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  <Instrument>
    <ResourceID>spase://CDPP/Instrument/AMDA/Galileo/PWS</ResourceID>
    <ResourceHeader>
      <ResourceName>Galileo PWS</ResourceName>    
      <AlternateName>Galileo Plasma Wave Spectrometer</AlternateName>
      <AlternateName>Galileo Plasma Wave Subsystem</AlternateName>
      <AlternateName>Galileo Plasma Wave Investigation</AlternateName>
      <AlternateName>Galileo Plasma Wave Receiver</AlternateName>
      <ReleaseDate>2012-11-27T21:10:13Z</ReleaseDate>
      <Description>

The Galileo Plasma Wave Receiver is described by     


      Gurnett, D. A., W. S. Kurth, R. R. Shaw, A. Roux, R. Gendrin, C. F. Kennel,
    F. L. Scarf, and S. D. Shawhan, The Galileo Plasma Wave Investigation, 
    Space Sci. Rev., 60, 341-355, 1992.
                          
                                                                              
Scientific Objectives:                                                        
                                                                              
The basic objective of this investigation is the study of plasma              
waves and radio emissions in the magnetosphere of Jupiter.  The               
Voyager 1 and 2 flybys of Jupiter have now clearly shown that many            
complex types of plasma wave and radio-emission phenomena occur in the        
Jovian magnetosphere.  These include electromagnetic whistler mode            
emissions called chorus and hiss, electromagnetic continuum radiation         
trapped in the magnetospheric cavity, electrostatic waves associated          
with harmonics of the electron cyclotron frequency, and a wide variety        
of escaping radio emissions.  Some of these waves, such as the whistler       
mode emissions, are believed to play an important role in the dynamics        
of the magnetosphere by controlling the pitch-angle scattering and loss       
of energetic charged particles.  In other cases plasma waves provide an       
important diagnostic tool by revealing various characteristic                 
frequencies of the plasma, from which quantities such as the electron         
density can be computed.                                                      
                                                                              
Since the Galileo spacecraft will be the first orbiter of Jupiter,            
this spacecraft will provide a much more comprehensive study of the           
Jovian magnetosphere than was possible with the previous Pioneer and          
Voyager flybys of Jupiter.  Specifically, the orbit of Galileo will           
provide a survey of the magnetotail at distances of up to 150 RJ over a       
range of local times near local midnight, a region that has never             
previously been explored; repeated passes through the plasma sheet, and       
the tail lobes; and numerous close flybys of the Galilean satellites.         
Of particular importance will be a very close pass by the satellite Io.       
The Voyager flybys showed that volcanic gases escaping from this moon         
are the main source of plasma in the Jovian magnetosphere.  The primary       
energization of plasma in the Jovian magnetosphere is believed to occur       
in a dense plasma torus that surrounds Jupiter near Io's orbit.  This         
energization is associated with many complex plasma wave phenomena,           
including the generation of intense kilometric and decametric radio           
emissions.                                                                    
                                                                              
In addition to exploring regions never previously investigated,               
Galileo, by virtue of its long lifetime in orbit around Jupiter, also         
provides a unique new capability for carrying out studies of temporal         
variations on time scales that cannot be investigated with a single           
flyby.  For example, it is known that the kilometric and decametric           
radio emissions associated with Io and its plasma torus have temporal         
variations on time scales of weeks and longer. With Galileo these             
temporal variations can be monitored over periods of several years and        
compared with other remote sensing instruments. These measurements            
should be able to tell us, for example, whether the variations are            
associated with changes in the volcanoes on Io. Considerable interest         
also exists in searching for evidence of magnetospheric substorm              
phenomena, possibly comparable to auroral substorms in the Earth's            
magnetosphere.  With the Galileo plasma wave instrument, it should be         
possible to provide remote sensing of substorms in a manner comparable        
to the remote sensing of terrestrial auroral kilometric radiation,            
which is known to be closely associated with terrestrial substorms.           
                                                                              
