# MAGSAT

NASA (U.S.) scientific satellite (1979-1980) made the first precise,
globally distributed measurements of the vector magnetic field near
Earth. From an altitude of 300-550 km, the space domain representation
of the internal field extended from spherical harmonic (q.v.) degrees 1
through 36 for the vector field (Lowe et al., 2001), with additional
resolution to degree 65 for the total field (Sabaka et al., 2002).
Magsat also contributed significantly to 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 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 (Figure 1). 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
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.

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. System errors in the determination
of the scalar field were 0.5 to 1.5 nT. 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.

Each observation has associated with it an attitude uncertainty, and
details on the sensors that were used to calculate the attitude. 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, corresponding to an attitude
error of 5 nT. A detailed error budget can be found in Langel and Hinze
(1998) as Table 3.4. The observations can be accessed through the
National Space Science Data Center, for example at
http://nssdc.gsfc.nasa.gov/ftphelper/magsat.html

# Bibliography

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

Langel, R.A., and Hinze, W.J., 1998, [The Magnetic Field of the Earth's
Lithosphere: The Satellite Perspective]{.underline}, Cambridge:
Cambridge University Press.

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}, 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.

Sabaka, T.J., Olsen, N., and Langel, R., 2002, A comprehensive model of
the quiet-time, near-Earth magnetic field, Phase 3, [Geophys. J.
Int]{.underline}., 151, 32-68.

# Cross References 

Langel, R.A., Champ, Ørsted, POGO, Spherical Harmonics

Michael E. Purucker