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212 MEMS and Microstructures in Aerospace Applications

The MEMS technologies used for these systems are similar to those discussed in
Chapter 8.
In general, there are several design considerations that must be considered in
the design of spacecraft attitude sensors. Chief among these are the specific nature
of the control system application. The constraints associated with predicted en-
vironmental conditions such as the prelaunch handling, launch loads (mechanical
vibration or shock as well as acoustic exposure), pressure venting profiles, on-orbit
operating temperatures, particle contamination, EMI or EMC effects, and radiation
exposure (both the total dose and heavy ions) must be well understood and docu-
mented prior to the detailed design phase of the sensor.
Other system level, but no less important, considerations come into play with
spacecraft attitude sensors such as the specific placement and orientation of the
device on the spacecraft or platform structure to be controlled. Inadequate attention
to these details, especially on very lightweight highly flexible structures, can lead to
destabilizing (and, in extreme cases, possibly destructive) controls–structures inter-
action (CSI) problems for the GN&C designer.
The imminent introduction of the MEMS-based GN&C sensor technology into
the spacecraft designer’s inventory will herald a breakthrough in how the function
of medium-to-high accuracy attitude determination will be implemented in future
space missions.

10.3.1 MEMS MAGNETOMETERS
MEMS magnetometers have already been discussed in Chapter 7, Microtechnologies
for Science Instrumentation Applications. A magnetometer measures the three com-
ponents of the magnetic field and provides a measurement of the attitude relative to
inertial coordinates. Since only the direction of the magnetic field is sensitive to the
attitude, another vector measurement such as a sun sensor is required for attitude
determination. For magnetometers, the largest component of the random noise for
attitude determination arises not from the sensor itself, but from the magnetic field
model, which, for LEO orbits, can cause an error of 0.58 at the equator, and up to 38
near the magnetic poles. Therefore, the sensitivity requirements for magnetometers
as an attitude sensor are relatively weak and provide an opportunity for insertion of
MEMS devices. The performance requirements for attitude determination magnet-
ometers are a range of about +60 mT, with a sensitivity of +10 nT.
A number of miniature magnetometer developments have occurred in recent
years. For the SUNSAT-1 satellite, the magnetic observatory at Hermanus manu-
factured a miniature fluxgate magnetometer with this performance at a size of about
130 mm 90 mm 36 mm and a weight of 295 g.
The University of California, Los Angeles, has developed a miniature fluxgate
magnetometer for NASA’s NMP ST5 small satellite mission. The magnetometer
mass and power is kept low with a dual core series drive circuit. The magnetometer
has two commendable ranges, 64,000 and 1000 nT. The dynamic range is changed
from 64,000 to 1000 nT by altering the closed loop response from 64,000 to 5000
nT, and then amplifying the signal to get to a 1000 nT range. This method keeps the

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