Digital star scanner unit for attitude updates in space - Design flexibility and performance

The paper describes the Bendix digital star scanner unit (SSU) designed to provide moderate-to-high-accuracy attitude updates to the guidance system of a rotating space vehicle, summarizes the performance flexibility of the baseline design, and provides supporting data from a test program conducted with a prototype unit. The SSU aims at applications ranging from the detection of a limited number of bright stars at a high rate of rotation to an increasing number of detectable stars at lower rotation rates. The device employs a photon-counting detector and a digital signal processor which implements a hard-wired detection algorithm. As stars pass through one of the slits located in the focal plane of the telescope, they are detected by the photon counter whose TTL compatible output is fed to the preprogrammed processor. The processing algorithm determines the time at which the star exits the slit and also the magnitude of the detected star. The purpose of the test program is to assess the effect of the error due to phase lag uncertainty for different rates and also to generate a performance data base for comparison with the estimated attitude and magnitude error.