The SSM is a triaxial fluxgate magnetometer system used as a space environment sensor for the Defense Meteorological Satellite Program (DMSP) spacecrafts 12 through 20. This instrument was designed and built at NASA/Goddard Spaceflight Center under the direction of Dr. Mario Acuña. The first of this series of magnetometers was placed in low Earth orbit in September 1994. A "proof-of-concept" SSM instrument was built from surplus magnetometers by Johns Hopkins Univ./Applied Physics Lab. and flown on the DMSP F7 spacecraft from Dec 1983 to late 1987.
The parameters measured by the SSM are the three components of the magnetic field vector. The SSM magnetometer takes and reports 12 readings per second for the Y and Z axes. Only 10 readings of the 12 readings per second are reported for the X axis due to telemetry limitations. (The SSM's axes are aligned with the spacecraft's axes where X is downward and aligned to local vertical within 0.01 degree, Y is parallel to the velocity vector for spacecraft with ascending node in the afternoon/evening sector, and Z is away from the solar panel and anti-parallel to the orbit normal vector.) The range is from +65535 to -65535 nT for each axis, with a one-bit resolution of 2 nT, (1 nT=1.0E-5 gauss=1.0E-9 Tesla).
The fluxgate sensors are constructed utilizing the ring core geometry.
The drawing shows the construction of one of the
three identical sensors. Important elements in the design of the sensor are
the feedback coil which nulls the ambient field within +/- 2000 nT, and the
sensor core itself since they can be shown to be the most significant sources
of error, both from the alignment stability point of view, as well as
temperature stability.
Here individual axis nulling is utilized, that is, each axis is nulled independently in only one direction. This is the most common approach but suffers from extreme sensitivity to cross-fields since the ambient field is nulled only along the sensor axis. Thus any distortion or motion of the sensor core within the feedback coil represents an effective change due to the presence of the transverse field.
A rigid attachment of the sensor core to the feedback coil usually results in differential expansion stresses being transmitted to the sensor core which can produce significant alignment shifts, zero level changes as well as dramatic increases in sensor noise levels. These problems have been solved by carefully matching the thermal expansion coefficients of the elements involved (core, windings, support structure) to minimize differential stresses and changes in the relative geometry between the core and the feedback coil.
The three sensor elements are mounted in three orthogonal directions in a
rigid frame which is covered with a fiber glass shell. This assembly is
attached to the mounting plate by four plastic legs. The legs provide
electrical and thermal isolation from the spacecraft's frame.
The mounting plate is aligned with the spacecraft's
axes with an accuracy of 0.5 degree/axis. Post-flight calibration can
determine this "mis-alignment" between the SSM's axes and the
spacecraft's axes to an accuracy of 0.1 degree/axis or better.
The complete SSM instrument consist of: (1) The sensor assembly with cable, (2) the electronics box, and (3) the regulator box for the heater on the sensor assembly. The sensor assembly is 112 by 90 by 67 mm and weighs 290 g with cable.
The SSM sensor was designed to be mounted on the exterior of the Earth facing panel of the Equipment/Service Module (ESM). That was found to be an inappropriate location for the sensor and it was moved to the truss which connects the ESM to the apogee-kick motor (AKM or 3rd stage rocket). In this move the sensor was rotated 180 deg about the Z axis. By changing the sign of the counts-to-field constants, the output of the sensor is still reported as 3 vectors aligned with the spacecraft axes. The SSM sensor is mounted on the truss structure only for F12, F13 and F14. For F15 to F20, the sensor will be placed at the end of a 5 meter Astromast boom which is deployed radially upward from the truss assembly. This will greatly decrease the magnetic fields from the spacecraft which are observed by the sensor.
updated: 30 May 1996
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