To carry out comprehensive studies of plasma waves and radio                  
emissions at Jupiter, the Galileo plasma wave instrument incorporates         
several new features that provide improvements over the previous              
Voyager 1 and 2 measurements.  These improvements include (1) nearly          
simultaneous electric and magnetic field measurements to distinguish          
electrostatic waves from electromagnetic waves, (2) direction finding         
measurements to determine source locations, and (3) better frequency          
and time resolution to resolve fine structure in the plasma wave and          
radio emission spectrum.  The main instrument package and the electric        
dipole antenna system were designed and constructed at the University         
of Iowa, and the search coil magnetic antenna was provided by the             
Centre de Recherches en Physique de l'Environnement Terrestre et              
Planetaire (CRPE).                                                            
                                                                              
                                                                              
Calibration:                                                                  
                                                                              
                                                                              
An extensive series of calibrations and performance checks were               
performed on the plasma wave instrument both on and off the spacecraft.       
Since the logarithmic compressors used in the spectrum analyzers do           
not give a true logarithmic response, the transfer function of the            
logarithmic compressors must be calibrated.  Because of the large             
number of channels, it is not practical to calibrate each frequency           
channel separately.  Instead, the transfer function is measured for           
each logarithmic compressor, and a frequency response calibration is          
performed at a fixed amplitude for all channels using that compressor.        
This procedure provides accurate calibrations for each frequency step         
because each band of the receiver uses a single filter and logarithmic        
compressor.                                                                   
                                                                              
A look-up table can be constructed which converts the telemetry               
data number to input signal strength.  When combined with the overall         
frequency response across each band, these calibrations are sufficient        
to determine the signal strength in all channels served by this filter        
band and compressor.                                                          
                                                                              
In addition to the amplitude response of the compressors, a                   
frequency response is also performed for each frequency channel. All          
frequency channels are checked to confirm that the filter bands have          
the proper shape and no spurious responses.  The effective noise              
bandwidths are measured by stimulating the instrument with a white            
noise signal of known spectral density.                                       
                                                                              
For the electric field antenna, the electric field strength is                
computed by assuming that the antenna has an effective length of              
Leff = 3.5 meters. This length is the distance between the geometric          
centers of the two dipole elements.  For the search coil magnetic             
antennas, the magnetic field sensitivity and frequency response was           
calibrated in the IPG magnetic field observatory at Chambon La Foret,         
France.  These calibrations were performed using a Helmholtz coil             
driven by a known AC current source.  The absolute accuracy of the            
sensitivity calibration is estimated to be about 3 percent. The               
magnetic noise levels were measured by placing the search coils in a          
mu-metal chamber, which shields the sensors from external noise               
sources.                                                                      
                                                                              
                                                                              
Operational Considerations:                                                   
                                                                              
                                                                              
Nominally, the instrument is operated any time low rate science               
(LRS) or greater data rate capability is available.  When LRS is the          
maximum data rate, the instrument is operated in its power-up mode,           
with the SA and SFR toggling back and forth between the electric and          
magnetic antennas.  When wideband data can be recorded or transmitted         
to the Earth, then the wide range of instrument modes and antenna             
configurations are utilized based on the science objectives for a given       
time interval.  The UVS instrument has a stepper motor that drives its        
grating which is a major source of magnetic interference in the               
frequency range from about 100 Hz to 2 kHz.  Every attempt is made to         
work with the UVS team to minimize the times during which the grating         
is moved while PWS is observing on the magnetic antenna.  In many             
cases, this requires a time- sharing arrangement which allows for some        
percentage of magnetic viewing time in an interference-free                   
environment, but which also allows UVS to observe with a moving grating       
in order to achieve its science objectives.                                   
                                                                              
Detectors:                                                                    
                                                                              
                                                                              
The plasma wave sensors on Galileo consist of one 6.6 meter                   
tip-to-tip electric dipole antenna and two search coil magnetic               
antennas.  The electric dipole antenna is mounted at the end of the           
magnetometer boom approximately 10.6 meters from the spacecraft, and          
the search coil magnetic antennas are mounted on the high gain antenna        
feed.  The electric antenna consists of two graphite epoxy elements           
with a root diameter of 2.0 cm, tapering to 0.3 cm at the tip.  The           
dipole elements are mounted perpendicular to the magnetometer boom to         
minimize electric field distortion effects due to the spacecraft              
structure.  The antenna axis is also oriented perpendicular to the            
spacecraft spin axis in order to permit direction finding.  Each              
element is hinged 1.8 meters from the tip so that the antenna can be          
folded for launch.  A housing at the base of the dipole elements              
contains two preamplifiers.  These preamplifiers provide low impedance        
signals to the main electronics package. Each element is grounded to          
the spacecraft structure through a 250 MegOhm resistance to limit             
differential charging effects.                                                
                                                                              
