A novel approach of correlating optical axes of spacecraft to the RF axis of test facility using close range photogrammetry

Compact Antenna Test Facility (CATF) is configured to carry out antenna characterization and also for checkout of the fully integrated spacecraft with antenna and other subsystems. The CATF is used for the measurement of radiation pattern, gain, return loss, cross polar isolation, precise identification of the bore-sight axis, EIRP and gain/temperature (G/T ratio) of the antenna system under the simulated zero-gravity environment. During the test, the spacecraft is positioned on the Device Under Test (DUT) Positioner, which is about 5 m above ground and the geometric axis of the spacecraft is represented by an optical mirror cube which is correlated with RF (Radio Frequency) axis of the facility in order to match the nominal reflector bore-sight with the Nominal Plane Wave axis (NPA) of the Facility. Special fixtures are used to simulate the zero-gravity environment. Novelty of this paper lies in the development of technique which synergies the optical alignment systems and Close Range Photogrammetry (CRP) for correlating the optical axis of the spacecraft with the RF axis of test facility. This paper describes the details of the new technique, general description of the facility and the various alignment measurement systems used for alignment of spacecraft and its subsystems. The paper further claims advantages of CRP over other methods of measurement in terms of time & effort saving, real time measurement capability, measurement of critical geometries in inaccessible orientations & development of methodology to derive RF & optical axis without the use of physical features as references on the spacecraft body.