A step-track enhancement to program-track for APL´s 18.3-meter dish antenna in support of Chandrayaan-1

Through an agreement between the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO), use of the Applied Physics Laboratory (APL) Satellite Communications Facility (SCF) has been made available to support the Chandrayaan-1 mission to the Moon, namely its 18.3-m dish antenna resource. The SCF, using the 18.3-m dish, served as the primary Western hemisphere ground station for the mission, principally to support downlink of the spacecraft´s science data at X-Band. Recent upgrades to the 18.3-m dish and ground station have been completed in order to meet the requirements of the mission, including an upgrade of the pointing system of the 18.3-m dish antenna. The historically observed accuracy of the dish´s pointing capability was sufficient for S-Band operations but not for the newly installed X-Band downlink capability. A number of potential solutions were considered and ultimately a step-track enhancement to program-track was selected as the best option for efficiently improving the pointing accuracy of the dish while requiring minimal funding support. This paper will describe the steps taken to quantify the pointing accuracy of the 18.3-m dish and the implementation of the step-track algorithm in order to achieve the required improvement in pointing performance.