Nadir describes the direction below a particular location, opposite the Zenith direction. Nadir-pointing is the action of pointing directly below the satellite perpendicular to Earth. This operation can increase the performance of the attitude control system, allowing more efficient downlinks and solar charge for power.

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W.F. Havens and H. Ohtakay. “Attitude Determination System for a Nadir-Pointing Satellite.” Journal of Guidance; Control; and Dynamics; Vol. 1; No. 5 (1978); pp. 352-358.