Exploring High Energy Astronomy


The Gamma-Ray Large Area Space Telescope

Opening the High-Energy World for Exploration
The Gamma-Ray Large Area Space Telescope (GLAST) is a next generation high-energy gamma-ray observatory that employs two instruments to observe celestial gamma-ray sources. The primary instrument is the Large Area Telescope (LAT), which detects gamma rays with energies of 20 million electron volts to greater than 300 billion electron volts. The secondary instrument is the GLAST Burst Monitor (GBM), which observes gamma ray bursts within its 2/3 sky field-of-view.

GLAST launched in 2008 and resides in a low-earth circular orbit (550 km altitude) at a 28.5 degree inclination. The mission is designed for a lifetime of 5 years and a goal of 10 years of operations.

The LAT instrument uses 3 primary and 2 redundant RAD750 radiation-hardened single-board computers to record and analyze information generated by the gamma-ray detectors.

The PTR Group worked directly with the U.S. Naval Research Laboratory (NRL) and the Stanford Linear Accelerator Center (SLAC) to design, write, and test flight software for these onboard computers in the following areas:

Boot code that configures the RAD750 CPU and PowerPCI bridge chip, verifies the operation of the onboard RAM, and validates and loads VxWorks operating system images. This software is written in the PowerPC assembly and C languages.

  • Embedded MicroController (EMC) assembly-level code that validates software images and selects appropriate images for execution. Assisted the spacecraft bus vendor with adapting this image validation software for use on the spacecraft bus.
  • Command and telemetry handlers that read and write RAM, EEPROM, and CPU registers.
  • LAT instrument management code that determines the current operating mode, manages commands and signals from the GBM, and coordinates pointing requests to the spacecraft bus. This code is written in the C language and operates as a VxWorks task.
  • CPU monitoring code that manages a watchdog timer and provides access to internal RAD750 CPU performance monitoring facilities, which count many types of low-level events such as cache misses, instruction dispatches, data loads and stores, and processor clocks.
  • Fault detection and recovery software that monitors critical errors and captures diagnostic information.
  • Ground-based programs to interpret and analyze data and telemetry received from the LAT instrument. The PTR Group used various tools during the development of the LAT software, including:
  • Corelis JTAG in-circuit emulators for low-level troubleshooting
  • COTS PowerPC single-board computers for initial testing of flight software
  • Sun and linux host machines for development of ground-based programs