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'PITCH CONTROL VIA MOMENTUM EXCHANGE.

"REACTION WHEELS AT NOMINAL ZERO BIAS FOR ROLL/YAW CONTROL.

'PITCH CONTROL VIA MOMENTUM EXCHANGE.

"REACTION WHEELS AT NOMINAL ZERO BIAS FOR ROLL/YAW CONTROL.

SEASAT-A, the large, symmetric Agena (the final stage of the launch vehicle) remains attached to the experimental module and provides a large gravity-gradient restoring torque; however, the O.S-deg pitch control requirement can be satisfied only by adding a pitch sensor and controlling the pitch wheel speed. For both HCMM and CTS, the effect of the gravity-gradient restoring torque is negligible, and pitch control is achieved by controlling the pitch wheel speed. Roll/yaw control, which is equivalent to maintaining the wheel angular momentum along the orbit normal, is accomplished as shown in Table 18-2. Except for GEOS-3, for which the gravity-gradient resoring torque is large, active control is achieved by using a sensed roll error to drive a torquing system. For all four spacecraft, yaw is controlled indirectly via quarter-orbit coupling with roll (see Section 18.2) because of the lack of simple, effective yaw sensors in Earth orbit.* Both SEASAT and CTS, however, use an indirect method of augmenting the yaw control, referred to as WHECON (an acronym for wheel control) by commanding a yaw torque based on the sensed roll error.

For spacecraft with magnetic coils, dampers, or residual dipoles, the geomagnetic field in the spacecraft coordinates is required. Assuming a dipole field and an orbit passing over the magnetic pole, the magnetic field in orbital coordinates (see Section 5.1 and Appendix H) is

where BM = 7.96X 106/*3 (Teslas) as defined in Eq. (H-18), J? is the geocentric distance in kilometers, and A is a continuous measure of the latitude (i.e., A is the

Table 18-2. Control Systems for Spacecraft in Table 18-1
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