| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set consists of electric field spectrum analyzer data
from the Voyager 2 Plasma Wave Subsystem obtained during the
entire mission. Data after 2019-01-02 will be added to the archive
on subsequent volumes. The data set encompasses all spectrum
analyzer observations obtained in the cruise mission phases
before, between, and after the Jupiter and Saturn encounter phases
as well as those obtained during the two encounter phases.
The Voyager 2 spacecraft travels from Earth to beyond 80 AU over
the course of this data set. To provide some guidance on when
some key events occurred during the mission, the following table
is provided.
Date Event
1977-08-20 Launch
1979-07-02 First inbound bow shock crossing at Jupiter
1979-08-03 Last outbound bow shock crossing at Jupiter
1981-08-24 First inbound bow shock crossing at Saturn
1981-08-31 Last outbound bow shock crossing at Saturn
1982-04-26 10 AU
1983-08-30 Onset of first major LF heliospheric radio event
1986-01-24 First inbound bow shock crossing at Uranus
1986-01-29 Last outbound bow shock crossing at Uranus
1986-05-26 20 AU
1989-08-07 30 AU
1989-08-24 First inbound bow shock crossing at Neptune
1989-08-28 Last outbound bow shock crossing at Neptune
1992-07-06 Onset of second major LF heliospheric radio event
1993-05-08 40 AU
1996-10-10 50 AU
2000-01-27 60 AU
2002-11-01 Onset of third major LF heliospheric radio event
2003-04-21 70 AU
2006-07-01 80 AU
2007-08-31 Termination shock crossing
2009-09-03 90 AU
2012-11-04 100 AU
2016-01-01 110 AU
Data Sampling
=============
This data set consists of full resolution edited, wave electric
field intensities from the Voyager 2 Plasma Wave Receiver spectrum
analyzer obtained during the entire mission. For each time
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. The time between spectra in this data set vary by
telemetry mode and range from 4 seconds to 96 seconds. During
data gaps where complete 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.
For the cruise data sets, the timing of samples is dependent upon
the spacecraft telemetry mode. In principle, one can determine
the temporal resolution between spectra simply by noting the
difference in time between two records in the files. In some
studies, more precise timing information is necessary. Here, we
describe the timing of the samples for the PWS low rate data as a
function of telemetry mode.
The PWS instrument uses two logarithmic compressors as detectors
for the 16-channel spectrum analyzer, one for the bottom (lower
frequency) 8 channels, and one for the upper (higher frequency) 8
channels. For each bank of 8 channels, the compressor
sequentially steps from the lowest frequency of the 8 to the
highest in a regular time step to obtain a complete spectrum. At
each time step, the higher frequency channel is sampled 1/8 s
prior to the lower frequency channel so that the channels are
sampled in the following order with channel 1 being the lowest
frequency channel (10 Hz) and 16 being the highest (56.2 kHz): 9,
1, 10, 2, 11, 3, ... 15, 7, 16, 8. The primary difference
between the various data modes is the stepping rate from one
channel to the next (ranging from 0.5 to 12 s, corresponding to
temporal resolutions between complete spectra of 4 s to 96 s).
In the following table, we present the hexadecimal id for the
various telemetry modes, the mode mnemonic ID, the time between
frequency steps, and the time between complete spectra. We also
provide the offset from the beginning of the instrument cycle (one
complete spectrum) identified as the time of each record's time
tag to the time of the sampling for the first high-frequency
channel (channel 9) and for the first low-frequency channel
(channel 1).
Time
Frequency Between High Freq. Low Freq.
MODE (Hex) MODE ID Step (s) Spectra (s) offset (s) offset (s)
01 CR-2 0.5 4.0 0.425 0.4325
02 CR-3 1.2 9.6 1.125 1.1325
03 CR-4 4.8 38.4 0.425 0.4325
04 CR-5 9.6 76.8 0.425 0.4325
05 CR-6 12. 96.0 0.9275 0.935
06 CR-7 NOT IMPLEMENTED
07 CR-1 0.5 4.0 0.225 0.2325
08 GS-10A SAME AS GS-3
0A GS-3 0.5 4.0 0.425 0.4325
0C GS-7 SAME AS GS-3
0E GS-6 SAME AS GS-3
16 OC-2 SAME AS GS-3
17 OC-1 SAME AS GS-3
18 **CR-5A 0.5 4.0 0.425 0.4325
19 GS-10 SAME AS GS-3
1A GS-8 SAME AS GS-3
1D **UV-5A SAME AS CR-5A
**In CR-5A and UV-5A, the PWS is cycled at its 0.5 sec/frequency
step or 4 sec/spectrum rate, but 4 measurements are summed on
board in 10-bit accumulators and these 10-bit sums are downlinked.
