CCSD3ZF0000100000001NJPL3IF0PDSX00000001 PDS_VERSION_ID = PDS3 RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 80 OBJECT = INSTRUMENT_HOST INSTRUMENT_HOST_ID = "MGN" OBJECT = INSTRUMENT_HOST_INFORMATION INSTRUMENT_HOST_NAME = "MAGELLAN" INSTRUMENT_HOST_TYPE = "SPACECRAFT" INSTRUMENT_HOST_DESC = " Spacecraft Overview =================== The Magellan spacecraft was built by the Martin Marietta Corporation. The spacecraft structure included four major sections: High-Gain Antenna (HGA), Forward Equipment Module (FEM), Spacecraft Bus (including the solar array), and the Orbit Insertion Stage. Spacecraft subsystems included those for thermal control, power, attitude control, propulsion, command data and data storage, and telecommunications. Design of the Magellan spacecraft was driven by the need for a low-cost, high-performance vehicle. Protoflight units that had been built for preflight tests or were spares from the Voyager spacecraft were available from storage at no cost. These included the 3.7 meter diameter high-gain antenna (HGA), the spacecraft bus, propulsion system components, thermal control louvers, and much of the radio subsystem. The stockpile of flight spares for the Galileo spacecraft provided Magellan's command and data system, tape recorders, attitude control processor, power subsystem and propulsion components. Further elements were drawn from other projects and from NASA standard designs. Only about 30% (by mass) of the Magellan spacecraft -- primarily the radar electronics and the solar panels -- was especially designed for the mission. The high-gain antenna (HGA) was used as the antenna for the synthetic aperture radar (SAR) and as the primary antenna for the telecommunications system. The HGA boresight was defined to be the +Z axis for the spacecraft-fixed coordinate system. The spacecraft bus was a ten sided structure containing the star scanner, medium-gain antenna (MGA), rocket engine modules (REMs), command data and data storage (CDDS) subsystem, attitude control monopropellant tank, and a nitrogen tank for providing propellant pressurization. The solar panel array was attached to the bus; its rotation axis defined the +X axis for the spacecraft-fixed coordinate system. The +Y axis of the coordinate system was in the nominal direction of the star scanner boresight, forming a right-hand coordinate system. The radar electronics, the reaction wheels, and various other spacecraft subsystem components were contained within the Forward Equipment Module, located between the bus and HGA. The orbit insertion stage contained a STAR-48 solid rocket motor (SRM) that was used to provide the impulse required to perform the Venus Orbit Insertion (VOI) maneuver. Thermal control of the spacecraft was accomplished by a combination of louvers, thermal blankets, passive coatings, and heat dissipating elements. The nominal operating temperature for the spacecraft components was between -5 and +40 degrees Celsius. The thermal control subsystem maintained these components at the appropriate temperatures for all orientations of the spacecraft orbit and sun-line and for all spacecraft operating modes. Electrical power was supplied by two large solar panels with a total area of 12.6 square meters. This array was capable of producing a minimum power of 1029 W at the end of the nominal mission; it could rotate about its axis to allow tracking of the Sun despite the changing Earth-Sun-spacecraft geometry during the mission. A dedicated sun sensor optimized power production. Bus voltage regulation was controlled by the power control unit (PCU) with a shunt regulator for diverting excess power from the solar arrays to maintain power as raw power (28-35 v), regulated power at 28 vDC +/-0.56 vDC, and as AC at 2.4 kHz through an inverter. Two 30 amp-hour, 26-cell nickel cadmium batteries provided power during times of solar occultation and allowed normal spacecraft operations independent of real-time solar illumination. These batteries were sized to allow a degraded mission in the event that one of them failed. The attitude of the Magellan spacecraft was controlled through the use of reaction wheels, with monopropellant rocket motors being used to desaturate the reaction wheels periodically. During both the interplanetary cruise and the orbital portions of the mission, attitude reference was provided by an inertial reference unit (IRU), updated each orbit using celestial references. During the mapping part of each orbit, the spacecraft was initially oriented with the HGA pointing down toward Venus, with the exact attitude