Instrument Host Overview
    ========================
 
      Planet-A and MS-T5 are identical except for their
      scientific payloads.  The main spacecraft body is a
      cylinder, 140 cm in diameter and 70 cm high.  The
      spinning spacecraft is about 250 cm high, from the
      top of the high- gain antenna dish to the bottom of
      the medium-gain antenna.  Its outer wall is covered
      by solar cells which produce 100 W of power under
      optimum conditions.
 
      An equipment platform mounted to a central thrust
      tube carries most of the subsystems, including the
      mechanically despun, high-gain antenna, two
      hydrazine fuel tanks, the thrusters, and the
      experiment sensors .  A star scanner and a nutation
      damper are attached to the thrust tube.  Planet-A
      and MS-TS weigh 139.5 kg and 138.1 kg,
      respectively, including lO kg of hydrazine
      propellant in each case.  Both spacecraft are
      spin-stabilized, with the facility to change the
      spin rate .  The spin axes of both spacecraft will
      be kept perpendicular to the ecliptic plane.  The
      spin-rate of Planet-A will be changed quite
      frequently.  It must be reduced, for example, to
      0.2 rpm during the UVI imaging operating using an
      onboard momentum wheel, while during communication
      periods and during solar-wind measurements a
      spin-rate of 6.3 rpm is preferable.  The
      spacecraft/Earth geometry and the data-rate
      requirements preclude the use of a single antenna
      for the duration of the mission.  The antenna
      subsystem therefore consists of a high-gain
      antenna, a medium-gain antenna as a backup, and a
      low-gain antenna for near-Earth operation.  The
      despun high-gain antenna has an offset- parabolic
      reflector with a diameter of 80 cm and a gain of
      21.5 dB in uplink and 22.5 dB in downlink
      transmission.  The communications subsystem
      operates in 5-band using a 5 W transmitter.
      Planet-A has two data formats: Format-1 provides
      Ultra-Violet Imager (UVI) data.  Format-2
      Solar-Wind Experiment (ESP) data (Table below).
      The data rates of both spacecraft are 2048 bit/s
      (without coding) in 'high bit rate' and 64 bit/s
      (with convolutional coding) in 'low bit rate' mode.
      The bit rate used will depend on the communication
      distance.
 
      The ground station for the mission has been built
      at Usuda, about 170 km northwest of Tokyo.  The
      station's 64 m diameter dish antenna has a gain of
      approximately 63 dB.  A 64 bit/s communication link
      (convolutionally encoded PCM) is expected to be
      possible from 1.1 AU, which will be the comet-Earth
      distance during the Planet-A encounter.
 
 
 
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Table.  The Planet-A scientific payload
 
 
Experiment   Mass  Power Data rate (bit/s)   Principal Investigator
             (kg)  (W)   Format 1/2
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                         H 1568/0           E. Kaneda, Geophys. Res. lab.,
UVI          7.5   8.9   L 98/0             Faculty of Science, Univ. of
                                            Tokyo
 
                         H 0/1408           T. Mukai, Res. Div. Planetary
ESP          4.7   4.9   L 0/88             Science, ISAS, Tokyo
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Format         UVI and UVI check        H = High bit rate
               ESP                      L = law bit rate
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