# MAGSAT

NASA (U.S.) scientific satellite (1979-1980) that captured a first,
precise snapshot of the near-earth vector magnetic field. From an
altitude of 300-550 km, the space domain representation of the internal
field extended from spherical harmonic degrees 1 through 36 for the
vector field (Lowe et al., 2001), with additional resolution to degree
48 for the total field. Magsat also contributed significantly to an
understanding of the ionospheric magnetic field, and the meridional
current systems through which the satellite flew. Magsat was preceded by
the POGO (q.v.) satellites that precisely measured the earth's scalar
magnetic field from altitudes as low as 400 km. Magnetic field
measurements have been made from near-earth space since 1958, when
Sputnik 3 (USSR) collected fluxgate magnetometer measurements of the
field with accuracies of about 100 nT. Magsat's measurements of the
near-earth vector field were not improved upon, or even repeated, until
the launchings of Ørsted (q.v) (1999) and CHAMP (q.v.) (2000), led by
Denmark and Germany, respectively. Follow-on magnetic field missions by
NASA focused on planetary magnetic fields (e.g. Mercury, Venus, Mars,
and the outer planets), on the earth's magnetosphere, and the dynamics
of the high-latitude ionosphere.

The Magsat project scientist was Robert A. Langel (q.v.), the project
manager was Gilbert Ousley, and the spacecraft design, development and
testing was done by the Applied Physics Laboratory (APL) of John Hopkins
University. The vector magnetometer was designed and built at Goddard
Space Flight Center's Laboratory for Extraterrestrial Physics , by Mario
Acuña (Instrument Scientist) and his group. Scientists and scientific
organizations in the US, Europe, and Japan formed the largest group of
users of Magsat observations. Magsat weighed in at 182 kg, and cost 19.2
million US dollars from inception through production of the finished
data products. Magsat was modeled on the Small Astronomical Satellite
(SAS-3), built by APL in 1975. SAS-3 utilized a Doppler tracking system
for position determination to within 60 m vertically and 200 m
horizontally, and had two star trackers that could provide attitude
information to 10 arc-seconds. However, the magnetic fields associated
with these star trackers and the spacecraft itself would have introduced
unacceptably large errors into the magnetic field determinations.
Therefore, a deployable scissors boom and an infrared attitude transfer
system (ATS) were developed for Magsat that put the scalar and vector
magnetometers six meters away from the star trackers, and the body of
the spacecraft. The deployable boom was designed, developed, and tested
by APL, and the attitude transfer system was based on one used for
submarines and adapted for Magsat by the Barnes Engineering Company. The
sun-synchronous orbit of Magsat, resulting in a sampling of the magnetic
field only at dawn and dusk local time, was a compromise dictated by the
carrying capacity of the Scout launch vehicle. The chosen orbit, and
mission lifetime (Fall, 1979 through Spring, 1980) allowed for a maximum
exposure of the fixed solar array to the sun, a near-constant thermal
environment, and a fixed flight attitude that allowed the star trackers
to always face away from the earth.

The orientation of the vector magnetometer was determined, at 0.25 s
intervals, using the two star cameras. If only one star camera was
tracking an identified star, a precision sun sensor immediately adjacent
to the vector magnetometer was used instead. Jumps in the vector data of
up to 10-15 nT can occur at locations where the method of determining
the orientation of the vector magnetometer changes. The system for
transferring the attitude determined by the star camera on the
spacecraft body to the vector magnetometer on the end of the boom
involved two optical systems, one for pitch and yaw (utilizing a plane
mirror), and a second for roll (using a dihedral mirror). As predicted,
the system for measuring roll had the largest errors, estimated at about
8 times the pitch/yaw error.

![](media/image1.jpeg){width="6.0in" height="2.4in"}Figure 1. Line
drawing of Magsat, it\'s orientation, and critical systems

Magsat carried both scalar and vector magnetometers. The scalar
magnetometer, a two-sensor, cesium 133 vapor optically pumped
magnetometer, was used to calibrate the vector magnetometer. The
two-sensor arrangement was chosen to minimize null zones, where the
magnetometers are incapable of sensing the ambient field. In practice,
intermittent noise in the cesium lamp excitation circuits caused the
tracking filters to lose lock, and reduced the amount of data returned
to well below the nominal 8 Hz rate. The vector instrument was a
triaxial fluxgate magnetometer with a dynamic range of +-2000 nT, and
digitally controlled current sources to increase its range to +-64000
nT. The vector instrument sampled the ambient field 16 times/second with
a resolution of +-0.5 nT. Each of the three orthogonal sensors was wound
with platinum wire on a highly permeable toroidal magnetic core. The
sensors were mounted on a stable ceramic block and temperature
controlled to minimize alignment shifts with respect to the reference
mirrors of the attitude transfer system.

While each observation has associated with it an attitude uncertainty,
and details on the sensors that were used to calculate the attitude, no
information has been retained on the anisotropy of these uncertainties.
The mission requirements specified that the three axes of the vector
magnetometer be determined within 60 arc-seconds, although in practice
it was often known within 20 arc-seconds. The vector magnetometer itself
was calibrated to an accuracy of better than 5 arc-seconds in GSFC's
Magnetic Test Facility. The proper handling of the attitude uncertainty,
and whether anything can be said of its anisotropy ex post facto, are
still open topics with respect to proper modeling of the Magsat data.

# Bibliography

Langel, R., Berbert, J., Jennings, T., and Horner, R., 1981, [Magsat
Data Processing: a Report for Investigators]{.underline}, NASA Technical
Memorandum 82160.

Lowe, D.A.J., Parker, R.L., Purucker, M,E., and Constable, C.G., 2001,
Estimating the crustal power spectrum from vector Magsat data, [Journal
of Geophysical Research]{.underline}, v.106, 8589-8598.

Potemra, T.A. et al., 1980, [John Hopkins APL Technical
Digest]{.underline}, The Magsat issue, July-September, 1980,

12 articles, pp. 162-232.

Michael E. Purucker