The Planetary Radio Astronomy (PRA) experiments' primary
objective is 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. They have also been successful at observing
solar radio emissions from the perspective of the outer solar
system.

  The Voyager Planetary Radio Astronomy experiment is designed to        
  investigate naturally-occurring radio emissions from the outer         
  planets and Sun. Radio emissions from Jupiter have been known          
  from Earth-based measurements since 1955 (Burke, B. F., and K. L. Franklin,
  Observations of a variable radio source associated with planet
  Jupiter, J. Geophys. Res., 60, 213–217, 1955.); PRA represents the           
  first attempt to survey those emissions, and to perform near-          
  encounter searches for radio emissions from the other gas              
  planets.                                                               
                                                                         
  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 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.                     
                                                                         
  PRA is also sensitive to impacts on the spacecraft by micron-          
  sized dust particles. Particularly in its high data rate modes,        
  the information obtained therefrom produces insights into the          
  processes which occur under such situations.                           

  Instrument Description                                                 
  ======================                                                 
  There were two receivers on each spacecraft, for the
  lower and higher frequency ranges, respectively. The low-band
  receiver had 70 channels of 1.0 kHz bandwidth each, with center
  frequencies spaced at 19.2 kHz intervals from 1.2 kHz to 1326kHz.
  The high-band receiver consisted of 128 channels of 200 kHz
  bandwidth each, with center frequencies spaced at 307.2 kHz
  intervals from 1.2 MHz to 40.4 MHz. The high-band receiver was
  designed especially for the observation of Jovian decametric radio
  emissions.
                                                                         
  The PRA receivers were 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.            

  For further details on the PRA instrument see
  Warwick, J.W. et al., Planetary Radio Astronomy Experiment for
  Voyager Missions, Space Science Reviews, 21, 309-327, 1977 and,

  Lang, G.J. and Peltzer, R.G., Planetary Astronomy Receiver,
  IEEE Transactions on Aerospace and Elecronics Systems, AES-13, 466-472, 1977.
                                                                         
  For further details on calibration see Wang, L. and Carr, T.D.,
  Recalibration of the Voyager PRA antenna for polarization sense
  measurement, Astron. Astrophys., 281, 945-954, 1994. and references therein.