Abstract
  ========
    The Cassini Radio and Plasma Wave Science instrument consists of
    three electric field sensors, three search coil magnetometers, and a
    Langmuir probe as well as an array of receivers covering the
    frequency range from 1 Hz to 16 MHz with varying degrees of spectral
    and temporal resolution.
 
    The text of this instrument description has been abstracted from the
    instrument paper [GURNETTETAL2004]:
 
    Gurnett, D. A., W. S. Kurth, D. L. Kirchner, G. B. Hospodarsky, T.
    F. Averkamp, P. Zarka, A. Lecacheux, R. Manning, A. Roux, P. Canu,
    N. Cornilleau-Wehrlin, P. Galopeau, A. Meyer, R. Bostrom, G.
    Gustafsson, J.-E. Wahlund, L. Aahlen, H. O. Rucker, H. P. Ladreiter,
    W. Macher, L. J. C. Woolliscroft, H. Alleyne, M. L. Kaiser, M. D.
    Desch, W. M.  Farrell, C. C. Harvey, P. Louarn, P. J. Kellogg, K.
    Goetz, and A.  Pedersen, The Cassini Radio and Plasma Wave Science
    Investigation, Space Sci. Rev., in press, 2002.
 
 
  Scientific Objectives
  =====================
    The primary objectives of the Cassini Radio and Plasma Wave
    investigation are to study radio emissions, plasma waves, thermal
    plasma, and dust in the vicinity of Saturn.
 
    Objectives concerning radio emissions include:
 
      Improve our knowledge of the rotational modulation of Saturn's
      radio sources, and hence of Saturn's rotation rate.
 
      Determine the location of the SKR source as a function of
      frequency, and investigate the mechanisms involved in generating
      the radiation.
 
      Obtain a quantitative evaluation of the anomalies in Saturn's
      magnetic field by performing direction-finding measurements of the
      SKR source.
 
      Establish if gaseous ejections from the moons Rhea, Dione, and
      Tethys are responsible for the low frequency narrow-band radio
      emissions.
 
      Determine if SKR is controlled by Dione's orbital position.
 
      Establish the nature of the solar wind-magnetosphere interaction
      by using SKR as a remote indicator of magnetospheric processes.
 
      Investigate the relationship between SKR and the occurrence of
      spokes and other time dependent phenomena in the rings.
 
      Study the fine structure in the SKR spectrum, and compare with the
      fine structure of terrestrial and Jovian radio emissions in order
      to understand the origin of this fine structure.
 
    Objectives concerning plasma waves include:
 
      Establish the spectrum and types of plasma waves associated with
      gaseous emissions from Titan, the rings, and the icy satellites.
 
      Determine the role of plasma waves in the interaction of Saturn's
      magnetospheric plasma (and the solar wind) with the ionosphere of
      Titan.
 
      Establish the spectrum and types of plasma waves that exist in the
      radiation belt of Saturn.
 
      Determine the wave-particle interactions responsible for the loss
      of radiation belt particles.
 
      Establish the spectrum and types of waves that exist in the
      magnetotail and polar regions of Saturn's magnetosphere.
 
      Determine if waves driven by field-aligned currents along the
      auroral field lines play a significant role in the auroral charged
      particle acceleration.
 
      Determine the electron density in the magnetosphere of Saturn,
      near the icy moons, and in the ionosphere of Titan.
 
    Objectives concerning lightning include:
 
      Establish the long-term morphology and temporal variability of
      lightning in the atmosphere of Saturn.
 
      Determine the spatial and temporal variation of the electron
      density in Saturn's ionosphere from the low frequency cutoff and
      absorption of lightning signals.
 
      Carry out a definitive search for lightning in Titan's atmosphere
      during the numerous close flybys of Titan.
 
      Perform high-resolution studies of the waveform and spectrum of
      lightning in the atmosphere of Saturn, and compare with
      terrestrial lightning.
 
    Objectives concerning thermal plasma include:
 
      Determine the spatial and temporal distribution of the electron
      density and temperature in Titan's ionosphere.
 
      Characterize the escape of thermal plasma from Titan's ionosphere
      in the downstream wake region.
 
      Constrain and, when possible, measure the electron density and
      temperature in other regions of Saturn's magnetosphere.
 
    Objectives concerning dust include:
 
      Determine the spatial distribution of micron-sized dust particles
      through out the Saturnian system.
 
      Measure the mass distribution of the impacting particles from
      pulse height analyses of the impact waveforms.
 
      Determine the possible role of charged dust particles as a source
      of field-aligned currents.
 
 
  Calibration
  ===========
    An extensive series of amplitude calibrations, frequency responses,
    phase calibrations, and instrument performance checks were carried
    out on the RPWS prior to launch, both before and after integration
    on the spacecraft. These tests and calibrations were performed at
    room temperature (25 deg C), -20 deg C, and 40 deg C. While there are
    calibration signals available in the instrument for in-flight
    calibration purposes, these are mainly used to check for drifts due
    to aging or radiation exposure. The primary calibration information
    to derive physical units (spectral density, etc.) is derived from
    the prelaunch tests.
 
