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Test
Attitude
control inputs are based on spacecraft orientation,
which is estimated through measurements. Various
methods are given below. Each has unique characteristics
that space systems sngineers match to their spacecraft's
design requirements and orbital attributes.
Attitude
Determination
Magnetometers
sense the magnetic field to determine spacecraft
attitude. This is most effective in Low Earth Orbit
(LEO), where the Earth's magnetic field is strong.
They are more accurate at the equator than the poles,
since polar magnetic field lines are more erratic
than equatorial field lines.
Sun
sensors
use the Sun to determine spacecraft attitude. This
is the most popular method of attitude determination.
Every spacecraft orbit has a unique period when
it is masked from Sun light by the Earth. This precludes
spacecraft attitude measurements during this time
and is taken into account by engineers.
Earth
horizon sensors
use the Earth's horizon to determine spacecraft
attitude. In LEO, they concentrate on merely telling
which direction is down; in Geosynchrounous Earth
Orbit (GEO), they focus on the actual horizon and
yield more accurate
attitude measurements.
Star
sensors
are very similar to Sun sensors.
Star cameras are star sensors that sense several
stars at once. Recent developments in charged-coupled
devices (CCDs) have reduced power requirements for
these considerably, making them more practical.
They are very accurate.
Gyroscopes
are
stabilized by their spin and resultant angular momentum.
If applied torque is zero, then angular momentum
is conserved. This means that an undisturbed gyro
will point the same direction in inertial space.
Hence, a stable platform is available to reference
attitude.
