Galileo Spacecraft Pages
Table of Contents:
Source: The Galileo Energetic Particles Detector, D. J. Williams et al., Space Science Reviews, 60, 385, 1992 (excerpts)
The EPD instrument contains a highly flexible command, control, and telemetry system. The "brains" of the system is a radiation hardened RCA 1802 microprocessor-based computer with 6 kbyte mask programmed ROM and 2.25 kbyte RAM and 22 8-bit input/output ports. The system is responsible for all instrument control, command decoding, and telemetry processing and formatting.
The command and telemetry interface with the spacecraft is made over 4 dedicated serial data lines, 3 going to the instrument and 1 going to the spacecraft. These data lines connect to high-speed direct memory address control electronics within the instrument. Through this electronics, the spacecraft is able to directly read and write into the contents of the EPD data system memory.
Commands intended for the EPD are thus written to a specific area in RAM, and formatted telemetry packets are likewise read from alternating buffers in RAM. This whole process is transparent to the instrument microprocessor electronics. Special instrument registers count the number of invalid bus transactions.
In addition, there is a low voltage power supply which converts the spacecraft 30 V DC power into 10 isolated output voltages needed in the instrument. The converter, based upon a buck/boost regulator, operates at 57.6 kHz and contains overcurrent crowbar protection (350 ma), load switching, and dual detector bias output levels. It operates at approximately 78% efficiency under normal load.
The converter also switches power to the instrument's cover release mechanism. The mechanism, based upon a bi-phase wax actuator and nine-watt heater element, successfully released two clamshell covers which protected the CMS and LEMMS detectors from chemical contamination and physical damage. The covers can not be reclosed during the mission.
The instrument may be calibrated in flight in two ways. Alpha particle and electron radioactive calibration sources are mounted on the foreground shield positioned in motor sector 0. These sources can be seen by both the CMS and LEMMS. An electronic pulser circuit also can be used to generate signals for the LEMMS and CMS analog channels. These pulses are injected after the preamplifier electronics, and are intended to verify channel operation and discriminator settings. A sophisticated feedback circuit enables the data system to automatically cycle the pulsers to each discriminator, measuring the 12% and 88% trigger levels in each to measure channel noise.
Instrument status is monitored through the use of an 8-bit analog to digital converter and multiplexer. Five temperatures, nine voltages, and the instrument input current are measured and reported in the telemetry stream once a minute.
Next: Galileo EPD Telemetry Format: R. C. Moore memo dated Aug. 3, 1978
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Updated 12/13/04, T. Hunt-Ward