Instrument Overview
  ===================
    (excerpted from [HURLEYETAL1992])
 
    The GRB experiment was proposed in 1977 with a twin experiment
    aboard the NASA spacecraft.  This approach had numerous
    advantages: a carefully intercalibrated pair of experiments at
    opposite ecliptic latitudes would survey the solar equatorial
    regions extensively, and stereoscopic observations would be
    relatively easy to compare with matched detectors.
    Triangulation baselines for cosmic gamma-ray bursts would be
    long, and the fact that two detectors in the network would be
    out of the ecliptic plane would provide a particularly
    favorable geometry for localization (non-coplanar detectors are
    required for arrival time analysis).
 
    With the unique mission opportunities, however, came unique
    design constraints.  A radiation-hardened microprocessor was
    required to survive the passage through the Jovian radiation
    belts; few were available during the design phase of the GRB
    experiment.  The limited performance of the unit chosen
    dictated simplified operating modes for the experiment.
    Electrical power aboard Ulysses is provided by a radioisotope
    thermoelectric generator (RTG) containing 10 kg of 238Pu (about
    10^5 Ci).  The GRB instrument would be the first cosmic
    gamma-ray burst detector to have to operate in this unfavorable
    background environment.  To minimize the interference from the
    RTG, the sensors had to be mounted on the magnetometer boom,
    and were required to be essentially amagnetic.  (More
    precisely, the remnant field could not exceed 2 x 10^-5 G at 25
    cm.) Finally, the mass and power allocations for the GRB
    experiment were small compared to inner planet missions-2 kg
    and 2.6 W, respectively.
 
 
    Scientific Objectives
    ---------------------
      The Ulysses solar X-ray/cosmic gamma-ray burst instrument
      (acronym: GRB) has three main scientific objectives.  The
      first is the study and monitoring of solar flare X-ray
      emission.  The second is the detection and localization of
      cosmic gamma-ray bursts.  The third is the in-situ detection
      of Jovian auroral X-radiation. . . .  For a review of
      experimental data on solar flare X-rays, see [DENNIS1985],
      [DENNIS1988].  Reviews of cosmic gamma-ray burst observations
      have appeared in [HURLEY1988], [HURLEY1989].
 
      Jovian X-radiation was first detected by [METZGERETAL1983]
      using the Einstein satellite.  X-rays in the 0.2-3 keV band
      were imaged with 4'' resolution, and provided evidence that
      the sources of X-radiation were the Jovian north and south
      auroral zones.  The origin of the X-rays could have been
      either electron bremsstrahlung in the Jovian upper
      atmosphere, or characteristic line emission from atmospheric
      atoms excited by heavy ions precipitating from the Io torus.
      These two models make very different predictions about the
      hard X-ray flux in the Ulysses GRB energy range.