Heaob

Payload Requirements Article to be pointed Pointing direction Pointing accuracy Pointing stability Slew rate Exclusion

Communication antennas

Nadir

0.25 deg

X-ray telescope Anywhere 1 arcmin 30 arcsec in 1 hr 10 deg/mln Within 15 deg of Sun

Other Requirements Sun pointing

Pointing during AV

Yaw (Z-axis) controlled to keep Y-axis normal to orbital plane Yaw (Z-axis) control as above

roll (X-axis) controlled to keep Sun in X-Z plane

Both FLTSATCOM and HEAO-B are 3-axis-controlled spacecraft. Both spacecraft had body-mounted payloads and controlled the body's attitude to point the payloads (see Table 10-13). Both spacecraft use one axis of body attitude control to orient a planar solar array toward the Sun. FLTSATCOM, which requires more than 1 kW of electric power, orients the array about a second axis.

FLTSATCOM employs a solid apogee kick motor to inject it into orbit Attitude is controlled during AKM firing by spin stabilization (see Tables 10-14 and 10-15). The spin direction can be rotated by using an off-axis thruster, the thruster fires pulses synchronized with the spin period. During normal operation and AV firing, FLTSATCOM is 3-axis stabilized. It uses Earth and Sun sensors for attitude sensing and thrusters and a reaction wheel for torque. HEAO-B did not have a AV mode. Under normal operation, it used gyroscopes and Sun sensors for attitude sensing with wheels and thrusters for torque. Its torqued gyroscopes slewed the reference-pointing direction, and payload star sensors allowed accurate reference and correction of gyro drift Radiation pressure causes the main disturbance torques for FLTSATCOM. Gravity gradient and aerodynamics were the chief torques for HEAO-B. See Table 10-18 and Sec. 11.1.

Table 10-39 shows weight and power values for components of the complete guidance, navigation, and control subsystems (including redundancy) for both FLTSATCOM and HEAO-B.

TABLE 10-39. Weight and Power for the Attitude-Control Component

Component

0 0

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