The IUE satellite was launched on 26 January 1978 into an
      elliptical geosynchronous orbit, in which it is always visible
      from Goddard Space Flight Center (GSFC). Science operations are
      conducted in real-time from GSFC for 16 hours each day. During
      the remaining 8 hours, operations are conducted by the European
      Space Agency at the Villafranca Satellite Tracking Station (VILSPA)
      near Madrid, Spain.
 
      Due to the ellipticity of the orbit and the subsequent daily
      variations in the satellite's elevation above the horizon,  VILSPA
      can view the satellite for only a  portion of each day. Since IUE's
      orbit is fixed with respect to sidereal time, the handover times
      between the GSFC and VILSPA observing shifts are normally changed by
      2 hours each month.
 
      Spacecraft power is provided by two large solar panels. At most
      orientations the power generated by the arrays is more than
      sufficient to run the spacecraft. When the spacecraft is tipped at a
      large angle from the sun, however, it may be necessary to draw on
      additional power from spacecraft batteries. The primary function of
      the batteries is to provide power to IUE during Earth shadow periods.
      Shadow seasons occur when a  portion of the IUE's orbit passes
      through the Earth's shadow once each day for 3 weeks in the late
      summer and winter. Additional observing constraints are invoked
      during these periods to ensure that the batteries are fully recharged
      before the next day's shadow.
 
      The spacecraft is three-axis stabilized with a  nominal 1 arc-second
      pointing accuracy. Control of the telescope pointing and execution
      of all spacecraft motion is the primary function of IUE's on-board
      computer (OBC). The OBC also performs other important functions, such
      as controlling camera exposures. Prior to August 1985, the OBC used
      data from three or more of IUE's six gyros to control attitude.
      Following the fourth gyro failure on August 17, 1985, a backup
      attitude-control system has been used. The new system (Femiano 1986)
      uses the two functional gyros and the spacecraft Fine Sun Sensor
      (FSS). The FSS measures the orientation of IUE with respect to the
      sun. In the two-gyro/FSS system, the FSS is used primarily to control
      the roll motion about the telescope optical axis. The two-gyro/FSS
      system has operational capabilities and accuracies essentially
      identical to the original three-gyro control system, with the
      exception that the telescope can no longer be pointed within 15
      degrees of the anti-solar position. The two-gyro/FSS has been
      described and compared with the original three-gyro system.
 
      The OBC processes data from the two functional gyros and the FSS and
      commands the reaction wheels so that either the telescope pointing
      stays fixed or the desired maneuver is performed. The reaction wheels
      act as flywheels to store angular momentum. By changing speed, the
      three reaction wheels cause the spacecraft to rotate about the
      desired axis. The spacecraft moves from target to target by executing
      a series of OBC controlled slews at a  rate of  3  to  6  degrees per
      minute. These spacecraft oriented axes are pitch,  yaw,  and roll.
      Only maneuvers preserving optimum illumination of the solar panels
      are allowed. Under the two-gyro/FSS system, maneuvers are executed as
      a sequence of slews in pitch (changing Beta, or sun angle) and at a
      constant sun angle (sunline slew). The wheel speeds must stay within
      certain limits, so after most maneuvers, angular momentum must be
      added to or removed from the wheels. This is done by firing small
      hydrazine jets and is known as a wheel unload. Since the wheel speeds
      also slowly change with time while maintaining telescope pointing,
      wheel unloads may also be needed prior to maneuvering after a shift-
      long exposure.
 
      At the Telescope Operations Center (TOC), the Telescope Operator (TO)
      under the supervision of the Resident Astronomer (RA) controls all
      aspects of the scientific instrument operation (target acquisition,
      telescope focus, exposures, and camera reads) and many spacecraft
      functions (maneuvering, gyro trims, offset guiding, etc.). The TO
      runs control programs, known as procedures, on the ground command
      computer to check ground system configuration and spacecraft
      telemetry for proper status of the relevant systems before
      transmitting the appropriate commands to the spacecraft. The
      procedures also record scientific and engineering data which are
      archived with each spectrograph or FES image. The commands are
      transmitted at VHF frequencies to the spacecraft. IUE transmits a
      continuous telemetry stream to the ground at S-band frequencies,
      normally at a  data rate of  20  kilobits/sec. The signal is received
      by an  18-meter antenna at the IUE tracking station located at the
      NASA Wallops Flight Facility, Wallops Island, Virginia. The helical
      VHF command antennas are also located at Wallops. The commands and
      incoming telemetry are relayed between GSFC and Wallops through a
      communications satellite. Once the telemetry reaches GSFC, it is
      processed by the ground computer and displayed on consoles in the
      Operations Control Center (OCC) for the various spacecraft subsystems
      as well as in the TOC.
 
      IUE's orbit is frequently monitored to provide accurate pointing
      predictions for the ground commanding and receiving antennas. A VHF
      signal is transmitted from the ground and retransmitted by the
      spacecraft, giving an accurate distance and velocity of the
      spacecraft (this is referred to as 'ranging'). Gravitational
      perturbations cause the gradual westward precession of the orbit's
      semi-major axis. A station-keeping maneuver is performed at intervals
      of 6 to 9 months to prevent the satellite from passing outside the
      range of ground antennas at GSFC and VILSPA. At this time, the large
      hydrazine jets are fired to slightly modify the semi-major axis so
      that the orbit drifts eastward again.