The search coil magnetic antenna consists of two high permeability            
rods, 25.5 and 27.5 cm long, one optimized for low frequencies, 5 Hz to       
3.5 kHz, and the other optimized for high frequencies, 1 kHz to 50 kHz.       
The winding on the low frequency search coil consists of 50,000 turns of      
0.07 mm diameter copper wire and the winding on the high frequency            
search coil consists of 2000 turns of 0.14 mm diameter copper wire.           
The two search coils are mounted orthogonally to minimize the electrical      
coupling between the sensors.  Both search coils are mounted                  
perpendicular to the spacecraft spin axis.  The high frequency sensor is      
perpendicular to the electric dipole antenna and the low frequency            
sensor is parallel to the electric dipole antenna.  Two preamplifiers         
mounted in a housing near the search coil are used to provide low             
impedance signals to the main electronics package.  Frequencies below         
2.4 kHz are obtained from the low frequency search coil, and                  
frequencies above 2.4 kHz are obtained from the high frequency search         
coil.                                                                         
                                                                              
Electronics:                                                                  
                                                                              
All of the signal processing for the plasma wave experiment is                
performed in a single main electronics package.  The main electronics         
package is mounted in the spacecraft body near the base of the                
magnetometer boom. Signals from the electric dipole antenna and the two       
search coils are processed by a wideband receiver and three spectrum          
analyzers:  a high frequency spectrum analyzer also called the High           
Frequency Receiver (HFR), a medium frequency spectrum analyzer also           
called the Sweep Frequency Receiver (SFR), and a low frequency spectrum       
analyzer also called simply the Spectrum Analyzer (SA).  The HFR              
provides 42 frequencies from 100.8 kHz to 5.645 MHz with a fractional         
frequency spacing of delta-f/f ~ 10.0% and a bandwidth of 2 kHz.  One         
spectral sweep is provided every 18.67 seconds with a dynamic range of        
100 db.  The SFR provides 112 frequencies from 40 Hz to 160 kHz with a        
fractional frequency spacing of delta-f/f ~ 8.0%.  This analyzer gives        
one spectral sweep every 18.67 seconds with a dynamic range of 100 db.        
The low frequency SA provides 4 logarithmically spaced frequency              
channels from 5.62 Hz to 31.1 Hz.  All four channels are sampled once         
every 2.67 seconds with a dynamic range of 110 db.  The data from the         
HFR, SFR, and SA (and survey wideband data as described below) are            
transmitted to the ground via the low rate telemetry at a bit rate of         
240 bits/sec.                                                                 
                                                                              
The wideband waveform receiver provides waveform measurements in              
three frequency bands, 5 Hz to 1 kHz, 50 Hz to 10 kHz, and 50 Hz              
to 80 kHz.  The frequency band to be used is controlled by the                
spacecraft Command and Data Subsystem (CDS).  An automatic gain control       
(AGC) circuit is used to control the amplitude of the output waveform.        
The AGC time constant is 0.1 seconds in the two high frequency bands          
and 1.0 second in the low frequency band.  The waveform from the              
wideband receiver is digitized by a 4-bit analog-to-digital converter         
(ADC).  The sample rate of the ADC is fixed at either 3,150, 25,200, or       
201,600 samples per second, depending on the frequency band selected.         
The waveform data can be transmitted in real time or recorded on the          
spacecraft digital tape recorder.                                             
                                                                              
