PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 1998-05-01 NOTE = "INST.TXT contains the instrument description." END_OBJECT = TEXT END INSTRUMENT: PLANETARY RADIO ASTRONOMY RECEIVER SPACECRAFT: VOYAGER 1 Instrument Information ====================== Instrument Id : PRA Instrument Host Id : VG1 Instrument Name : PLANETARY RADIO ASTRONOMY RECEIVER Instrument Type : RADIO SPECTROMETER PI Name : JAMES W. WARWICK Build Date : UNK Instrument Mass : 7.700000 Instrument Height : UNK Instrument Length : UNK Instrument Width : UNK Instrument Manufacturer Name : MARTIN MARIETTA Instrument Serial Number : UNK Science Objectives ================== The Planetary Radio Astronomy (PRA) experiments' primary objective was to locate and explain kilometric, hectometric, and decametric radio emissions from the planets; to measure plasma resonances near the giant planets; and to detect lightning on the giant planets. The instrument was also successful at observing solar radio emissions from the perspective of the outer solar system. Radio emissions can be used to determine the rate of rotation of the inner core of a planet, to determine the existence of a magnetic field, and to search for magnetic anomalies. Radio emissions are often the only remote diagnostic for interactions occurring in the portions of magnetospheres through which a spacecraft does not pass. This is particularly true for the inner magnetosphere, which usually goes unsampled. Further information on the instrument and the investigations performed can be found in [WARWICKETAL1997]. Instrument Description ====================== The Voyager Planetary Radio Astronomy (PRA) instrument consisted of two superheterodyne receivers, one for the range from 1326.0 to 1.2 kHz (center frequency) and a second for the range 40.55 to 1.53 MHz (center frequency). The channels are numbered so that the lowest frequency channel (1.2 kHz) is channel number 198. Channels in the range 1 to 128 are referred to as 'high band', and the remaining channels as 'low band'. In low band, the channel spacing is 19.2 kHz. In high band, the channel spacing is 307.2 kHz. The PRA receiver is driven by two orthogonal antennas mounted on the spacecraft body. Each antenna element is made of BeCu hollow tubes 0.5 inches in diameter and is 10 meters in length. By combining the signals from the two antennas in a 90 degree hybrid, the PRA instrument can distinguish between the opposing states, left hand and right hand, of circular polarization of an incoming wave. The Planetary Radio Astronomy (PRA) receivers were calibrated under environmentally-controlled conditions and over the entire frequency and dynamic range of the instruments. This calibration consisted in application of a known narrow-band signal across the inputs and recording the receiver outputs. The laboratory calibrations provided power levels for each data number (DN) and each frequency in terms of known inputs across the antenna terminals of each of the experiment's two monopoles. Calibrations were carried out over a range of receiver temperatures, but in practice the stability of the receiver as a function of temperature and the stability of the temperature of the receiver as a function of mission phase and the status of the overall spacecraft were such that a single calibration for each DN at each frequency could be used. Receiver output levels were quantized. The minimum value for the wave flux density was frequency dependent varying from 5.E-20 W M**-2 Hz**-1 at frequencies below 1.5 MHz to 5.E-19 at frequencies above 1.5 MHz. The maximum wave flux density was typically 50 dB above the minimum value. The instrument noise level also was frequency dependent. It was about 1.E-19 W M**-2 Hz**-1 below 1.5 MHz. The noise at 10 MHz was still about 1E-19 W M**-2 Hz**-1, increased to about 1.E-17 W M**-2 Hz**-1 at 25 MHz, and then decreased to an intermediate value at 40 MHz. The low-band and high-band operation of the receiver differ. In low-band the receiver operated with a sharply tuned filter only 1 kHz broad at the 3 dB points and in high-band, with a 200 kHz filter. The gain of the receivers was designed in such a way that the output increased discontinuously by 23 dB (corresponding to the 200:1 bandwidth ratio) between the lowest frequency of high-band and the highest frequency of low-band. This caused the instrument output to remain constant across the high-band to low-band transition point if its input was broadband noise. If unpolarized radiation fell orthogonally on each monopole, the total unpolarized flux density for signals below about 5 MHz could be roughly estimated to be S = So (10**(m/1000)), where m was the channel reading in millibels and So is So = 1.5E-21 (W/Hz m**2). No reliable method for estimating the flux density exists for frequencies above 5 MHz due to the increasing effect of antenna resonances. Although the PRA instrument had 14 possible operating modes, in practice the mode called POLLO was used more than 95% of the time. In POLLO, the receiver swept through all 198 channels in sequence from the highest frequency to the lowest. At each frequency step, data were produced every 30 msec, consisting of 25 msec of integration and 5 msec of switching and settling time. Thus, a full sweep through all 200 channels took 6 sec (including 60 msec for two status words). Between steps the 90 degree hybrid was switched such that the receiver was sensitive to the alternate sense of circular polarization. This toggling between left hand and right hand polarization itself alternated with each 6 sec receiver sweep. Thus, for a given frequency, a pair of left hand and right hand measurements were 6 sec apart.