 
  Operational Considerations
  ==========================
    The different types of receivers used to cover the spectral and
    temporal range covered by the RPWS does not lend itself to a
    monolithic, synchronous mode of operation. Nevertheless, to reduce
    the magnitude of the in-flight operations to an acceptable level
    requires that many of the measurements be scheduled in a systematic
    way. The approach is to attempt to acquire survey information in the
    form of uniform spectral and temporal observations at a low enough
    data rate, ~1 kbps, to ensure that such coverage is available for
    the entire Saturnian tour and for a large portion of the cruise and
    approach to Saturn. The survey data set will support spatial
    mapping, statistical studies, and studies of dynamical effects in
    the magnetosphere and their possible correlation with solar wind
    variations. In addition to the survey information, special
    observations will be added (sometimes at greatly increased data
    rates) at specific locations or times to provide enhanced
    information required by several of the science objectives. The
    special observations may include full polarization and
    direction-finding capability or high spectral or temporal resolution
    observations by the high frequency receiver, wideband measurements
    at one of the possible bandwidths, acquisition of delta-ne/ne
    measurements, or intensive wave-normal analysis afforded by
    acquiring five-channel waveforms on an accelerated schedule. While
    minimizing the number of different modes in which the instrument is
    operated both simplifies operations and yields a more manageable
    data set, flexibility (for example in the duty cycle of wideband
    measurements) increases the likelihood that enhanced measurements
    can be integrated successfully with the resource requirements of the
    other instruments. One of the resources which will be most limited
    on Cassini is the overall data volume; RPWS requires large data
    volumes for some of its measurements.
 
 
  Detectors
  =========
    The RPWS utilizes three 10-m electric antennas, three magnetic
    antennas, and a Langmuir probe for detectors.  Three monopole
    electric field antennas, labeled Eu, Ev, and Ew, are used to provide
    electric field signals to the various receivers. The physical
    orientations of these three antennas relative to the x, y, and z
    axes of the spacecraft are given below. However, the electrical
    orientations of these are strongly affected by the asymmetric nature
    of the ground plane of the spacecraft chassis.  These electrical
    orientations are incorporated into the calibrations, primarily of
    the High Frequency Receiver.  By electronically taking the
    difference between the voltages on the Eu and Ev monopoles, these
    two antennas can be used as a dipole, Ex, aligned along the x axis
    of the spacecraft.
 
    Physical orientations of the electric monopole antennas:
    Antenna    theta (degrees) phi (degrees)
        Eu        107.5           24.8
        Ev        107.5          155.2
        Ew         37.0           90.0
 
    The angle theta is the polar angle measured from the spacecraft +z
    axis.  The angle phi is the azimuthal angle, measured from the
    spacecraft +x axis.
 
    The tri-axial search coil magnetic antennas, labeled Bx, By, and Bz,
    are used to detect three orthogonal magnetic components of
    electromagnetic waves. The search coil axes are aligned along the x,
    y, and z axes of the spacecraft.
 
    The spherical Langmuir probe is used for electron density and
    temperature measurements.  This is extended from the spacecraft in
    approximately the -x direction, in spacecraft coordinates.
 
 
  Electronics
  ===========
    The electronics consists of five receivers. These receivers are
    connected to the antennas described above by a network of switches.
    The high frequency receiver (HFR) provides simultaneous auto- and
    cross-correlation measurements from two selected antennas over a
    frequency range from 3.5 kHz and 16 MHz. By switching the two inputs
    of this receiver between the three monopole electric antennas, this
    receiver can provide direction-of-arrival measurements, plus a full
    determination of the four Stokes parameters. The high frequency
    receiver includes a processor that performs all of its digital
    signal processing, including data compression. The high frequency
    receiver also includes a sounder transmitter that can be used to
    transmit short square wave pulses from 3.6 to 115.2 kHz. When used
    in conjunction with the high frequency receiver, the sounder can
    stimulate resonances in the plasma, most notably at the electron
    plasma frequency, thereby providing a direct measurement of the
    electron number density.  The medium frequency receiver (MFR)
    provides intensity measurements from a single selected antenna over
    a frequency range from 24 Hz to 12 kHz. This receiver is usually
    operated in a mode that toggles every 32 seconds between the Ex
    electric dipole antenna and the Bx magnetic search coil, thereby
    providing spectral information for both the electric and magnetic
    components of plasma waves. The low frequency receiver (LFR)
    provides intensity measurements from 1 Hz to 26 Hz, typically from
    the Ex electric dipole antenna and the Bx magnetic antenna. The
    five-channel waveform receiver (WFR) collects simultaneous waveforms
    from up to five sensors for short intervals in one of two frequency
    bands, either 1 to 26 Hz, or 3 Hz to 2.5 kHz. When connected to two
    electric and three magnetic antennas, this receiver provides wave
    normal measurements of electromagnetic plasma waves. The wideband
    receiver is designed to provide nearly continuous wideband waveform
    measurements over a bandwidth of either 60 Hz to 10.5 kHz, or 800 Hz
    to 75 kHz. These waveforms can be analyzed on the ground in either
    the temporal domain, or in the frequency domain (Fourier
    transformed) to provide high-resolution frequency-time spectrograms.
    In a special frequency-conversion mode of operation, the high
    frequency receiver can provide waveforms to the wideband receiver in
    a 25-kHz bandwidth that is tunable to any frequency between 125 kHz
    and 16 MHz.
 
    The Langmuir probe controller is used to sweep the bias voltage of
    the probe over a range from -32 to +32 V in order to obtain the
    current-voltage characteristics of the probe, and thereby the
    electron density and temperature. The controller can also set the
    bias voltage on the Eu and Ev monopoles over a range from -10 to +10
    V in order to operate them in a current collection mode for
    delta-ne/ne measurements.
 
    The RPWS data processing unit consists of three processors. The
    first processor, called the low-rate processor, controls all
    instrument functions, collects data from the high frequency
    receiver, the medium frequency receiver, the low frequency receiver,
    and the Langmuir probe, and carries out all communications with the
    spacecraft Command and Data System (CDS). The second processor,
    called the highrate processor, handles data from the wideband and
    five-channel waveform receivers and passes the data along to the
    low-rate processor for transmission to the CDS. The third processor,
    called the data compression processor, is primarily used for data
    compression, but can also perform specialized operations such as
    on-board dust detection by using waveforms from the wideband
    receiver.