PI: Prof. Donald A. Gurnett
RPWS General Description:
The major functions of the Radio and Plasma Wave Science (RWPS) instrument are to measure the electric and magnetic fields and electron density and temperature in the interplanetary medium and planetary magnetospheres.
RPWS Scientific Objectives:
- To study the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR).
- To monitor and map the sources of SKR.
- To study daily variations in Saturn's ionosphere and search for outflowing plasma in the magnetic cusp region.
- To study radio signals from lightning in Saturn's atmosphere.
- To investigate Saturn Electric Discharges (SED).
- To determine the current systems in Saturn's magnetosphere and study the composition, sources, and sinks of magnetospheric plasma.
- To investigate the dynamics of the magnetosphere with the solar wind, satellites, and rings.
- To study the rings as a source of magnetospheric plasma.
- To look for plasma waves associated with ring spoke phenomena.
- To determine the dust and meteroid distributions throughout the Saturnian system and interplanetary space.
- To study waves and turbulence generated by the interaction of charged dust grains with the magnetospheric plasma.
- To investigate the interactions of the icy satellites and the ring systems.
- To measure electron density and temperature in the vicinity of Titan.
- To study the ionization of Titan's upper atmosphere and ionosphere and the interactions of the atmosphere and exosphere with the surrounding plasma.
- To investigate the production, transport, and loss of plasma from Titan's upper atmosphere and ionosphere.
- To search for radio signals from lightning in Titan's atmosphere, a possible source for atmospheric chemistry.
- To study the interaction of Titan with the solar wind and magnetospheric plasma.
- To study Titan's vast hydrogen torus as a source of magnetospheric plasma.
- To study Titan's induced magnetosphere.
RPWS Sensing Instruments:
- Electric Field Sensor
- Magnetic Field Sensor
- Langmuir Probe
- High, Medium, and Wide Band Receivers
RPWS Instrument Characteristics:
- Mass (current best estimate) = 6.80 kg
- Average Operating Power (current best estimate) = 7.00 W
- Average Data Rate (current best estimate) = 0.90 kilobits/s
The Radio and Plasma Wave Science (RPWS) instrument will be used to investigate electric and magnetic waves in space plasma at Saturn. Plasma is essentially a soup of free electrons and positively charged ions, the latter being atoms that have lost one or more electrons. Plasma makes up most of the universe and is created by the heating of gases by stars and other bodies in space. Plasma is distributed by the solar wind, and it is also "contained" by the magnetic fields (i.e., the magnetoshperes) of bodies such as Saturn and Titan. The Cassini RPWS instrument will measure the a.c. electric and magnetic fields in the interplanetary medium and planetary magnetospheres and will directly measure the electron density and temperature of the plasma in the vicinity of the spacecraft.
The major components of the RPWS Subsystem are the electric field sensor, the magnetic search coil sensor assembly, the Langmuir probe sensor assembly, and the instrument main electronics.
The electric field sensor is made up of three deployable antenna elements, an associated preamplifier, and antenna deployment mechanism drive electronics. The antennas are composed of interlocking sections made from beryllium copper, and each antenna element is deployable separately to 10 meters with its own 400-Hz a.c. motor. The electric field preamplifier is used to add gain to the output signals from the antennas. The antenna deployment mechanism electronics convert 15 volt primary power to 400-Hz a.c. power for the antenna drive motors.
The magnetic search coil sensor assembly is composed of a triaxial sensor assembly and an associated preamplifier. The triaxial sensor consists of three orthogonal (i.e., perpendicular) metallic alloy cores with two sets of windings each, one to produce flux in the core and another to detect the flux. The magnetic search coil preamplifier adds gain to the output signal from the sensor assembly.
The Langmuir probe sensor assembly consists of a sensor, a preamplifier, and associated control electronics. The Langmuir probe sensor is a 5-cm diameter sphere located at the end of a rod approximately 1 meter in length. The sensor rod is folded in a stowed state until deployed in flight. The probe sensor preamplifier adds gain to the output from the probe.
The RPWS main electronics includes a digital data processing unit, a high-frequency receiver, a wideband receiver, a medium-frequency receiver, a low-frequency five-channel waveform receiver, the Langmuir probe bias circuitry, and a power converter.
The data processing unit (DPU) will control all instrument functions and will handle all communications with the orbiter. It will contain a large block of RAM to be used as waveform storage for the five-channel waveform receiver. Software in the DPU will be used to enhance the scientific return of the instrument by performing various analysis and data compression operations.
The high-frequency receiver is a digital waveform processor that operates by digitizing a portion of the bandwidth received from the electric field sensor antennas and deriving spectral and waveform vector information from the waveforms using digital signal processing techniques.
The wideband receiver will obtain very high-resolution electric or magnetic field waveforms for selected time intervals that vary from under a minute to as much as an hour or more. The receiver has two selectable passbands: 50 Hz to approximately 10 kHz and 10 kHz to approximately 80 kHz. The wideband receiver uses high-rate telemetry to transfer waveform information in a given bandwidth from a selected sensor directly. The input signal is selectable from one of five inputs: two electric, one magnetic, a frequency-converted output from the high-frequency receiver, and the Langmuir probe.
The medium-frequency receiver provides spectrum measurements over the frequency range from 25 Hz to 12.6 kHz. This receiver is attached to one of four sensor inputs (two electric and two magnetic) and uses double frequency conversion to convert the input bandwidth down to a low-frequency constant frequency band, where it is detected by an amplitude detector.
The low-frequency five-channel waveform receiver provides high-resolution spectral measurements of electric and magnetic fields over the frequency range from 0.1 Hz to 2.5 kHz. It provides simultaneous waveforms from all five antennas (three magnetic axes and two electric axes). This receiver captures blocks of waveform data simultaneously from the five sensors and maintains a high-degree of phase and amplitude accuracy. The data is processed through five parallel amplifier and filter channels.
The power converter converts d.c. power from the spacecraft power supplies to a.c. power to operate the instrument. The conversion frequency will be fixed at 100 kHz by locking onto a signal from the spacecraft bus interface unit.
For additional information, see: Cassini RPWS Home Page