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Microsystems in Spacecraft Guidance, Navigation, and Control 213

noise low in both ranges. One gain change command line switches both elements.
Total mass of the ST5 magnetometer is approximately 600 g and it consumes
approximately 0.55 W of power.
The magnetometers are calibrated in-orbit to correct the prelaunch gain and
offset parameters. Misalignment of the orthogonal pickup coils and sensor mount-
ing errors are also determined once in orbit. Examples of possible MEMS magnet-
ometers are based on Lorentz force using resonating bars and membranes. 15–17

10.3.2 MEMS SUN SENSORS
A sun sensor determines the vector direction of the Sun, and can be either a coarse
or even a very fine attitude sensor. Many sun sensors rely, much like a sun dial, on
the shadowing effects of some masks. When reducing the size of the masks to
MEMS dimensions, problems arise due to diffraction as well as the reduced angular
deflection at these small dimensions. Two categories of conventional sun sensors
exist — digital and analog types. The digital sun sensors illuminate a geometric
pattern on the detector plane. The presence or absence of light in these well-defined
areas defines a digital signal that can be translated into the sun angle. An analog sun
sensor outputs analog currents, from which the sun angles can be derived. This
simple approach of the digital sun sensor, where the mask design creates a digital
read out of the sun position, will no longer work. A typical approach to reduce the
dimensions of a sun sensor is to use an imager and determine the centroid of a
shadow pattern generated by a mask. The mask can be produced using microma-
chining technology, which, if it can be inserted into the same process steps, could
increase the accuracy and reduce handling when producing such a sensor. One
example of such a sun sensor has been produced at JPL. 18–20 This micro sun sensor
is essentially a pinhole camera with multiple holes, and is comprised of a silicon
wafer mask with several hundred small apertures placed on top of a charge coupled
device (CCD) focal plane array at a distance of 750 mm. An image of the apertures
is formed on the focal plane when the Sun illuminates this setup. Sun angles can be
derived by analyzing the image. The experimental data presented indicate that
this sun sensor can achieve accuracies in the order of a few arcminutes or better.
It is projected that this type of sun sensor will be the size of three dimes stacked
on top of each other. It will have a mass of less than 30 g and consume less than
20 mW.

10.3.3 EARTH SENSORS
Earth horizon sensors use the Earth’s horizon to determine spacecraft attitude. In
LEO, they concentrate on merely telling which direction is down; in geosynchron-
ous earth orbit (GEO), they focus on the actual horizon and yield more accurate
attitude measurements. Since they are typically based on an IR detector, bolo-
meters, and uncooled imagers based on MEMS fabrication technology, as described
in Chapter 7 under spacecraft instrumentation, can be effectively used. An example
of such a device is the Micro Infrared Earth Sensor (MIRES) developed at LAAS-
CNRS in France. 21–23 It uses an uncooled 320 240 infrared sensor array with a

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