Control

Accuracy

How well the vehicle attitude can be controlled with respect to a commanded direction

0.25 deg, 3 o; Includes determination and control errors, may betaken with respect to an Inertlal or Earth-fixed reference

Range

Range of angular motion over which control performance must be met

All attitudes, within 50 deg of nadir, within 20 deg of Sun

Jitter

A specified angle bound or angular rate limit on short-term, high-frequency motion

0.1 deg over 1 mln, 1 deg/s,1 to 20 Hz; usually specified to keep spacecraft motion from blurring sensor data

Drift

A limit on slow, low-frequency vehicle motion. Usually expressed as angle/lime.

1 deg/hr, 5 deg max. Used when vehicle may drift off target wtth infrequent resets (especially if actual direction Is known)

Settling Time

Specifies allowed time to recover from maneuvers or upsets.

2 deg max motion, decaying to < 0.1 deg in 1 mln; may be used to limit overshoot, ringing, or nutation

* Definitions vary with procuring and designing agencies, especially In details (e.g., 1 or 3 a, amount of averaging or filtering allowed). It is always best to define exactly what Is required.

• Repoint the payload's sensing systems to targets of opportunity

• Maneuver the attitude control system's sensors to celestial targets for attitude determination

* Track stationary or moving targets

* Acquire the desired satellite attitude initially or after a failure

Spacecraft Subsystems

Mission

• Earth-Pointing or Inertial-Polntlng?

• Control During AV Bums?

• Separate Payload Platform?

• Accuracy/Stability Needs?

• Slewing Requirements?

• Onboard Navigation Data Required?

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