PDS_VERSION_ID        = PDS3                                                  
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  PUBLICATION_DATE      =  " "                                                
  NOTE                        = "Experiment description for the               
                                 Galileo Gravitational Wave Experiments       
                                 conducted in 1993 (jointly with Ulysses      
                                 and Mars Observer from DOY 081 through DOY   
                                 102), 1994 (from DOY 118 through DOY 158),   
                                 and 1995 (from DOY 140 through 160).         
                                 Formatted for display or printing with up to 
                                 78 constant-width characters per line."      
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      The Galileo Gravitational Wave Experiment is an effort to search for    
low-frequency gravitational waves generated by massive astrophysical systems. 
Gravitational waves--waves of space-time curvature--are transverse, carry     
energy and momentum, and propagate from their sources at the speed of light.  
The strength of the waves is characterized by the strain amplitude, h, which  
measures the fractional change in the separation of test masses and the       
fractional change at which separated clocks keep time.  In a spacecraft       
gravitational wave experiment, the earth and a distant spacecraft act as      
separated test masses, with the transponded 2- or 3-way Doppler signal        
continuously measuring the relative dimensionless velocity delta-v/c between  
the Earth and the spacecraft.  The metric perturbation due to the gravity     
wave, h, produces a signature in the Doppler time series that is of order h in
delta-f/f0 and is replicated three times in the Doppler time series: once when
the wave "shakes" the Earth, once when the wave shakes the spacecraft         
(suitably delayed by a one-way light time) and once when the initial shaking  
of the earth is transponded back to the earth a two-way light time later.     
This three pulse response is crucial in discrimination of gravitational waves 
from a noise background.                                                      
                                                                              
      The Gravitational Wave Experiment will be most sensitive to waves having
periods ~100-1000 seconds.  Waves with these periods are generated by         
supermassive astrophysical systems undergoing violent dynamics.  Searches will
be made for gravitational waves of differing temporal character: bursts (e.g.,
produced during formation, collision, and coalescence of supermassive black   
holes), periodic waves (e.g., produced by black holes orbiting each other) and
stochastic waves ( e.g., produced at the Big Bang).  Hybrids in this          
classification scheme (e.g. chirp waves from coalescing binaries) are also    
possible signals.                                                             
                                                                              
       During the gravitational wave experiment, care must be taken to        
maximize sensitivity.  This leads to the following general requirements:      
                                                                              
       (1)   To the extent practical, observations should be done in          
       the antisolar direction in order to minimize solar wind phase          
       scintillation noise.                                                   
                                                                              
       (2)   Tracking should be done with the highest radio frequencies       
       possible, again to minimize solar wind scintillation noise.            
                                                                              
       (3)   Tracking should be done in the two- and three-way coherent       
       modes.                                                                 
                                                                              
       (4)   Stations should be configured for maximum Doppler                
       stability.                                                             
                                                                              
       (5)   Tracking should be done during the highest elevatio angles       
       practical at each station.                                             
                                                                              
       (6)   Data should be taken using both the closed and open loop         
       receivers with the Doppler sample rate set to be as large as is        
       practical to minimize aliasing of thermal noise into the digital       
       band.                                                                  
                                                                              
       (7)   Where practical, an independent assessment of station            
       stability as well as the tropospheric and ionospheric noise            
       should be done.                                                        
                                                                              
       (8)   The spacecraft should be in quiet, minimum-dynamics modes.       
                                                                              
       (9)   Engineering telemetry from the spacecraft, logs of station       
       and spacecraft events, etc. should be gathered to create a             
       master file of "veto signals".