processing unit. This approach was taken because of the limitations inherent in
communicating between the spacecraft body and despun platform.

The IPS measures protons over the energy range from ~10 keV to 1 MeV using a
spectrometer which incorporates a Microstrip Solid State Detector (MSSD) having
a planar configuration with six individual elements. The IES measures electrons
over the energy range from ~ 25 to 400 keV using a spectrometer which
incorporates a Microstrip Solid State Detector (MSSD) having a 0.5 x 2.1 cm
planar configuration with five individual elements. The MSSD forms the image
plane for a sensor segment with a "pin-hole" aperture. The MSSD has a thick
"dead layer" and thus does not respond to protons with energies below ~ 250
keV. However the dead layer is sufficiently thin as that it allows the IES to
be sensitive to electrons with energies ~ 25 KeV or more. The "pin-hole"
aperture accepts electrons over a 60 degrees angular segment in a plane
containing the satellite spin axis. Each of the five detector e1ements in the
MSSD detects electrons in a ~12 degrees angular sub-interval of the 60 degrees
field-of-view (FOV). The complete IES system has a nominal geometric factor of
6 x 10^-3 cm2. The nominal angular resolution of a detector element is 12
degrees x 12 degrees and its elemental geometric factor is approximately 3.8 x
10^-4. Three of these detector segments provide the desired electron
measurements over a FOV of +/- 12 degrees x 180 degrees. The satellite spin is
used to obtain measurements over the full 4p steradians.

All detector elements in each segment are attached to a dedicated preamplifier.
In the normal mode of operation, the detector elements with fields of view
perpendicular, parallel and anti-parallel to the spin axis are connected to a
dedicated amplifier chain at all times. There are a total of 6 amplifier and
pulse height analyzer chains in the IES signal conditioning unit (SCU). The
remaining four detector elements in each sensor segment are then sampled on a
time-share basis. Each of the remaining three amplifier chains are attached to
one of the four detector elements in each segment under DPU control. In
general, the DPU can control which 6 of the 15 detector elements are being
sampled at any given time by the amplifier chains. The responses of the
selected detector elements are pulse-height analyzed by 8-bit Analog-to-Digital
Converters (ADCs) to obtain the energy for each detected particle.

The DPU samples the 5 detector elements in each ES segment according to
pre-programmed sampling schemes which utilize the sectored satellite spin to
obtain data samples over the unit sphere. In the normal mode of operation, the
combined data from the 3 dedicated detector elements and the 3 program-control
sampled detectors strips are used to build a three dimensional (3D) "image" of
the medium energy electron angular distribution. For each electron detected,
the polar and azimuthal angular sector and energy are known. The DPU
accumulates the result into an array which contains the electron fluxes in 15
(polar) by 32 or 16 (azimuthal) angular intervals for each of up to 12 energy
ranges. Within 24 seconds (4 spin periods), the complete 4p steradians can be
sampled and within one spin period a 2D distribution function can be obtained.

For more information, see:
http://leadbelly.lanl.gov/ccr/
http://www.css.tayloru.edu/~physics/seps.html
and
http://pwg.gsfc.nasa.gov/polar/polar_inst.shtml