NOAA KLM User's Guide
The Attitude Determination and Control Subsystem (ADACS) provides, in conjunction with the RCS and Command and Control Subsystems (CCS), the functions of the on-orbit attitude control and ascent guidance. It is a zero-momentum system consisting of reaction wheels and Earth, Sun, and inertial reference sensors. Figure 1.2.4-1 provides a simplified block diagram of the subsystem.
In the subsystem's attitude-control mode, the Earth Sensor Assembly (ESA) and the Sun sensor, together with rates derived from the Inertial Measurement Unit (IMU), furnish the primary attitude reference. Control torquing is accomplished by an orthogonal set of Reaction Wheel Assemblies (RWA's) backed up by a fourth skewed reaction wheel. The momentum accumulation in the wheels is unloaded by means of magnetic coils, which, in turn, are backed up by the RCS cold-gas thrusters. The subsystem requires ephemeris data for orbital operation, and this typically can be satisfied with a ground update once per week. In all other respects, the subsystem is autonomous, including the capability for Earth acquisition and reacquisition.
The IMU, which provides yaw, pitch, and roll rate information in orbital mode, is the key component in the ascent guidance phase. The same closed-loop guidance scheme previously used on TIROS-N and ATN missions will be retained. The IMU will furnish a navigation reference from liftoff until orbit insertion and closed-loop guidance for all satellite maneuvers following separation from the launch vehicle.
Satellite attitude-control accuracy in mission orbit is +0.2 degrees with respect to the local geodetic reference frame; knowledge of attitude is obtainable through on-ground processing to an accuracy of ±0.10 degrees in all axes. Attitude rates do not exceed 0.035 degrees per sec in pitch and yaw and 0.015 degrees per sec in roll. Attitude determination with a maximum 3-sigma variation better than 0.14 degrees is provided onboard the satellite.
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