NSSDCA::ANON_DIR:[COHO.P11MAG]P11HVM_15M.SFD
Note: this document is an extract from the SFDU metadata text for the 15-minute
averaged IMF data in NSSDC data set 73-019A-01H. The mission, experiment, and
data processing details also apply to the 1-hour data, except that the
one-hour data in 73-019A-01I have been supplied to NSSDC all in RTN coordinates
with SCET-UT times.
JFC 7/17/95
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/* VOLDESC.SFD file */
Technical_Contact: Joyce Wolf
Mail Stop 169-506
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
Electronic Mail (SPAN): JPLSP::JWOLF
Phone: 818-354-7361
Prev_Vols: None
/* or USA_NASA_NSSD_P11A_0001
USA_NASA_NSSD_P11A_0002, etc. */
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Data_Set_Name: Pioneer 11 HVM Cruise Data Archive
Data_Source: Pioneer 11 Helium Vector Magnetometer
Scientific_Contact: Dr. Edward J. Smith
Jet Propulsion Laboratory
Mail Stop 169-506
4800 Oak Grove Drive
Pasadena, CA 91109
Electronic Mail: JPLSP::ESMITH
Telephone: 818-354-2248
Spacecraft_Characteristics: Launched in April of 1973, Pioneer 11 made its
closest approach to Jupiter on Dec. 3, 1974. During late 1975 and early 1976,
Pioneer 11 attained heliographic latitudes of 16 deg and higher; the sector
structure of the IMF disappeared at these latitudes. On Sept. 1, 1979, Pioneer
11 made its closest approach to Saturn. Since then, it has been heading out
of the Solar System, upstream with respect to the direction of the
interstellar wind. It passed Neptune's orbit in 1990.
The spacecraft spins at about 7.8 rpm, with the spin axis directed toward the
Earth. It carries 12 instruments for measuring fields and particles, and is
powered by radioisotope thermal generators (RTG's).
Investigation_Objectives: The primary investigation objectives for the
Pioneer 11 Helium Vector Magnetometer cruise data are to determine the large-
scale structure and dynamics of the interplanetary magnetic field in the outer
solar system and to study how they are influenced by changing solar activity,
and the interaction of the solar wind with the interstellar medium.
Instrument_Attributes:
A. Instrument_Description: The Helium Vector Magnetometer produces
measurements of the 3 orthogonal components of the ambient magnetic field in a
0-3 Hz passband. The instrument switches automatically among 8 ranges, plus
or minus 4, 14, 42, 144, 640, 4000, 22000, and 140000 nT. The measurements
are digitized to 8 bits and a sign bit, giving a sensitivity of 1/256 of full-
scale in each range. For more information, refer to Smith, E. J., B. V.
Connor, and G. T. Foster, Jr., "Measuring the magnetic fields of Jupiter and
the outer solar system," IEEE Trans. Magn., vol. MAG-11, pp. 962-980, 1975.
B. Instrument_Performance: The instrument has functioned normally throughout
the mission.
C. Measured_Parameters: Three orthogonal components of magnetic field; the
third component is parallel to the spacecraft spin axis.
D. Instrument_Accuracy: Two factors determine the accuracy with which each
component of the field is determined. One is the scale factor relating the
change in field to the corres-ponding change in output voltage. The straight
line representing this scale factor intercepts the B axis (V = 0) at a non-
zero value (the instrument "offset" or "zero level").
The HVM is operated in a feedback mode so that the scale factor is highly
linear and very stable. An in-flight calibration (IFC) is incorporated into
the instrument such that, on receipt of a command, carefully calibrated step
field changes are applied to the sensor to produce an end-to-end calibration
of all three axes. During the lifetime of Pioneer 10 and 11, we performed an
in-flight calibration approximately every two weeks. No change in instrument
response as large as one bit has ever been observed on either instrument.
Thus, the scale factors are considered known to within 0.25 percent, and,
accordingly, the errors are negligible.
Accurate estimates of the offsets must be determined in flight. Since Pioneer
11 is a spinning spacecraft with two magnetometer axes lying in the spin
plane, two of the offsets can be continuously monitored by averaging the data
on a given axis over a large number of revolutions. By analyzing the results
over many long intervals, it is estimated that these two offsets are being
determined to within 0.005 nT. Since the spin axis is continuously oriented
toward Earth and is very nearly radial at large distances (about 30 AU) the
two transverse components (B-Theta and B-Phi) are extremely well known. At 30
AU, the field magnitude, B, is typically 0.2 nT and the relative accuracy is,
therefore, about 2.5 percent.
