# MAGSAT

NASA (U.S.) scientific satellite (1979-1980) made the first precise,
globally distributed measurements of the vector magnetic field near
Earth. The satellite flew at an altitude of 300-550 km, in a near-polar
inclination. Researchers modeling the Magsat data have developed space
domain representations of the internal field extending from spherical
harmonic (q.v.) degree 1 through 36 for the vector field (Lowe et al.,
2001), with additional resolution to degree 65 for the scalar field
(Sabaka et al., 2004). Magsat also contributed significantly to
understanding of the ionospheric magnetic field, and the meridional
current systems through which the satellite flew. The most demanding
aspect of making vector magnetic measurements from space is in their
precise alignment, which must be at the arc-second level. 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 (Potemra et al., 1981). 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 (Oct, 1979
through June, 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.

![](media/image1.jpeg){width="6.0in" height="2.4in"}Figure 1. Line
drawing of Magsat, its orientation, and critical systems. The roll axis
is along the orbital velocity vector, the yaw axis is vertical, and the
pitch axis completes the orthogonal coordinate system.

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 (Langel
et al., 1981). 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 (as
daily files) at the ftp server
([ftp.spacecenter.dk/data/magnetic-satellites/Magsat](ftp://ftp.spacecenter.dk/data/magnetic-satellites/Magsat))
of the Danish National Space Center

# 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 Purucker, M., 2004, Extending Comprehensive
Models of the Earth's Magnetic Field with Oersted and CHAMP data,
[Geophys. J. Int]{.underline}., 159, 521-547.

# Cross References 

Langel, R.A., Champ, Ørsted, POGO, Spherical Harmonics, Uniqueness of
solutions to Laplace's equations

Michael E. Purucker