Controlling the orientation of the TOPEX/Poseidon satellite in space is the responsibility of the onboard Attitude Control System (ACS). The satellite is stabilized along three axes: X(roll), Y(pitch), and Z(yaw). The ACS determines the orientation from star trackers, the digital fine sun sensor, gyros, magnetometers, and the Earth Sensor Assembly Module.[Figure above] Then, it uses reaction wheels, magnetic torquer bars, or the thrusters on the Propulsion Module to control the pitch, yaw, and roll attitudes of the satellite, which allows the altimeter antenna to be pointed accurately at the surface of the Earth right underneath the satellite for the data acquisition at all times. Also, it allows the solar array to be aimed at the Sun properly and helps the high-gain antenna to track the TDRSS satellites.
A key parameter in the ACS algorithm that governs the orientations of the altimeter antenna and the solar array is β'(betaprime), the angle between the orbital plane and the sunline. The angle is positive when the Sun is above the orbital plane and negative when it is below the orbital plane. It varies sinusoidally between positive and negative over periods of 56 days as the orbital plane fully rotates about Earth. The peak values of variations over a year become as large as 88 degrees.
The satellite has several yaw control modes, depending on the value of β'. All the modes are intended to steer the satellite about Z axis. When β' is greater than 15º satellite performs "positive yaw steering." When β' is near 15º, the satellite is put into a "fixed yaw" position to limit peak yaw angular rate. When β' is between 0º and 15º, the satellite yaw angle is maintained at 0º, which means it flies forward with its bow pointed in the direction of motion. When β' is between -15º and 0º, the satellite yaw angle is maintained at 180º, which means it flies backward with its stern pointed in the direction of motion. When β' is smaller than -15º, the satellite performs "negative yaw steering."
|β' > 15ƒ||positive yaw steering|
|β' = 15º||fixed yaw position|
|0º< β'< 15º||0º yaw angle, the satellite flies forward|
|-15º< β'< 0º||180º yaw angle, the satellite flies backward|
|β'< -15º||negative yaw steering|
Back to Main Page
This page is created by Masaharu Suzuki The University of Texas at Austin
Last Modified: Wed Feb 11, 1999