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Navigation: Attitude Control
The TOPEX/Poseidon satellite is stabilized along three axes. Thinking of
attitude control in terms of axes is a bit complicated, however. It is easier if we
use the more familiar aeronautical terms "pitch," "yaw,"
and "roll." Pitch is when the nose of an aircraft moves up or down.
Yaw is when the nose moves from side to side. Roll is when the wings dip to
one side or the other. Although we usually do not talk about satellites in terms
of nose, tail, and wings, the principles are the same.
A key parameter governing the Attitude Control System (ACS) algorithm
is "betaprime," the angle between the orbit plane and the
sunline. Betaprime is positive when the sun is above the orbit plane and
negative when it is below. Betaprime varies sinusoidally between positive
and negative over periods of ~56 days as the orbit plane fully rotates about
Earth; variations over a year result in peak values as large as ~88 deg. Proper
orientations of the satellite's altimeter antenna and solar array are governed
by the value of betaprime.
The TOPEX/Poseidon Attitude Determination and Control Subsystem
(ADCS)(shown here) controls the satellite's pitch, yaw, and roll attitudes to
ensure that the altimeter antenna (shared by the NASA and CNES altimeters)
is always accurately pointed along the "local nadir." This means
that the antenna must be aimed at the point on the Earth directly
beneath the spacecraft at all times. In addition, the ADCS "yaw
steers" the spacecraft about the nadir axis in order allow the solar
array to continuously track the sun and to facilitate tracking of the TDRSS
satellites by the high-gain antenna.
Attitude control is, in fact, a complex task, so the ADCS has several
components. First, there must be some way to determine or
"characterize" the existing spacecraft orientation. To accomplish
this, the spacecraft has two optical sensors on its top (the side farthest from
the Earth) called advanced star trackers, or ASTRAs (shown here.) The two
ASTRAs are pointed at different areas of the sky. In the onboard computer
(OBC) is a stored catalog of 327 of the brightest stars. The input from the star
trackers is compared to the star catalog by the computer to determine the
attitude of the spacecraft in all three axes.
Spacecraft attitude is adjusted using a system of reaction wheel assemblies
(RWAs), electromechanical devices that use the momentum of spinning wheels
to move the satellite in one direction or another. In addition, there are
magnetic torquer bars, which provide damping action to prevent excessive
rotation or torque from the RWAs.
All of this hardware and software is brought to bear in the service of the
science observations. To find out more about how these observations are
made and how they are turned into scientific data, see the next section
called Science Observations.
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