Voyager Wave Subsystem (PWS)
Plasma Wave Receivers (PWS) built and operated by The University of Iowa
are among the science instruments flown on Voyagers 1 and 2. Each
identical PWS instrument consists of both a 16-channel spectrum analyzer
covering the range of 10 Hertz to 56.2 kiloHertz and a wideband waveform
receiver which returns the waveform of waves in the frequency range of 40
Hertz to 12 kiloHertz. The spectrum analyzer provides data on a continual
basis with a maximum temporal resolution of one spectrum per 4 seconds.
The waveform receiver returns 4-bit samples of the electric field measured
at a rate of 28,800 samples per second. Because of the very high data
rate, the waveform samples must be transmitted in the same manner as the
Voyager imaging information. At Jupiter, some 10,000 48-second waveform
frames were obtained. At Saturn, Uranus, and Neptune, the number of frames
obtained was very small due to the lower telemetry rates available at the
greater distances of those planets.
Visit the official Voyager
Plasma Wave Experiment home page.
Science Objectives
The primary science objective of the Voyager plasma wave investigation is
to make the first surveys of the plasma wave and low frequency radio wave
spectra in the magnetospheres of the outer planets: Jupiter, Saturn,
Uranus, and Neptune. Plasma waves participate in a fundamental manner in
the dynamics of planetary magnetospheres and in the interactions of that
magnetosphere with the external solar wind and internal perturbations such
as those induced by satellites interior to the magnetosphere. Plasma waves
also provide diagnostic information about the plasma environment near the
planets including such parameters as electron density and sometimes
temperature. The instrument is also sensitive to low frequency radio
emissions and, therefore, acts as a low frequency extension to the
Planetary Radio Astronomy investigation. Radio waves are often the only
means of remotely observing regions of plasma not accessible to the
spacecraft and also lead to remote diagnostics of plasma conditions. The
plasma wave receivers are also sensitive to the results of small dust
particles impacting on various parts of the spacecraft at high velocities
and, hence, provide a direct measure of the rate of impact, the density of
the dust, and an estimate of the mass distribution of dust in the vicinity
of the large planets, especially those with rings and otherwise dusty
environments. Finally, the Plasma Wave Receiver will characterize the
plasma wave and radio wave spectrum of the outer heliosphere and perhaps
beyond, extending our understanding of solar wind plasma processes and
wave-particle interactions to several tens of Astronomical Units.
Operational Considerations
The primary operational considerations of the PWS include maintaining the
proper operating mode and obtaining waveform samples as often as the
spacecraft tape recorder/downlink capabilities allow. The standard
instrument mode is with Waveform Power On and Input Gain State Hi. For
encounter periods, this corresponds to GS3GAINHI/WFMPWRON. Since there has
never been a period when the signal levels were so high as to require the
Low input gain state, and it is highly unlikely that such levels will ever
be encountered, Low Input Gain State should never be selected. As long as
there is power margin available, it is most straightforward to leave the
Waveform Receiver Power on. The power consumption is less than 0.5 Watt
for this section, hence, the power savings afforded by turning it off is
not large. The most involved operational consideration is providing for
the transmission of waveform data to the ground. At Jupiter, the majority
of the waveform data could be sent directly to the ground via the 115200
bps downlink. This capability disappeared after Jupiter, however, because
of the greater distance to the spacecraft, hence, lower telecon rates.
Since operating the A/D converter at a rate less than 28800 Hertz would
result in aliasing, it is necessary to record the data at the 115200 bps
rate on the spacecraft tape recorder using the appropriate data mode and
playback the recorded data at a lower rate, commensurate with the link
capabilities. Again, a choice of the proper playback mode is required.
Since the data modes available on the spacecraft are highly dependent on
mission phase, these modes are not described here.
Calibration Description
The Voyager plasma wave receiver spectrum analyzers were calibrated by
first establishing a relationship between input voltage (of a sine wave at
the filter center frequency) and output voltage and second by measuring
the effective bandwidth of the filter. The bandwidth is measured by
applying a random noise signal of known spectral density and by measuring
the output voltage which, by the first part of the calibration, is related
to the rms voltage of a sine wave. Dividing the equivalent sine wave
voltage squared by the input spectral density gives a bandwidth. This
procedure is repeated for each of the frequency channels.
Voyager |
Rings Node Home
Last updated 26 August 2003