| DATA_SET_DESCRIPTION |
This data set consists of 4-second edited, wave electric
field intensities from the Voyager 2 Plasma Wave Receiver
spectrum analyzer obtained in the vicinity of the Uranian
magnetosphere. For each 4-second interval, a field strength
is determined for each of the 16 spectrum analyzer channels
whose center frequencies range from 10 Hertz to 56.2
kiloHertz and which are logarithmically spaced in
frequency, four channels per decade. The time associated
with each set of intensities (16 channels) is the time of
the beginning of the scan. During data gaps where complete
4-second spectra are missing, no entries exist in the file,
that is, the gaps are not zero-filled or tagged in any
other way. When one or more channels are missing within a
scan, the missing measurements are zero-filled. Data are
edited but not calibrated. The data numbers in this data
set can be plotted in raw form for event searches and
simple trend analysis since they are roughly proportional
to the log of the electric field strength. Calibration
procedures and tables are provided for use with this data
set; the use of these is described below.
Use of Voyager PWS Calibration Tables
The Voyager PWS calibration tables are given in two plain
ASCII text files named VG1PWSCL.TAB and VG2PWSCL.TAB (for
Voyagers-1 and -2, respectively). These provide information
to convert the uncalibrated `data number' output of the PWS
16-channel spectrum analyzer to calibrated antenna voltages
for each frequency channel. Following is a brief
description of these files and a tutorial in their
application.
The first column lists an uncalibrated data number followed
by the corresponding value in calibrated volts for each of
the 16 frequency channels of the PWS spectrum analyzer.
Each line contains calibrations for successive data number
values ranging from 0 through 255. (Data number 0 actually
represents the lack of data since the baseline noise values
for each channel are all above that.)
A data analysis program may load the appropriate table into
a data structure and thus provide a simple look-up scheme
to obtain the appropriate voltage for a given data number
and frequency channel. For example, the following VAX
FORTRAN code may be used to load a calibration array for
Voyager 2 PWS:
real*4 cal (16,0:255)
open ( unit=10, file='vg2pwscl.tab', status='old' )
do i=0,38
read (10,*) ! skip 38 lines of label info
end do
do i=0,255
read (10,*) idn, (cal(ichan,i),ichan=1,16)
end do
close (10)
Then, given an uncalibrated data value idn for the
frequency channel ichan, the corresponding calibrated
antenna voltage would be given by the following array
reference:
volts = cal (ichan, idn)
This may be converted to a wave electric field amplitude by
dividing by the effective antenna length in meters, 7.07m.
That is:
efield = cal(ichan, idn) / 7.07
Spectral density units may be obtained by dividing the
square of the electric field value by the nominal frequency
bandwidth of the corresponding spectrum analyzer channel.
specdens = (cal(ichan,idn)/7.07)**2 / bandwidth(ichan)
Finally, power flux may be obtained by dividing the
spectral density by the impedance of free space in ohms:
pwrflux=((cal(ichan,idn)/7.07)**2/bandwidth(ichan))/376.73
Of course, for a particular application, it may be more
efficient to apply the above conversions to the calibration
table directly.
The center frequencies and bandwidths of each PWS spectrum
analyzer channel for the Voyager 2 PWS are given below:
VOYAGER 2 PWS SPECTRUM ANALYZER
Voyager-2
Channel Center Frequency Bandwidth
1 10.0 Hz 2.16 Hz
2 17.8 Hz 3.58 Hz
3 31.1 Hz 4.50 Hz
4 56.2 Hz 10.7 Hz
5 100. Hz 13.8 Hz
6 178. Hz 28.8 Hz
7 311. Hz 39.8 Hz
8 562. Hz 75.9 Hz
9 1.00 kHz 75.9 Hz
10 1.78 kHz 151. Hz
11 3.11 kHz 324. Hz
12 5.62 kHz 513. Hz
13 10.0 kHz 832. Hz
14 17.8 kHz 1260. Hz
15 31.1 kHz 2400. Hz
16 56.2 kHz 3800. Hz
A failure in the Voyager 2 Flight Data System which occurred
about 3 months after launch has adversely affected the
calibration of PWS channels 9 through 16. An algorithm has
been devised to partially correct for this failure, and has
proven useful for Voyager 2 Jupiter, Saturn, and Uranus
encounters, but is not valid for Earth-Jupiter cruise and
may be modified in the future. The following implementation
of this correction algorithm in VAX FORTRAN assumes that
uncalibrated data numbers are stored in a 16-element
integer array, idn, with the array index equal to the PWS
channel number:
real*4 tonl(9:16) ! the old noise level for v2
data tonl/2.0,1.0,-1.0,-2.0,-3.0,1.0,2.0,1.0/
do ichan=9,16
if (idn(ichan).gt.0) then
if(idn(ichan).lt.64) idn(ichan)=64
if (idn(ichan).le.72) then
idn(ichan)=int(tonl(ichan)-530.4+8.6
*real(idn(ichan)))
else
idn(ichan)=int(tonl(ichan)+20.113+0.99
*real(idn(ichan)))
end if
end if
end do
This correction should not be applied permanently to the
Voyager 2 calibration table since it is valid for a limited
time span and may be modified in the future.
Additional information about this dataset and the
instrument which produced it can be found elsewhere in this
catalog. An overview of the data in this data set can be
found in Gurnett et al. [1986] and a complete instrument
description can be found in Scarf and Gurnett [1977].
SAMPLING_PARAMETER_NAME = TIME
SAMPLING_PARAMETER_RESOLUTION = 4.000000
MINIMUM_SAMPLING_PARAMETER = 197708201553.000000
MAXIMUM_SAMPLING_PARAMETER = N/A
SAMPLING_PARAMETER_INTERVAL = 4.000000
MINIMUM_AVAILABLE_SAMPLING_INT = 4.000000
SAMPLING_PARAMETER_UNIT = SECOND
DATA_SET_PARAMETER_NAME = PLASMA WAVE SPECTRUM
NOISE_LEVEL = 0.000005
DATA_SET_PARAMETER_UNIT = VOLT/METER
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