Dataset Overview

This dataset contains full resolution magnetometer data from the Pioneer
Venus Orbiter spacecraft beginning with orbit 3602 (Oct 16, 1988) and
continuing until the orbiter was lost in the Venusian atmosphere (orbit
5055, Oct 7, 1992). The dataset differs from the high resolution magnetometer
dataset from theprevious orbits due to an anomaly in the multiplexor that
controlled the sampling of the magnetometer sensors. During orbit 3602, and
for the remainder of the mission, the spacecraft downlink contained
magnetometer data from only the P sensor. The P sensor is the sensor which
is aligned with the spacecraft spin axis, nominally in the direction of the
ecliptic normal.

Version 2.0 of this data set includes new versions of some of the data
products. The ancillary engineering (ENGR) were regenerated
setting bad data to flag values. The original, unflagged data values
are available in the binary version of the same data.

After the anomaly was first detected, the magnetometer team made a
substantial effort to recover the average magnetic field vector using the
single sensor data. Since the sensor is not perfectly aligned with the
spacecraft spin axis, a small amount of spin modulation is present in the
sensor data. In theory, the amplitude of the spin plane field can be
determined from the amplitude of the spin modulation and knowledge of the
true sensor mounting geometry. In practice, this turned out to be a very
difficult task because the spin axis of the spacecraft is not sufficiently
constant in body-fixed coordinates.


Data Processing

This dataset has been calibrated to account for sensor zero levels, gains,
and geometry. The data are given in units of nanoTesla. The data are at the
full downlink sampling resolution. They have not been resampled during the
data processing procedure. After the anomaly, the onboard calibrate signal
continued to function and the calibration of the instrument was repeatedly
checked. No change in gain was detected. The technique used to check the zero
levels earlier in the mission could not be used after the anomaly occurred.
Since the operations were limited to the low power available on the
spacecraft, and since the magnetometer became a low priority instrument, it
was not possible to check the zero levels to the levels desirable. However,
it is clear that towards the end of the mission the zero levels had drifted
significantly. It is recommended that these data be used principally in a
variometer mode, i.e. for changes only.

The dataset contains the following columns:

Column  Description 
___________________________________________________________________________
UT      S/C event time (UT) of the sample 
P1      Measured value of the P sensor at the sample time 
P2      Measured value of the P sensor at the sample time + 38 microsecs 
P2      Measured value of the P sensor at the sample time + 76 microsecs 
Bz      The derived value of the spacecraft z-axis aligned magnetic
          field component at the sample time .

Data Sampling

The data 'P' columns are values which have been sampled from the P
sensor directly. The magnetometer samples each sensor 38 microseconds after
the previous sensor sample. The data processing procedure uses the P1 sample
to determine the Bz value and hence it is sampled at the start of sweep
(time-tag) time.

Ancillary Products

  Other datasets on the CD-ROM are the Ephemeris which contains spacecraft
position in Venus Solar Orbital coordinates, spacecraft altitude, solar
zenith angle, Venus centered longitude and latitude, spacecraft spin axis
components, celestial longitude and latitude of the spacecraft, celestial
longitude of the earth, and the Sun-spacecraft range.
  Other ancillary datasets are the: 1) phase and offset which contains the
phase amplitude of sun synchronous modulation of the 4 signals (E100, E730,
E5.4 and E30K), and offsets of the G sensor. 2) The engineering dataset which
contains temperatures, magnetometer modes, magnetometer sample format,
magnetometer spin average select, telemetry data format, telemetry bit rate,
spacecraft spin period, pulse time, the difference between the Sun pulse time
and the Rip pulse time, and the pulse time flag.

References:

      Russell, C.T., R.C. Snare, J.D. Means and R.C. Elphic, 'Pioneer
      Venus Orbiter Fluxgate Magnetometer', IEEE Trans. Geo. Elec.,
      vol. GE 18, no. 1 p. 32, 1980.

These data have been reprocessed several times during the course
of the mission. Each successive reprocessing has eliminated some
of the problems found in earlier version of the data.  Please bring
to the attention of the original investigator any suspicious section
of data.  No zero level correction has been applied to the data.