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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. reference.SANTOETAL1995 NEAR EARTH ASTEROID RENDEZVOUS Spacecraft NEAR 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.