INSTRUMENT OVERVIEW
      ===================
      The Dust Impact Detector System consists of six subsystems with the
      primary aim of recording the impacts of all particles of significant
      mass incident on the Giotto probe.  Mounter on Giotto's dust shields,
      the detectors determine the mass spectrum.  The capacitor impact
      system records the discharge of a parallel plate capacitor when
      impacted by a particle whose mass exceeds the threshold value.  The
      sensor is bonded to the outer surface of the front shield.  The MSM
      and RSM subsystems of the DIDSY were designed and built at the
      University of Kent at Canterbury but the acoustic design of the front
      shield was the responsibility of the Rutherford Applied Physics
      Laboratory.  Three 200 kHz longitudinally resonant PZT 5A piezoelectric
      microphone elements are attached to the rear of the spacecraft front
      shield. Each crystal is contained in a hermetically sealed stainless
      steel container. Impacts on the spacecraft rear kevlar shield are
      sensed by a fourth (RSM) sensor identical to the front ones.  The IPM
      sensor consist of two impact ionization detectors each with a sensitive
      area of 59.6 cm**2 and a piezoelectric detector supplied by the
      University of Kent at Canterbury. The entire system was designed and
      developed by the Max-Planck-Institute-Fuer-Kernphysik with the help
      of the Technical University of Munich.  The sensor is attached to the
      front shield of the spacecraft.
 
 
      SCIENCE OBJECTIVES
      ==================
      DIDSY attempts to measure the mass efflux from a comet.   In order
      to determine the mass spectrum, a collection of instruments,
      sensitive to different properties are used.   Two broad groups measure
      the particles directly (gas and dust) or the plasma generated by both.
      In the former case, the discrete data represent a set measured values
      for a single event.   These can be collected to form the mass spectrum
      of the particles.  In the latter case, a 1 cm**2 gold-plated electrode
      measures the plasma.
 
      CALIBRATION DESCRIPTION
      =======================
      A program of laboratory measurements was carried out to determine
      the performance of different shield designs.   A revised design
      incorporates a 35 degree sector of the annular shield which remains
      'clean' (DID 4). It is then possible to get a relatively true signal.
      There is also a large sector (325 degrees) that is treated as a
      single surface and measured by detectors DID 2 and DID 3.
 
      DETECTORS
      =========
      DETECTOR_ID                   : DID 7
      DETECTOR_TYPE                 : PARALLEL PLATE CAPACITOR
 
      DETECTOR_ID                   : DID 5
      DETECTOR_TYPE                 : PIEZOELECTRIC
 
      (A PZT 5A piezoelectric microphone element attached to the rear
      shield.)
 
      DETECTOR_ID                   : DID 4, DID 3, DID 2
      DETECTOR_TYPE                 : PIEZOELECTRIC
 
      (PZT 5A piezoelectric microphone elements attached to the rear of the
      spacecraft front shield.)
 
      DETECTOR_ID                   : DID 1
      DETECTOR_TYPE                 : PIEZOELECTRIC
 
      (A PZT 5A piezoelectric microphone element attached to the front
      shield which is used in combination with the plasma detectors (1A
      and 1B) below.)
 
      DETECTOR_ID                   : DID 1A, DID 1B
      DETECTOR_TYPE                 : IMPACT PLASMA
 
      (Impact ionization detectors for submicron and micron size particles.)
 
      ELECTRONICS
      ===========
      The Cetral Data Formatter (CDF) performs functions for each detector.
      The CIS electronics exhibits one of two processes.   If a particle
      strikes the sensor, a conduction path is set up by the formation of
      a plasma. The RSM/MSM electronics subsystem performs both analog and
      digital processing of the signals arising at each sensor from a
      particle impact. The signals from the IPM subsystems are processed in
      parallel to reduce dead-time.   The IPM/M electronics also performs
      both analog and digital processing.
 
      PLATFORM MOUNTING DESCRIPTION
      =============================
      DIDSY is mounted inside the dust shields along the front of the
      spacecraft.
 
      INSTRUMENT SECTION DESCRIPTION
      ==============================
      The DIDSY system has been designed so that, as far as possible, the
      individual subsystems may operate independently of each other for
      operational reliability. At the same time, by operating in overlapping
      ranges of incident particle mass, comparison of results from the
      different subsystems will yield greater 'scientific' reliability.
 
      Despite independent operation, some coincidence information for
      individual impacts is exchanged between Subsystems for event
      validation. However, the failure of one detection subsystem should
      not in any way affect the correct operation of another. The Central
      Data Formatter (CDF) performs a primary task of collecting data from
      each subsystem, processing it as required, and passing it into the
      telemetry stream. In addition to handling data formatting the CDF
      also supplies power to each subsystem via a separate current-limiter
      so that, for example, a short-circuit in one subsystem does not
      affect operation of the others. Unregulated power is supplied by the
      spacecraft power subsystem on a single current-tripped supply.
 
      Data within the DIDSY subsystem are divided into two broad categories,
      namely: (i) discrete data, which represent a set of measured values
      for single event detected by one subsystem, and (ii) cumulative data,
      which represent the total event amplitude distribution from each sensor
      and are generated by accumulation of discrete data events within the
      CDF. Such an approach results from the telemetry allocation, which
      makes it impossible to transmit full information on all detected
      impacts, except at very low impact rates. A limited quantity of
      discrete data are mixed with the cumulative data and passed into the
      spacecraft telemetry stream by the CDF every 1.3 s during the last
      hour before closest approach and every 2.83 s prior to this. A brief
      description of the constituent elements of the DIDSY system is given
      below.
 
      -----------------------------------------------------------------------
      Constituent elements of DIDSY
 
      Designation      Description                Location          Subsystem
      -----------------------------------------------------------------------
 
       DID 1,1A/B Impact plasma, momentum      Located in cut-out in   IPM/M,
                  and ambient plasma sensor    front shield            IPM/P
       DID 2,3,4  Piezoelectric momentum       Various locations       MSM
                  sensors                      on front shield
       DID 5      Piezoelectric momentum       Rear shield             RSM
                  sensors
       DID 6      Central Data Formatter       Experiment platform
       DID 7      Capacitor Impact Sensor      Front shield            CIS
       DID 8      Roll-up cover                Front shield over DID 1
      -----------------------------------------------------------------------