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.
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
Obtain a quantitative evaluation of the anomalies in Saturn's
magnetic field by performing direction-finding measurements of the
Establish if gaseous ejections from the moons Rhea, Dione, and
Tethys are responsible for the low frequency narrow-band radio
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
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
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
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.
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.
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.
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.
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
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
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., 114, 395-463, December 2004.