The plasma wave instrument has several modes of operation and                 
methods of data transmission.  These modes are also controlled by             
the spacecraft CDS. The medium and low frequency spectrum analyzers and       
the wideband waveform receiver can be connected to either the electric        
dipole antenna or the search coil magnetic antennas.  In the normal           
mode of operation, the SFR and SA are cycled between the electric and         
magnetic antennas so that alternate electric and magnetic spectrums are       
obtained.  Since the search coils do not provide signals in the               
frequency range covered by the HFR, this analyzer is always connected         
to the electric antenna.  In the cycling mode of operation, the time          
required for a complete set of electric and magnetic field spectrums is       
37.33 seconds.  The SFR and SA can also be locked on either the               
electric or magnetic antennas to provide improved time resolution at          
the expense of complementary electric and magnetic field coverage.  In        
all cases the HFR, SFR, and SA outputs consist of an 8-bit binary             
numbers that are approximately proportional to the logarithm of the           
received signal strength.  In the ground data processing the data from        
the HFR, SFR, and SA will be displayed in the form of color                   
frequency-time spectrograms.  The frequency scale of the Galileo              
spectrograms will extend from 5.6 Hz to 5.65 MHz, and variable time           
scales will be available, ranging from 30 minutes to more than 24             
hours, depending on the application.  Normally, 24-hour spectrograms          
will be used to survey the plasma wave data.  These survey spectrograms       
will be used to select specific intervals for more detailed analysis,         
such as comparison with charged particle or magnetic field data, or           
direction-finding analyses.                                                   
                                                                              
The greatest flexibility in the operation of the plasma wave                  
instrument is available in the wideband waveform receiver.  This              
receiver provides very high resolution measurements of electric and           
magnetic field waveforms during times of special interest, such as the        
pass through the Io torus and satellite encounters.  The waveform data        
provide the highest possible frequency and time resolution, subject           
only to the constraints of Fourier analysis, delta-f*delta-t ~ 1.             
Although the waveform receiver has only three frequency bands, with bit       
rates of 12.6, 100.8, and 806.4 kbits/sec, several spacecraft modes are       
available for recording and transmitting the data to the ground.  In          
the highest time resolution mode, an essentially continuous sample of         
the electric or magnetic field waveform can be obtained over a                
bandwidth of 50 Hz to 80 kHz for periods of up to 18 minutes (the time        
required to fill the spacecraft tape recorder).                               
                                                                              
On the ground the waveform data will be Fourier transformed in                
discrete packets, usually consisting of 1024 samples, and                     
displayed in the form of a frequency-time spectrogram.  These                 
frequency-time spectrograms provide the highest time resolution data          
available from the Galileo plasma wave instrument.  In certain modes of       
operation, such as MPW, XPW, and PW4, the duration of the wideband            
recording can be extended at the expense of reduced duty cycle,               
frequency coverage, or analysis bandwidth.  To provide some wideband          
telemetry even when the high rate telemetry link is not available, a          
waveform survey output is included in the regular low rate telemetry          
data. This waveform survey output provides one block of 280 waveform          
samples every 18.67 seconds in two frequency bands, 5 Hz to 1 kHz and         
50 Hz to 10 kHz.                                                              
                                                                              
Filters:                                                                      
                                                                              
The following three tables describe the 158 frequency channels                
which make up the low rate science portion of the Galileo PWS.                
                                                                              
Table 1.  Spectrum Analyzer (SA) Channels                                     
                                                                              
Channel   MOD(mf,4) Center Frequency (Hz)    Bandwidth (Hz)                   
1         0 (4)           5.62               0.832                            
2         3              10.0                1.86                             
3         2              17.8                2.75                             
4         1              31.1                4.79                             
                                                                              
mf is the minor frame counted from 1 through 28.                              
                                                                              
                                                                              
Table 2.  Sweep Frequency Receiver (SFR) Channels                             
                                                                              
Chan mf        Freq. (Hz)     Bandwidth (Hz)                                  
                                                                              
Band 1                                                                        
1     1       42.1             4.26                                           
2     2       45.6                                                            
3     3       49.0                                                            
4     4       52.5                                                            
5     5       56.0                                                            
6     6       59.6                                                            
7     7       66.7                                                            
8     8       70.4                                                            
9     9       77.7                                                            
10    10      81.5                                                            
11    11      89.0                                                            
12    12      96.7                                                            
13    13     104.5                                                            
14    14     112.5                                                            
15    15     120.6                                                            
16    16     128.9                                                            
17    17     137.3                                                            
18    18     150.2                                                            
19    19     158.9                                                            
20    20     172.5                                                            
21    21     186.4                                                            
22    22     200.7                                                            
23    23     215.5                                                            
24    24     235.9                                                            
25    25     251.7                                                            
26    26     268.0                                                            
27    27     290.6                                                            
28    28     314.1                                                            
                                                                              
