PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = GIO INSTRUMENT_ID = DID OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "DUST IMPACT DETECTOR" INSTRUMENT_TYPE = "DUST IMPACT DETECTOR" INSTRUMENT_DESC = " 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. Mounted 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 and Located in cut-out in IPM/M, ambient plasma sensor front shield IPM/P DID 2,3,4 Piezoelectric momentum sensors Various locations MSM on front shield DID 5 Piezoelectric momentum sensor Rear shield RSM 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 ----------------------------------------------------------------------------- " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ESA-SP1077" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END