The offset on the sensor axis parallel to the spin axis is more difficult to
determine. We use the method developed by Davis and Smith (also independently
developed by Hedgecock), as improved upon by Belcher. The basis of this so-
called variance method is to choose the BZ offset so as to reduce the
variations in B-magnitude to a minimum in the least squares sense. Experience
indicates that the method yields a relative accuracy of greater than 5
percent. For Pioneer 11 we estimate that the offset is known to about 0.02 nT
so that in a typical field of 0.2 nT, the offset is determined to within about
10 percent. This number is probably conservative and we may actually be doing
slightly better.
Data_Set_Parameters: Averages of field components (BX, BY, BZ); averages of
squares and crossproducts of components (BX2, BY2, BZ2 and BXBY, BXBZ, BYBZ);
averages of field direction cosines (BXCOS, BYCOS, BZCOS); average of field
magnitude (BMAG) and average of square of field magnitude (BMAG2).
Heliocentric positions of the spacecraft and Earth, referred to the ecliptic
and equinox of date, are also included.
Data_Set_Quality: There are no significant known errors in the data;
however, in 1987 and afterward, the cyclic switching on and off of other
instruments on Pioneer 11 caused random fluctuations of the instrument's Z
axis zero levels. These fluctuations are of the order of 0.05 nT, with a
period of several days. We have been able to remove most, but not all, of
these fluctuations.
Data_Processing_Overview: Data reduction until 1976 was done on an IBM 7044
and a Univac 1108; after that time a SEL/32 was used.
Raw data points consisted of Ground Received Time, triaxial magnetometer
measurements in counts (0 to 511), and magnetometer range (0 to 7). Each
measurement was converted into nanotesla using range-dependent scaling factors
and offsets. Spin-plane offsets were calculated daily by averaging spinning
data. The other offset (parallel to the spin axis) was estimated over periods
of several weeks using either the Leverett Davis method of minimizing the
variance of the square of the magnitude, or John Belcher's variation of that
method.
The magnetic field vectors were then despun into spacecraft inertial
coordinates and rotated through the roll angle CKAH (Clock Angle of Sun,
provided by Ames in File 3 on our EDR tapes). In this PE coordinate system,
field vectors from 1976 and later were written onto RDR (Reduced Data Record)
tapes. Prior to 1976, data were transformed from PE to SH (or SJ during
Jupiter Encounter) before being written onto the RDR tapes. (In the PE
system, Z is along the Pioneer spin axis, nominally toward Earth, and X lies
in the plane containing the directions from Pioneer to the Earth and Sun and
is orthogonal to the spin axis, Z. In SH, also known as Radial-Tangential-
Normal, X is the Sun-to-Pioneer direction and Y is parallel to the Sun's
equatorial plane. For a complete definition of these systems, see the
description of COORDSYS in file FORMAT.SFD.)
From the RDR tapes, Ground Received Time 1-minute, 1-hour, and 1-day averages
of the field components, crossproducts of components, squares of components,
direction cosines of components, field magnitude, and square of field
magnitude were calculated and submitted on tapes to NSSDC.
For this cruise data archive, each 15-minute (or 1 hour) Spacecraft Event Time
parameter average has been constructed from those GRT 1-minute parameter
averages whose midpoints (converted to SCET) lie within the SCET averaging
interval. The number of seconds in each minute for which data existed was
used as a weighting factor. Averages in PE coordinates have been converted to
SH.
Lit_References: Smith, E. J., B. V. Connor, and G. T. Foster, Jr., "Measuring
the magnetic fields of Jupiter and the outer solar system,"
IEEE Trans. Magn., vol. MAG-11, pp. 962-980, 1975.
Smith, E. J., et al., "Saturn's Magnetosphere and Its
Interaction with the Solar Wind," J. Geophys. Res., vol. 85,
pp. 5655-5674, 1980.
Smith, E.J., D. Winterhalter, and J. A. Slavin, "Recent
Pioneer 11 observations of the distant heliospheric magnetic
field," Solar Wind 6, eds. V.J. Pizzo, T.E. Holzer, and D.G.
Sime, NCAR/TN-306, HAO/NCAR, Boulder, Colorado, 1988, p. 581.
Other references may be found in these articles.
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