ACCURATE ANTENNA REPOINTING FOR PATTERN MAPPING

Abstract

The attitude control system of the European Space Agency’s Orbital Test Satellite (OTS) was originally designed for nominal earth pointing with only limited bias capability (of up to 2.5° in pitch and roll) which is more than adequate to remove earth sensor/wheel misalignments that could be incurred during or following launch. Subsequently, a need was expressed to support off-nominal coverage missions and antenna mapping tests. A method was thus defined that would provide greater repointing capability (up to 4.6°) while retaining the accuracy available with the precision infrared sensor. The method is outlined, and the repointing limitations indicated that are inherent to the OTS sensor design (13° in roll and 7° in pitch). Operations and conditions are stated that enable these extremes to actually be reached. The error budget is presented for the case of antenna mapping, demonstrating that attitude restitution can be made so that beam centre position can be determined to an accuracy of 0.1° half cone angle. The significant advantages of the described method are that only one ground station is required, and that results can be available within 24 hours following completion of the test. Results obtained with OTS are referred to, that support the claims. Finally desirable design modifications are discussed that could allow further increases in repointing capability of future satellites.