An 11-m ground station from Allied Signal and EMP, currently under construction at UCB, will provide command transmission and data reception. The Mission Operations Center and Science Operations Center at UCB will operate the spacecraft and instrument, write the data onto CD-ROMs, and distribute the data to the Solar Data Analysis Center (SDAC) at GSFC and the High Energy Data Center (HEDC) in Zurich. The SDAC will archive and distribute both data and analysis software to outside users in the U.S., and coordinate access to context observations from other spacecraft and ground instruments. The HEDC will perform the same functions in Europe. A program of ground observations is supported directly by HESSI to provide the most critical context data. All users will have equal access to the ground-based data.
 
 
 
 

3.0 Science Instrumentation

      The HESSI scientific objectives will be achieved with high resolution imaging spectroscopy observations from soft X-rays to g -rays, using a single instrument consisting of an Imaging System, a Spectrometer, and Instrument Electronics. The Imaging System is made up of nine Rotating Modulation Collimators (RMCs), each consisting of a pair of widely separated grids mounted on the rotating spacecraft. Pointing information is provided by the Solar Aspect System (SAS) and Roll Angle System (RAS).

      The Spectrometer has nine segmented GeDs, one behind each RMC, to detect photons from 3 keV to 15 MeV. Each detector is made from a single germanium crystal, which is electrically divided into independent front and rear segments to provide an optimum response for low and high energy photons, respectively. The GeDs are cooled to <~75 K by a space-qualified long-life mechanical cryocooler to achieve the highest spectral resolution (Table 2) of any presently available g -ray detector. As the spacecraft rotates, the RMCs convert the spatial information from the source into temporal modulation of the photon counting rates of the GeDs. The Instrument Electronics amplify, shape, and digitize the GeD signals, provide low-voltage power and GeD high voltage, format the data, and interface to the spacecraft electronics.