMMS or CMS sensor data; each rate channel consists of a 10-bit log compressed number which originates as a 3-byte binary integer from the EPD rate channel accumulators.

11. Bit 8 of byte 8 in the EPD packet determines whether the rate channel being viewed is in the J mode. (CMS telescope 180 degree end in use) or the J' mode (CMS telescope 0 degree end in use). The EPD system can be commanded into a state in which it alternates between the J (bit=0) and J' (bit=1) modes from one logical record to the next.

Next: 2.0 Software System Overview

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Updated 8/31/04, T. Hunt-Ward