Instrument Host Information
This description contains excerpts from:
  Santo, A.G., S.C. Lee, and R.E. Gold, NEAR Spacecraft and
  Instrumentation, The Journal of the Astronautical Sciences, Vol. 43,
  No. 4, pp. 373-397, October-December 1995 [SANTOETAL1995] describing
  the NEAR spacecraft.

   Instrument Host Overview
     The NEAR spacecraft design is mechanically simple, and geared toward
     a short development and test time. Except for the initial deployment
     of the solar panels and protective instrument covers, the spacecraft
     has only one moveable mechanism. A distributed architecture allows
     parallel development and test of each subsystem, yielding an
     unusually short spacecraft integration and test period. Several
     innovative features of the NEAR design include the first use of an
     X-band solid state power amplifier for an interplanetary mission,
     the first use of a hemispherical resonator gyroscope in space, and
     extremely high-accuracy high voltage power supply control.

   System Description
     Most electronics are mounted on the forward and aft decks. The
     science instruments, except for the magnetometer, are hard-mounted
     on the outside of the aft deck with co-aligned fields-of-view. The
     magnetometer is mounted on the High Gain Antenna (HGA) feed. The
     interior of the spacecraft contains the propulsion module.

     The spacecraft design was selected for its mechanical simplicity.
     The solar panels, the HGA, and the instruments are all fixed. The
     solar panels and HGA can be fixed because throughout most of the
     mission the Sun-spacecraft-Earth  angle is less than 40 degrees.
     While the HGA is pointed to the Earth, the Sun to solar panel angle
     is small and the energy output from the resultant solar illumination
     incident on the panels is sufficient. During the first two months
     after launch and for a short time during the Earth flyby, the
     Sun-spacecraft-Earth angle is greater than 40 degrees. At these
     times, the telecommunication link is sufficient to allow
     communications with the Earth through the medium or low gain
     antennas while keeping the solar panels pointed within 40 degrees of
     the Sun. During asteroid operations, the geometry allows the
     spacecraft to be oriented as needed for scientific data collecting,
     while maintaining the required Sun to solar panel angle. While
     mechanically simple and reliable, hard-mounting the HGA, solar
     panels, and instruments drives other areas of spacecraft and mission
     design. The resultant spacecraft moments-of-inertia are such that
     closed-loop control of the vehicle pointing must be maintained
     throughout the mission. Because the power system is designed for
     100% sunlight operation, the 450 N thruster location is chosen such
     that the panels can be oriented towards the Sun during all large
     delta(V) maneuvers. Finally, scientific operations and high speed
     downlink to Earth cannot always be carried out simultaneously.

   Command and Data Handling Subsystem
     The Command and Data
     Handling (C&DH) subsystem comprises redundant APL-built command and
     telemetry processors, redundant Solid State Recorders (SSR), a power
     switching unit to control spacecraft relays, and an interface to-a
     redundant 1553 standard bus for communicating with other
     processor-controlled subsystems. The redundant components are
     cross-strapped among themselves, and among the redundant uplink
     chains of the telecommunications subsystem.  The functions provided
     by the C&DH subsystem are command management, telemetry management,
     and autonomous operations.

     The command function operates on cross-strapped inputs from the two
     CDUs at either of two rates: 125 bps (normal mode) or 7.8 bps
     (emergency rate). The format of the uplinked commands is
     Consultative Committee for Space Data Systems (CCSDS) compliant,
     with a separate virtual channel for each side of the redundant C&DH
     subsystem. Four types of commands are supported: relay commands are
     directed to the power switching unit to change the state of the
     spacecraft relays; dedicated data commands are directed over
     specific serial interfaces to control SSR and telecommunications
     subsystem operation; 1553 data commands are directed to a specified
     remote terminal on the 1553 bus; and C&DH-specific commands are
     interpreted within the C&DH to control its own operation.  Some of
     the C&DH-specific commands provide facilities for storing commands
     for later execution, either at a specified Mission Elapsed Time
     (MET), or when the spacecraft conditions warrant autonomous action.
     A series of commands that perform a specific function can be stored
     as a command macro; the entire series of commands can be invoked by
     a single macro execute command. During normal operations, the C&DH
     will invoke command macros that have been scheduled for execution at
     a specific MET.  In this way, operations are carried out when the
     spacecraft is out of ground contact. Command macros can also be
     invoked by the autonomy function of the C&DH, to place the
     spacecraft in a safe condition. Approximately 56 K bytes of memory,
     4000 commands, is available for stored commands in each processor.

     The telemetry function collects engineering status and science data
     from the housekeeping interface, from dedicated serial interfaces,
     from the remote terminals on the 1553 bus, and from the C&DH
     internal event history buffers. This data is packetized where
     necessary, and placed into CCSDS-compliant transfer frames. The
     transfer frames are directed to the SSRs, the downlink, or both.
     Data recorded on the SSRs is read back, packed into transfer frames
     and placed into the downlink on command. Recorder playback data can
     be interleaved with realtime data on the downlink, and data can be
     recorded on one of the redundant SSRs while the other recorder is
     read back.

     SEAKR provides the SSRs. These recorders are constructed out of 16
     Mbit IBM Luna-C DRAMs. One recorder has 0.67 Gbits of storage; the
     other recorder has 1.1 Gbits of storage because it contains an
     additional memory board which is designated as the flight spare, and
     will be used to replace either of the other memory boards in the
     event of a ground test failure.

     The downlink data rate is selectable among eight rates ranging from
     26.5 kbps to 9.9 bps to match the communication link capability
     throughout the mission. For all except the highest downlink rate,
     the recorder capacity exceeds the downlink capacity, so bandwidth is
     limited by the downlink.  While the C&DH subsystem controls the rate
     of collection of realtime data to match the downlink rate, the rate
     at which data is placed on the recorder is under the control of the
     subsystems. Each remote terminal on the 1553 bus can request the
     C&DH to pick up and record up to 5336 bits of data per second. This
     feature allows the spacecraft operators complete flexibility with
     respect to the bandwidth used by each instrument.

   Guidance and Control Subsystem
     The Guidance and Control (G&C) subsystem is composed of a suite of
     sensors for attitude determination, actuators for attitude
     corrections, and processors to provide continuous, closed loop
     attitude control.  In
     operational mode, the attitude is controlled to a commanded pointing
     scenario. In safe modes, the G&C maintains the solar panels pointed
     to the Sun for maximum power, and attempts to place the Earth within
     the medium-gain antenna pattern to establish ground communications.
     The G&C subsystem also controls the thrusters for delta(V)
     maneuvers. Finally, the G&C subsystem recognizes many internal
     failure modes and initiates autonomous actions to correct them.