Abstract: Disclosed is a method and apparatus (1'; 1") for tracking a stellar body (22) using a telescope (9; 32) of a spacecraft (e.g., a satellite) (10; 10"). In accordance with an embodiment of the invention, the telescope (9; 32) is provided with gimbal supports (18a; 18b), and is maneuverable relative to the spacecraft (10; 10'). The stellar body (22) is acquired by the telescope (9; 32) so that the stellar body (22) is within the field of view (FOV) of the telescope (9; 32). After the stellar body (22) is acquired, an operation is performed for controlling the attitude of the spacecraft (10; 10') to within pre-established deadband limits, and, as a result, the spacecraft (10; 10') and telescope (9; 32) are each assumed to have a desired orientation relative to the stellar body (22).

Abstract: Phase detection between interfering light beams in an interferometric sys is accomplished by frequency shifting the reference beam prior to combination with the interrogation beam and electro-optically scanning the resulting pattern to produce an FM electrical signal having the shift frequency as a carrier and the interrogated parameter as the modulation data. A phase lock loop is responsive to the FM signal to provide a D.C. signal that is analogous of rate of change of the interrogated parameter with scan, and which is integrated to provide an output voltage signal that is proportional to the interrogated parameter and the corresponding optical phase difference. The method and system employing it are operative for a wide range of phase differences including multiple fringes.