Band 2                                                                        
29    1      337.              6.76                                           
30    2      364.                                                             
31    3      392.                                                             
32    4      420.                                                             
33    5      448.                                                             
34    6      476.                                                             
35    7      534.                                                             
36    8      563.                                                             
37    9      622.                                                             
38    10     652.                                                             
39    11     712.                                                             
40    12     774.                                                             
41    13     836.                                                             
42    14     900.                                                             
43    15     965.                                                             
44    16       1.031k                                                         
45    17       1.098k                                                         
46    18       1.201k                                                         
47    19       1.272k                                                         
48    20       1.380k                                                         
49    21       1.491k                                                         
50    22       1.606k                                                         
51    23       1.724k                                                         
52    24       1.887k                                                         
53    25       2.013k                                                         
54    26       2.144k                                                         
55    27       2.325k                                                         
56    28       2.513k                                                         
                                                                              
Band 3                                                                        
57    1        2.70k           120.                                           
58    2        2.91k                                                          
59    3        3.14k                                                          
60    4        3.36k                                                          
61    5        3.58k                                                          
62    6        3.81k                                                          
63    7        4.27k                                                          
64    8        4.50k                                                          
65    9        4.98k                                                          
66    10       5.21k                                                          
67    11       5.70k                                                          
68    12       6.19k                                                          
69    13       6.69k                                                          
70    14       7.20k                                                          
71    15       7.72k                                                          
72    16       8.25k                                                          
73    17       8.78k                                                          
74    18       9.61k                                                          
75    19      10.17k                                                          
76    20      11.04k                                                          
77    21      11.93k                                                          
78    22      12.85k                                                          
79    23      13.79k                                                          
80    24      15.09k                                                          
81    25      16.11k                                                          
82    26      17.15k                                                          
83    27      18.59k                                                          
84    28      20.10k                                                          
                                                                              
Band 4                                                                        
85    1       21.6k            1520.                                          
86    2       23.3k                                                           
87    3       25.1k                                                           
88    4       26.9k                                                           
89    5       28.7k                                                           
90    6       30.5k                                                           
91    7       34.2k                                                           
92    8       36.0k                                                           
93    9       39.8k                                                           
94    10      41.7k                                                           
95    11      45.6k                                                           
96    12      49.5k                                                           
97    13      53.5k                                                           
98    14      57.6k                                                           
99    15      61.7k                                                           
100   16      66.0k                                                           
101   17      70.3k                                                           
102   18      76.9k                                                           
103   19      81.4k                                                           
104   20      88.3k                                                           
105   21      95.4k                                                           
106   22     102.8k                                                           
107   23     110.3k                                                           
108   24     120.7k                                                           
109   25     128.9k                                                           
110   26     137.2k                                                           
111   27     148.8k                                                           
112   28     160.8k                                                           
(Note that the same bandwidth applies to the entire set of channels in        
band.)                                                                        
                                                                              
                                                                              
Table 3.  High Frequency Receiver (HFR) Channels                              
                                                                              
HFR                  Center Frequency                                         
Channel    mf             (MHz)                                               
1          1, 2          0.1008                                               
2          5, 6          0.1134                                               
3          9, 10         0.1260                                               
4          13, 14        0.1386                                               
5          17, 18        0.1512                                               
6          21, 22        0.1638                                               
7          25, 26        0.1764                                               
8          3, 4          0.2016                                               
9          7, 8          0.2268                                               
10         1, 12         0.2520                                               
11         5, 16         0.2772                                               
12         9, 20         0.3024                                               
13         3, 24         0.3276                                               
14         7, 28         0.3528                                               
15         1             0.4032                                               
16         5             0.4536                                               
17         9             0.5040                                               
18         13            0.5544                                               
19         17            0.6048                                               
20         21            0.6552                                               
21         25            0.7056                                               
22         2             0.8060                                               
23         6             0.9070                                               
24         10            1.008                                                
25         14            1.109                                                
26         18            1.210                                                
27         22            1.310                                                
28         26            1.411                                                
29         3             1.613                                                
30         7             1.814                                                
31         11            2.016                                                
32         15            2.218                                                
33         19            2.419                                                
34         23            2.621                                                
35         27            2.822                                                
36         4             3.226                                                
37         8             3.629                                                
38         12            4.032                                                
39         16            4.435                                                
40         20            4.838                                                
41         24            5.242                                                
42         28            5.645                                                
(The bandwidth for all channels is 1340 Hz)                                   
                                                                              
