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.
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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.
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.
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.