being a function of the spacecraft altitude and the SAR mapping objectives. During the downlink transmission part of the orbit, the spacecraft was oriented with the HGA slightly off the Earth-line. The low gain antenna (LGA) was mounted coaxially with the HGA and did not require pointing since it had an omnidirectional beam pattern. The altimeter horn (ALTA) was mounted so that a portion of the fan-shaped beam nominally pointed in the nadir direction during the mapping part of an orbit. The Magellan propulsion subsystem consisted of two parts. The first, a Star 48 SRM, provided the impulse for VOI. Following that maneuver, the empty casing and parts of its support structure were ejected from the spacecraft. The second part consisted of monopropellant hydrazine thrusters that were used for trajectory correction maneuvers (TCMs) during inter- planetary cruise, thrust vector control (TVC) during VOI, orbit trim maneuvers during the mapping mission, and attitude control when the reaction wheels were being desaturated. The rocket motors were clustered in modules located on the end of outrigger booms in order to increase their moment arms and thus decrease attitude control propellant requirements. Twelve 0.9-N (Newton) and four 22-N rocket motors were used for attitude control, with thrust being provided by eight 445-N rocket motors or by the 0.9-N motors for small TCMs. All engines pointed in the -Z direction, with the exception of the roll motors. The 0.9-N motors were used for tip-off control following separation of the inertial upper stage (IUS), reaction wheel desaturation, roll control for all times other than VOI, to back up any failed reaction wheels, and for small TCMs or orbit trim maneuvers (OTMs). The 22-N motors were used for roll control during VOI. The 445-N motors were used for controlling the spacecraft rotational axis during VOI, and to provide impulses during all propulsive maneuvers. The monopropellant motors also provided the impulses needed to trim the VOI maneuver. The command, data and data storage (CDDS) system received uplink commands via the radio frequency subsystem (RFS) and controlled the spacecraft in response to those commands. It also controlled the acquisition and storage of scientific data and sending that data, along with supplemental engineering data, to the RFS for downlink transmission to Earth. The commands were sent to the spacecraft as time-event pairs for storage and later execution, and also in the form of blocks which the CDDS later expanded into spacecraft executable commands. In the Venus orbit phase, commands for up to three days of radar operation were stored. There was also a provision for receiving and executing discrete commands sent from the ground. SAR data were stored on two multi-track digital tape records (DTRs) for later playback over the high-rate X-band link; there was no provision for real-time transmission of the SAR data. Data storage capacity of the two DTRs was approximately 1.8 billion bits. Engineering data were normally transmitted to Earth over a real-time S-band link. During those times when a real-time link was not possible, the engineering data were recorded on a DTR and played back via the X-band high-rate link with the SAR data. The recorded data stream was alternately switched between the two DTRs so that the data would not be lost during the DTR track change. The Magellan telecommunications subsystem contained all the hardware necessary to maintain communications between Earth and the spacecraft. The subsystem contained the radio frequency subsystem, the LGA, MGA, and HGA. The RFS performed the functions of carrier transponding, command detection and decoding, and telemetry modulation. The spacecraft was capable of simultaneous X-band and S-band uplink and downlink operations. The S-band operated at a transmitter power of 5 W, while the X-band operated at a power of 22 W. Uplink data rates were 31.25 and 62.5 bps (bits per second) with downlink data rates of 40 bps (emergency only), 1200 bps (real-time engineering rate), 115.2 kbps (kilobits per second) (radar downlink backup), and 268.8 kbps (nominal). For more information on the Magellan spacecraft see the papers by [SAUNDERSETAL1990] and [SAUNDERSETAL1992]. " END_OBJECT = INSTRUMENT_HOST_INFORMATION OBJECT = INSTRUMENT_HOST_REFERENCE_INFO REFERENCE_KEY_ID = "SAUNDERSETAL1990" END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO OBJECT = INSTRUMENT_HOST_REFERENCE_INFO REFERENCE_KEY_ID = "SAUNDERSETAL1992" END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO END_OBJECT = INSTRUMENT_HOST END