Mounting Offsets:                                                             
                                                                              
The electric antenna is mounted at the end of the magnetometer                
boom such that its effective axis is parallel to the spacecraft X             
axis (perpendicular to both the magnetometer boom and the spacecraft          
spin axis.  The low frequency magnetic search coil is mounted with its        
effective axis parallel to the spacecraft X axis and the high frequency       
search coil is parallel to the spacecraft Y axis (perpendicular to the        
X axis and to the spacecraft spin axis.                                       
                                                                              
Field of View:                                                                
                                                                              
The field of view of the PWS, whether from the electric dipole                
antenna or one of the magnetic search coils is a standard dipole              
antenna pattern which has a maximum sensitivity to the field along the        
axis of the sensor.  For radio waves which propagate above the                
characteristic frequencies of the plasma and which do not interact with       
the local plasma, this means maximum sensitivity is to sources in a           
plane perpendicular to the antenna axis, since the electric and               
magnetic fields of a radio wave are normally perpendicular to the             
propagation vector.                                                           
                                                                              
Data Rates:                                                                   
                                                                              
The basic low rate science (LRS) data rate of the instrument is               
240 bps. Wideband waveform receiver data rates range from 19.2                
kbps to 806.4 kbps, depending on the telemetry mode.  Rates of 94._           
kbps and lower can be either recorded on the spacecraft tape recorder         
or transmitted directly to the ground; rates of 403.2 and 806.4 kbps          
can only be recorded onboard for later playback at lower rates.               
                                                                              
Instrument Modes:                                                             
                                                                              
The PWS has several modes of operation.  The SA and SFR can either            
monitor only the electric antenna, only the magnetic antenna, or toggle       
back and forth between the two, obtaining a complete spectral scan from       
each antenna before switching to the other.  This toggling mode is the        
most commonly utilized mode.  There is also a mode which enables the          
magnetic search coil calibration tone.  When enabled, the calibration         
signal is excited for 56 mf (37.33 sec) at the beginning of each              
MOD(RIM,8)=0 until disabled.                                                  
                                                                              
The wideband receiver has three basic modes (analysis bandwidths)             
and can be attached to either the electric antenna or the magnetic.           
The three modes provide analysis bandwidths of 10 kHz, 80 kHz, and            
1 kHz although there is an additional mode which toggles between the          
10 kHz and 1 kHz mode.  In this mode, the waveform data is collected          
for 14 mf (9.33 sec) in one bandwidth and then for 14 mf in the other         
bandwidth.                                                                    
                                                                              
A wide range of telemetry formats are available for the wideband              
data. For any one of the three wideband modes, the instantaneous              
data rate is fixed (806.4 kbps for the 80 kHz mode, 100.8 kbps for the        
10 kHz mode, and 12.6 kbps for the 1 kHz mode.  However, the different        
telemetry modes differ primarily in the number of consecutive samples         
collected during an RTI.  This results in a variation in the duty cycle       
depending on the data rate allocated to this data stream in the               
selected telemetry mode.                                                      
                                                                              
Phase 2 Software Implications:                                                
                                                                              
In response to the failure of the high gain antenna and the                   
resulting reduction in downlink telecommunications capabilities for the       
Galileo spacecraft, the Galileo Project undertook a massive                   
re-programming of onboard software in order to enable science                 
observations in Jupiter orbit.  Coupled with flight software changes,         
modifications to the Deep Space Network were also undertaken to improve       
the overall downlink capability from the spacecraft.  Together, these         
actions increased the actual bit-to-ground capability from a maximum of       
about 10 bps to a maximum of 160 bps.  In addition, onboard data              
compression was implemented which increased the information content of        
the downlink by roughly a factor of 10.                                       
                                                                              
The PWS instrument has no microprocessor; all of its functions                
are hardwired, hence, no reprogramming of the instrument itself was           
possible.  One result of this is that the basic timing of the                 
instrument is identical to that described in the section above.               
However, significant software additions in the CDS and AACS were              
implemented which enable the PWS to perform its basic observations at         
dramatically lower data rates.  These changes can be categorized simply       
as editing and compression. The LRS observations are edited as                
described below to reduce the observations retained for downlink from         
240 bps to 65 bps.  The remaining 65 bps LRS data stream is then              
compressed using the same integer cosine transform (ICT) algorithm as         
used for the Solid State Imaging (SSI) data.  The severity of this            
compression is variable and can range from 40 bps to 5 bps, with 5 bps        
being the most often utilized data rate. The wideband data are not            
compressed and are minimized through editing functions only.  In              
addition, a new wideband telemetry mode was developed which uses bit          
allocations in the original LRS telemetry frame originally reserved for       
Golay encoding to produce a mode with drastically reduced data rate           
requirements.                                                                 
                                                                              