On the ground, the sums are divided by 4, hence providing, in a
sense, 16-second averages. One of every 12 sets of sums is
dropped on board in order to avoid LECP stepper motor
interference.
Data Processing
===============
The spectrum analyzer data are a continuous (where data are
available) low resolution data set which provides wave intensity as
a function of frequency (16 log-spaced channels) and time (one
spectrum per time intervals ranging from 4 seconds to 96 seconds,
depending on telemetry mode). The data are typically plotted as
amplitude vs. time for one or more of the channels in a strip-chart
like display, or can be displayed as a frequency-time spectrogram
using a gray- or color-bar to indicate amplitude. With only sixteen
channels, it is usually best to stretch the frequency axis by
interpolating from one frequency channel to the next either linearly
or with a spline fit. One must be aware if the frequency axis is
stretched that more resolution may be implied than is really
present. The Voyager PWS calibration table is given in an ASCII
text file named VG2PWSCL.TAB (for Voyager-2). This provides
information to convert the uncalibrated 'instrument data number'
output of the PWS 16-channel spectrum analyzer to calibrated antenna
voltages for each frequency channel. Following is a brief
description of this file and a tutorial in its application.
Descriptive headers have been removed from this file. The columns
included are IDN, CHAN_01, CHAN_02, CHAN_03, CHAN_04, CHAN_05,
CHAN_06, ... CHAN_16.
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 mapping from insturment data numbers
to voltages for each frequency channel. For example, the following
C code may be used to load a calibration array for Voyager 2 PWS:
int idn;
double cal[256][16];
FILE* file = fopen(''VG2PWSCL.TAB'', ''rb'');
for(int iamp = 0; iamp < 256; ++iamp){
fscanf(file, '' %3d'', idn);
for(int ichan = 0; ichan < 16; ichan)
fscanf(file, '',%8lE'', cal[iamp][ichan]);
}
( Here two single quotes, '', are use in place of double-quote
characters due to PDS documentation limitations. )
Then, given an instrument data value idn for the frequency channel
with index, ichan, the corresponding calibrated antenna voltage
would be given by the following array reference:
volts[ichan] = cal[idn][ichan];
This may be converted to a wave electric field amplitude by
dividing by the effective antenna length in meters, 7.07 m. That
is:
efield[ichan] = volts[ichan] / 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[ichan] = (efield * efield) / bandwidth[ichan];
The center frequencies and bandwidths of each PWS spectrum
analyzer channel for the Voyager 2 spacecraft 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
Finally, power flux may be obtained by dividing the spectral
density by the impedance of free space in ohms:
pwrflux[ichan] = specdens[ichan] / 376.73;
Of course, for a particular application, it may be more efficient
to apply the above conversions to the calibration table directly.
Voyager 2 PWS SA Data Number Correction
---------------------------------------
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, Uranus, and Neptune encounters, but is not valid
for Earth-Jupiter cruise and may be modified in the future. The
following implementation of this correction algorithm in C
assumes that uncalibrated data numbers are stored in a 16-element
integer array, named 'idn', with the array index equal to the PWS
channel number minus one:
float tonl[8] = {2.0, 1.0, -1.0, -2.0, -3.0, 1.0, 2.0, 1.0};
/* Instrument data number correction for upper 8 channels from:
*
* 1977-267T00:47 to 1977-283T16:00
* 1978-010T20:04 to 2006-324T20:50
*/
for(int ichan = 8; ichan < 16; ++ichan){
if( idn[ichan] <= 0 ) continue;
if( idn[ichan] < 64) idn[ichan] = 64;
if( idn[ichan] <= 72 )
idn[ichan] = (int)(tonl[ichan-8] - 530.4 + 8.6*idn[ichan] );
else
idn[ichan] = (int)(tonl[ichan-8] + 20.133 + 0.99*idn[ichan]);
/* Telemetry bit errors can result in corrections above 255 */
if(idn[ichan] > 255) idn[ichan] = 255;
}
/* Instrument data number correction for upper 8 channels from:
*
* 1977-283T16:00 to 1977-312T20:12
* 1977-335T21:54 to 1978-010T20:04
* 2006-324T20:50 to present (at least 2016-245)
*/
for(int ichan = 8; ichan < 16; ++ichan){
if( idn[ichan] <= 0 ) continue;
if( idn[ichan] < 64) idn[ichan] = 64;
if( idn[ichan] <= 86 )
idn[ichan] = (int)(tonl[ichan-8] - 650.8 + 8.6*idn[ichan] );
else
idn[ichan] = (int)(tonl[ichan-8] + 6.253 + 0.99*idn[ichan] );
/* Telemetry bit errors can result in corrections above 255 */
if(idn[ichan] > 255) idn[ichan] = 255;
}
These correction should not be applied permanently to the Voyager 2
calibration table since they are valid for a limited time span and
may be modified in the future. These corrections are applicable to
data in the UTC time ranges noted in the source comments above and
should be applied as if all data values are 8-bit measurements.
