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Apollo spacecraft sextant
From: Jim Thompson
Date: 2004 May 2, 08:47 -0300
From: Jim Thompson
Date: 2004 May 2, 08:47 -0300
Here is a picture of the sextant used on the Apollo spacecraft in the 1960's: http://www.nmsi.ac.uk/piclib/imagerecord.asp?id=10253577 Appolo's guidance and navigation system is described here, in an excerpt from the Apollo operations manual: http://rocinante.colorado.edu/~wilms/computers/apollo.html. "The sextant is a highly accurate optical instrument capable of measuring the included angle between two targets. Angular sightings of two targets are made through a fixed beam splitter and a movable mirror located in the sextant head. The sextant lens provides 1.8-degree true field of view with 28X magnification. The movable mirror is capable of sighting a target to 57 degrees LOS from the shaft axis. The mechanical accuracy of the trunnion axis is twice that of the LOS requirement due to mirror reflection which doubles any angular displacement in trunnion axis." They used the sextant for determining position in space, spacecraft attitude, measuring angles on the moon, and for taking pictures of potential landing spots. That page explains the role of the sextant ("SXT") within the spacecraft's G&N system: The optics provide accurate star and landmark angular measurements. Sightings are accomplished by the navigator using the SXT (sextant) and SCT (scanning telescope). The optics are positioned by drive motors commanded by the optics hand controller or manually using a universal tool, as desired. The shaft axes are parallel. Trunnion axes may be operated in parallel or offset, as desired. The SCT is a unity power instrument providing an approximate 60-degree field of view. It is used to make landmark sightings and to acquire and center stars or landmarks prior to SXT use. The SXT provides 28-power magnification with a 1.8-degree field of view. The SXT has two lines of sight, enabling it to measure the included angle between two objects. This requires two lines of sight which enable the two viewed objects to be superimposed. For a star-landmark sighting, the landmark line of sight is centered along the SXT shaft axis. The star image is moved toward the landmark by rotating the shaft and trunnion axes until the two viewed objects are superimposed. The shaft and trunnion angles are repeated by the optic CDUs. When the navigator is satisfied with image positions, he issues a mark command to the AGC (Apollo guidance computer). The AGC reads the optics CDU angles, IMU CDU angles, and time, and computes the position of the spacecraft. The AGC bases the computation on stored star and landmark data which may also be used by the AGC to request specific stars or landmarks for navigational sightings. Two or more sightings, on two or more different stars, must be taken to perform a complete position determination. Now THAT's astronavigation. This is a highly significant milestone in the evolution of the sextant, and in the history of human navigation. It means that humans first navigated to the moon in a similar way to how Cook opened up the Pacific Ocean centuries ago -- by doing celestial navigation with an optical instrument. Although their peripherals were vastly more sophisticated than Cook's. Jim Thompson jim2@jimthompson.net www.jimthompson.net Outgoing mail scanned by Norton Antivirus -----------------------------------------