Realtime Science                                                              
                                                                              
LRS Editing and Compression:  The original PWS LRS data                       
stream is edited to reduce the data rate before compression from 240          
bps to 65 bps.  First, the 120 bits of waveform survey data are edited        
out of the data stream.  Second, all housekeeping and status                  
information is removed; if one assumes that the sequenced commands are        
executed properly and the instrument timing is maintained, the status         
of the instrument can be unambiguously determined from the most recent        
mode command and the current spacecraft clock.  A recorded mode like          
the original LRS but now called LPW preserves the full, original PWS          
LRS data stream and can be used to compare realtime science with the          
full data steam at limited times in case questions arise about the            
assumptions of correct command execution or instrument timing.  Third,        
only one sample per channel is preserved in a given 18.67-second              
instrument cycle.  Accordingly, only the first sample of the four SA          
channels are retained and only the first sample of the lower frequency        
HFR channels are retained.  Further, since there is an overlap between        
the upper frequency range of Band 4 of the MFR with the lower range           
of the HFR, and since the HFR has better sensitivity over this range,         
the highest frequency 6 channels of MFR band 4 are edited out.  What          
remains is a single sample of each of 152 of the original 158 channels        
in the low rate portion of the instrument.  This is the data stream           
which is forwarded to the AACS for ICT compression.                           
                                                                              
The ICT compression works on 8x8 pixel blocks of an image.                    
Since The PWS dynamic spectrogram can be thought of as an                     
image, the compression algorithm can be used to compress the                  
spectrogram prior to transmission to the ground.  One complication is         
that the PWS generally alternates between a magnetic and electric             
spectrum (at least for the SA and MFR channels) and the resulting             
alternating spectra add entropy to the 'image' and thereby reduce the         
compressibility of the data set.  Therefore, the electric and magnetic        
spectra are separated or 'unzipped' prior to compression.  Also, it is        
necessary to build up 8x8 blocks of the spectrogram.  152 channels can        
be broken into 19 8-channel segments, hence, 8 electric and 8 magnetic        
spectra are accumulated in order to make an 8x152 pixel strip (19 8x8         
blocks) of electric field data and a similar strip for the magnetic           
data.  (Note that the electric HFR data is included with the magnetic         
strip so that the 18.67-sec samples of the HFR channels are preserved.)       
 Hence, data is collected on 16 x 18.67 second time intervals (about 5        
minutes) in order to build the two 8x152 pixel strips.  Each of the           
strips is compressed individually with all 19 8x8 blocks in each being        
used to generate a downlink packet.  Since transmission errors will           
make decompression impossible for all data following the error, a             
5-minute gap will appear for any packet with a telemetry error.               
Fortunately, telemetry errors are very infrequent and the data which          
reaches the ground intact is virtually immune from the spikes typical         
of bursty bit errors in an uncompressed telemetry stream.                     
                                                                              
Once on the ground, the electric and magnetic strips are                      
decompressed and 'zipped' back together in the original time order.           
This allows sequential electric (or magnetic) spectra to be maintained        
together in the event the instrument is not cycled between E and B            
sweeps. The net result of this compression/decompression scheme is that       
the full temporal and spectral resolution of the PWS instrument is            
maintained even though the real data rate to the ground is as low as 5        
bps.  Of course, as in any lossy compression scheme, information is           
lost.  By maintaining the spectral and temporal resolution, the loss in       
the resulting data set is in amplitude inaccuracy.  Based on both             
ground experimentation and analysis of the Jupiter data, we believe the       
amplitude errors at 5 bps are no more than about 6 dB and are much less       
at the higher data rates (less severe compression).  At 5 bps these           
amplitude errors appear as 'tiling' in which all pixels in a given 8x8        
block have similar values but are different from adjoining 8x8 blocks         
or else the 8x8 blocks take on a checkerboard appearance.  The errors         
do not appear to be systematic, hence, we believe that averaging pixels       
in frequency and time over regions of the spectrogram where the               
spectrum appears to be 'simple' could provide a better estimate of the        
true signal strength.  Generally, however, the 5 dB accuracy will             
enable a wide range of studies without need to know the absolute              
amplitude better than a few dB.                                               
                                                                              