For 10-bit sums (modes CR-5A and UV-5A, i.e. TELETMETRY_MODE column
values 0x18 and 0x1D) data numbers must first be divided by 4, and
then corrected with the algorithms above.
Additional information about this data set and the instrument
which produced it can be found elsewhere in this catalog. A
complete instrument description can be found in
[SCARF&GURNETT1977].
Data
====
The spectrum analyzer data are a continuous (where data are
available) low resolution data set which provides wave intensity as
a function of frequency (16 log-spaced channels) and time (one
spectrum per time intervals ranging from 4 seconds to 96 seconds,
depending on telemetry mode). Each sample is nominally an 8-bit
value which is roughly proportional to the log of the signal
strength. In telemetry modes CR-5A and UV-5A the values are 10-bit
sums of 4 original 8-bit instrument samples. Zero values indicate
missing samples and negative values indicate samples flagged as
contaminated by interference (see below).
Ancillary Data
==============
None
Coordinates
===========
The electric dipole antenna detects electric fields in a dipole
pattern with peak sensitivity parallel to the spacecraft x-axis.
However, no attempt has been made to correlate the measured field
to any particular direction such as the local magnetic field or
direction to a planet. This is because the spacecraft usually
remains in a 3-axis stabilized orientation almost continuously.
The only exception to this are a small number of occasions during
calibration turns when the modulation of the low-frequency
heliospheric radio emission could be used to do direction-finding
on the source of these emissions [GURNETTETAL1998].
|
| CONFIDENCE_LEVEL_NOTE |
Overview
========
This data set includes all available spectrum analyzer data acquired
during the Voyager 2 mission to date. The data set has been cleaned
as well as possible for periodic noise spikes due to a stepper motor
operating on the LECP and a modulated grid within the PLS. The
'bad' points remain in the data set as negative numbers with the
same absolute value as the original data point so that the point can
be skipped in normal data processing by testing for negative values
or recovered for special inspection by converting the sign back to a
positive one. Other possible sources of noise which have not been
eliminated include random intense spikes of noise below 1 kiloHertz
due to the operation of attitude control thrusters. Other randomly
occurring spikes or time periods of intense spikes over the entire
frequency range are indicative of telemetry errors. No attempt has
been made to remove spikes since some could be valid data, i.e. real
bursts of wave activity. The 17.8-Hz channel is sometimes
contaminated by interference from the PRA instrument, depending on
that instrument's mode. This interference is at a relatively
constant level.
A failure in the Voyager 2 flight data system on September 24, 1977
has decreased the sensitivity and the calibration accuracy of the
upper 8 spectrum analyzer channels (i.e. 1 kiloHertz and higher).
One aspect of this failure is that voltages on other inputs to a
multiplexer shared by the PWS upper 8 channels can affect the PWS
data. Hence, if all 8 upper channels move up and down together, it
is likely this is a side effect of the failure and not a real effect
in the PWS data.
An intermittent failure appeared in the Voyager 2 17.8 Hz channel
beginning on November 6, 1995. The channel now exhibits three
states. In the original failed state, the channel becomes very
noisy, outputting an abnormally large signal. In a second failed
state, the output goes to zero. However, for the majority of the
time since the failure, the channel outputs credible measurements in
the correct amplitude range.
Beginning in May 2001 the Voyager 2 wideband receiver began to
display degraded operation and by September of 2002, the receiver
showed virtually no response. Attempts to playback wideband data
from Voyager 2 have ceased.
Review
======
This archival data set was examined by a peer review panel prior to
its acceptance by the Planetary Data System (PDS). The peer review
was conducted in accordance with PDS procedures.
Prior to creation of the final version of the archival data set, key
elements of the archive were distributed for preliminary review.
These included electronic versions of example PDS labels, CATALOG
files, and Software Interface Specifications. These materials were
distributed to PDS personnel, the experiment investigator, and
others, as appropriate.
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