Automatic theodolite for pre-launch azimuth alignment of the saturn space vehicles

The inertial guidance system in the Saturn 1B and Saturn 5 space vehicles is aligned in azimuth prior to lift-off by a Perkin-Elmer high precision, automatic alignment theodolite. This special theodolite, designated the AALT-SV-M2, acquires and locks onto the autocollimated images from each of two porro prisms mounted within the instrument unit on top of the S4-B booster stage of the vehicle. A separate retroreflecting prism on the skin of the instrument unit near the porro prisms is also tracked to eliminate the effects of vehicle sway. The theodolite itself is located in an underground hut between the crawler-ways about 232 m from the base of the vehicle. Six of these theodolite systems have been built by Perkin-Elmer under contract to NASA. These units have been used successfully in all the Saturn launches to date; they have consistently achieved better than the required alignment accuracy of +/-2 sec of arc for all missions. In this paper, we describe the theodolite and its function as an integrated electrooptical system. The means employed to separate the various return images into the proper channels and to generate the required error signals are discussed.