Recorded Data                                                                 
                                                                              
LRS data:  When PWS data are recorded, the full LRS data stream               
is recorded and played back.  This includes all status bits,                  
additional samples of multiply-sampled channels (e.g. SA channels), and       
the waveform survey data.  The discussion of data in the original             
instrument description is fully applicable in this case.                      
                                                                              
Wideband data:  The wideband data suffered the most through the               
process of reducing the downlink requirements.  Nevertheless, a minimal       
wideband capability was retained.  Beginning with the Io flyby, a new         
data mode was introduced which replace the Golay encoding bits formerly       
used for the LRS data format with PWS wideband data in the LPW format.        
This mode is called LPW and is usually modified with the term Golay           
bits to distinguish these data from the low rate data.  All three of          
the bandwidths (wideband modes) can be used with the LPW format and for       
each, a total of 832(????) contiguous 4-bit samples are acquired.             
However, these 832 samples are recorded only once per 2 minor frames,         
or 2.67 seconds, hence, the best spectral temporal resolution (temporal       
resolution of Fourier transforms assuming 1 transform per set of              
contiguous samples) is 2.67 seconds.  The spectral resolution (416            
spectral components is very competitive with the HPW (94._ kbps mode)         
but the temporal resolution is poorer by a factor of about 50.  It            
is this mode, however, which is used for virtually all of the orbital         
tour recording.                                                               
                                                                              
Some of the original wideband telemetry modes were eliminated                 
and those which remain (in addition to the LPW/Golay bits) are MPW,           
MPP, and HPW.  These provide 7.68, 19.2, and 94._ kbps, respectively.         
In practice, the LPW/Golay bits provides superior spectral resolution         
over the MPW mode at significantly reduced bit rate (due to the               
poor temporal resolution) and the MPW mode is not utilized in the             
tour.  Limited use of the MPP and HPW modes is included in the tour           
data set, however.                                                            
                                                                              
For all the wideband telemetry modes, an additional capability                
for reducing the downlink requirements was implemented.  This is              
an editing function often referred to as '1 of n-line editing' and            
consists of returning 1 of every n sets of contiguous samples recorded.       
Most of the tour data were returned with n = 2 or n = 4 so that               
the temporal resolution of the Fourier transformed spectra are a factor       
of 2 or 4 poorer than the original recorded data.  For example, in            
the LPW/Golay bits mode, if n = 2, the time between returned spectra          
is not 2.67 seconds, but 5.33 seconds.  Reducing the number of samples        
in a contiguous set of samples by returning every nth sample was              
never considered because this would under sample the waveform and             
lead to aliasing problems.  Likewise, truncating the number of                
samples in a set would reduce the spectral resolution, thereby                
defeating the remaining attribute of the wideband data."                      


    </Description>
      <Acknowledgement/>
            <Contact>
                <PersonID>spase://SMWG/Person/Donald.A.Gurnett</PersonID>
                <Role>PrincipalInvestigator</Role>
            </Contact>
            <Contact>
                <PersonID>spase://SMWG/Person/William.S.Kurth</PersonID>
                <Role>CoInvestigator</Role>
            </Contact>
      <InformationURL>
        <Name>Instrument home page at The University of Iowa</Name>
        <URL>http://www-pw.physics.uiowa.edu/galileo/</URL>
      </InformationURL>
      <InformationURL>
        <Name>Experiment Details at the National Space Science Data Center (NSSDC)</Name>
        <URL>http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1989-084B-07</URL>
      </InformationURL>
    </ResourceHeader>
    <InstrumentType>Antenna</InstrumentType>
    <InstrumentType>SearchCoil</InstrumentType>    
    <InvestigationName>Plasma Wave Spectrometer</InvestigationName>
    <ObservatoryID>spase://CDPP/Observatory/AMDA/Galileo</ObservatoryID>
    <Caveats/>
  </Instrument>
</Spase>