Instrument Host Overview    ========================      For most Magellan experiments, data were collected by      instruments on the spacecraft.  Those data were then relayed      via the telemetry system to stations of the NASA Deep Space      Network (DSN) on the ground.  Radio Science experiments (such      as radio occultations) required that DSN hardware also      participate in data acquisition.  The following sections      provide an overview first of the spacecraft and then of the      DSN ground system as both supported Magellan science      activities.      Instrument Host Overview - Spacecraft    =====================================      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].      Instrument Host Overview - DSN    ==============================      The Deep Space Network is a telecommunications facility managed      by the Jet Propulsion Laboratory of the California Institute of      Technology for the U.S.  National Aeronautics and Space      Administration.       The primary function of the DSN is to provide two-way      communications between the Earth and spacecraft exploring the      solar system.  To carry out this function it is equipped with      high-power transmitters, low-noise amplifiers and receivers, and      appropriate monitoring and control systems.       The DSN consists of three complexes situated at approximately      equally spaced longitudinal intervals around the globe at      Goldstone (near Barstow, California), Robledo (near Madrid,      Spain), and Tidbinbilla (near Canberra, Australia).  Two of the      complexes are located in the northern hemisphere while the third      is in the southern hemisphere.       The network comprises four subnets, each of which includes one      antenna at each complex.  The four subnets are defined according      to the properties of their respective antennas: 70-m diameter,      standard 34-m diameter, high-efficiency 34-m diameter, and 26-m      diameter.       These DSN complexes, in conjunction with telecommunications      subsystems onboard planetary spacecraft, constitute the major      elements of instrumentation for radio science investigations.       For more information see [ASMAR&RENZETTI1993].