National Aeronautics and Space Administration

The Ulysses spacecraft can operate in 4 different telemetry modes: 128, 256, 512, and 1024 b/s. The time resolution of the gamma-ray burst instrument varies with the spacecraft data rate. The maximum telemetry allocation for the instrument is 40 b/s.

The hard X-ray detectors operate in the range 15-150 keV. The detectors consist of 2 3-mm thick x 51-mm diameter CsI(Tl) crystals mounted via plastic light tubes to photomultipliers. The hard detector changes its operating mode depending on (1) measured countrate, (2) ground command, or (3) change in spacecraft telemetry mode. The trigger level is generally set for 8-sigma above background and the sensitivity is ~1e-6 erg/sq-cm. When a burst trigger is recorded, the instrument switches to record high resolution data, recording it to a 32-kbit memory for a slow telemetry read out. Burst data consist of either 16 s of 8-ms resolution count rates or 64 s of 32-ms count rates from the sum of the 2 detectors. There are also 16 channel energy spectra from the sum of the 2 detectors (taken either in 1,2,4,16,or 32 second integrations). During 'wait' mode, the data are taken either in 0.25 or 0.5 s integrations and 4 energy channels (with shortest integration time being 8 s). Again, the outputs of the 2 detectors are summed.

The soft X-ray detectors consist of 2 500-micron thick x 0.5 sq-cm area Si surface barrier detectors. A 100 mg/sq-cm beryllium foil front window rejects the low energy X-rays and defines a conical field of view of 75 degrees (half- angle). These detectors are passively cooled and operate in the temperature range -35 to -55 degrees Celsius. This detector has 6 energy channels, covering the range 5-20 keV.