Instrument Overview
  ===================
    The Lightning and Radio Emission Detector (LRD) instrument will
    be carried by the Galileo Probe into Jupiter's atmosphere.  The
    LRD will verify the existence of lightning in the atmosphere and
    will determine the details of many of its basic characteristics.
    The instrument, operated in its magnetospheric mode at distances
    of about 5, 4, 3, and 2 planetary radii from Jupiter's center,
    will also measure the radio frequency (RF) noise spectrum in
    Jupiter's magnetosphere.  The LRD instrument is composed of a
    ferrite-core radio frequency antenna (~100 Hz to ~100 kHz) and
    two photodiodes mounted behind individual fisheye lenses.  The
    output of the RF antenna is analyzed both separately and in
    coincidence with the optical signals from the photodiodes.  The
    RF antenna provides data both in the frequency domain (with three
    narrow-band channels, primarily for deducing the physical
    properties of distant lightning) and in the time domain with a
    priority scheme (primarily for determining from individual RF
    waveforms the physical properties of closeby-lightning).
 
    The LRD instrument has been designed to take into account large
    uncertainties in the nature of possible Jovian lightning.  For
    example, since Jupiter has no well-defined surface close to the
    cloud system, there will be no cloud-to-ground discharges, which
    are the best understood type of lightning on Earth.  Lightning in
    general, and cloud discharges specifically, are very complex
    physical phenomena and can generate a large variety of RF pulse
    types and trains: unipolar pulses, bipolar pulses, asymmetric
    pulses, groups and bursts of pulses.  The LRD instrument is
    designed as a compact and versatile instrument which allows a
    characterization of these signals with maximum sensitivity and
    maximum dynamic range.  During the design phase, prototype
    instruments have been intensively tested with Earth lightning
    during several measuring campaigns.  The final instrument
    characteristics have been set with acceptable margins for the
    unknown conditions to Jupiter.
 
    Modelling of the propagation of RF signals in the frequency range
    of the LRD instrument in Jupiter's atmosphere shows that direct
    propagation of signals will occur to distances of order 10^4 km
    (Rinnert et al., 1979).  Hence, it is likely that Jovian
    atmospheric discharges with the energy of a typical
    cloud-to-ground discharge on Earth (order 10^8 J) will be
    detected at 10^4 km or more distance within the atmosphere with
    the LRD instrument.  As noted below, the LRD instrument also
    includes a 'superbolt' channel, in order to count extremely large
    events.
 
    Hence, in light of all the above, the flown lightning detector
    instrument must be designed to be as sensitive as possible,
    limited only by spacecraft noise.  The instrument must also cover
    as large a dynamic range as possible.
 
 
  Principal Investigator
  ======================
    The Principal Investigator for the LRD instrument was Louis
    Lanzerotti.
 
 
  Scientific Objectives
  =====================
    Radio frequency measurements are made in a reduced mode of
    operation at altitudes of ~5, 4, 3, 2 planetary radii from the
    center of Jupiter.  These data are stored in the Probe memory and
    then read out during the atmospheric descent phase of the
    mission.  During the atmospheric descent, the full complement of
    LRD data are acquired until the loss of the Probe signal by the
    over-flying Orbiter and/or the demise of the Probe due to
    atmospheric pressure and heat.
 
    The RF data obtained in the magnetosphere will be analyzed also
    jointly with the Probe Energetic Particle Instrument (EPI) data
    to gain understanding of magnetospheric particle dynamics.  In
    the magnetosphere, statistics on the characteristics of
    individual waveforms measured during a sampling interval will be
    accumulated at the four different altitudes.  In addition, noise
    levels at three different spectral frequencies (3, 5, 90 kHz)
    will be determined during the measurement intervals.
 
    In the atmosphere mode, in addition to statistics on the
    waveforms and the spectral noise levels at the three
    narrow-banded frequencies, individual waveforms will be detected,
    saved, and transmitted to Earth.  Such waveforms will provide
    powerful additional diagnostic capabilities for Jovian RF
    signals.
 
    The LRD instrument, as noted above, has been designed to be as
    sensitive as possible, limited only by the spacecraft noise, and
    to be as versatile as possible, limited only by the imposed
    limitations on power, bit rate, and reliability considerations.
    It is within these constraints that the scientific objectives
    will attempt to be achieved.  Extensive measurements with Earth
    lightning have been made and these will be continued in order to
    gain the maximum understanding of the operational characteristics
    of the instrument, and therefore the maximum science from the
    Probe descent through Jupiter's atmosphere.
 
    In both the magnetosphere and atmosphere modes the component of
    the Jovian magnetic field perpendicular to the Probe spin axis
    will be determined.  These data will be used for analyses of EPI
    data and for determining the spatial distribution of the sources
    of some of the detected lightning signals.  Further, these data
    will give engineering data on the Probe spin rate.
 
 
  Calibration
  ===========
    Because of severe constraints as to weight and power for Probe
    subsystems, the LRD instrument is very compact.  Further,
    extensive on-board compression of the data is necessary because
    of the limited available data rate.
 
    All sensors and instrument characteristics, of course, have been
    extensively tested and calibrated.  For example, radiation tests
    were carried out on the sensor electronics and pressure tests
    were made of the vented electronics box.  These latter tests
    caused a stiffening piece to be added to the microprocessor chip.
    The calibrations could be verified over long periods because of
    the delays of the launch of the Galileo spacecraft.  A further
    verification of the instrument parameters is provided by the
    on-board implemented test generator (ITG).
 
 
  Operational Considerations
  ==========================
    Magnetosphere mode.  The LRD instrument will operate in the
    pre-entry phase at distances from the planet's center of about 5,
    4, 3, and 2 Rj.  The instrument is switched on by the Probe timer
    at these locations.  In this 'magnetosphere mode' the EPI is also
    in operation.  As the Probe is still encapsulated within the heat
    shield, the MS is less sensitive and the optical sensors are
    covered.  The outputs of the LRD instrument are as noted in the
    previous section, but without the waveform snapshots.  The
    magnetosphere mode data set at each of the four locations
    consists of a 64 byte data frame with statistics and the 3 kHz
    spectral channel subdivided into parallel and perpendicular (to
    the magnetic field) channels.  The data are stored in the Probe
    memory for transmission during the atmospheric descent phase of
    the mission.
 
    Atmosphere mode.  When the LRD instrument is switched on at
    descent the instrument begins with a test cycle (IFT) and the
    first data set contains the test pulse data.  After that, the
    instrument runs continuously until the end of the mission and
    outputs a complete data set every four major frame periods, 256
    s.  These data sets contain spectral data (the 15 kHz channel
    being sectored), waveform statistics data, a 1 ms time interval
    with a selected waveform, optical data and miscellaneous data
    such as magnetic field component, spin period, and engineering
    data.  The number of complete data sets achieved during descent
    depends upon the length of time that the Probe survives and/or
    the length of time that the Probe relay signal is successfully
    acquired by the over-flying Orbiter.  For example, if the total
    atmosphere data time is ~48 min, then 10 data sets would be sent
    back (the 11th would be acquired but there would be no time for
    transmittal).  The 10 data sets would contain one test data set
    and 9 science data sets.