Fundamental Technologies

Galileo Spacecraft Pages


Listed below are the major functions that are performed by the Galileo Energetic Particle Detection (EPD) software system:

In performing the above functions the EPD software operates as an interrupt driven system. These interrupt signals are generated by the Galileo spacecraft at the rate of 15 interrupts per second. The 76 bytes of data that constitute one low rate science (LRS) frame or packet are collected and processed during ten consecutive interrupt intervals. Thus one LRS frame is generated every 2/3 seconds. Two consecutive LRS frames constitute one logical record. One of the low rate science frames in the pair is referred to as an "even packet" and the other as an "odd packet." The leftmost bit of byte 9 within a packet indicates whether it is even (bit=0) or odd (bit=1). The 10 real time intervals within a packet time are numbered 0 through 9.

In addition to being either even or odd, each packet is associated with a number (the modulo 91 counter) between 0 and 90. This number can be computed from the modulo 7 counter (bits 2-4 of packet byte 9) and the modulo 13 counter (bits 5-8 of byte 9) as follows:

MOD91 = 7 x MOD13 + MOD7

The Mod 7 counter increments by 1 for each packet and the MOD 13 counter increments by 1 for every 7 packets. A spacecraft major frame consists of 91 consecutive packets (0 - 90).

The EPD software establishes and maintains synchronization with the spacecraft via real time clock information that arrives during real time interval number 8 of each packet. This clock information contains the real time interval number, the current MOD 91 value and an indication as to whether the packet is even or odd. Scheduling of the tasks to be performed by the EPD software system is based on the above interrupt structure and on the values of the modulo counters.  Figure 2.1 below shows this scheduling scheme for some of the more important tasks.

Figure 2.1:


Return to EPD Flight Software System Documentation table of contents page.
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Updated 8/23/04, T. Hunt-Ward