Development of a scalable Payload Downlink Chain for highly agile earth observation missions in Low Earth Orbit

This paper describes the development of a highly capable Payload Downlink Chain (PDC) for use on the Surrey Satellite Technology Limited 300 kg (SSTL-300) platform which has been designed to support small, low cost science and Earth Observation (EO) satellite missions operating in Low Earth Orbit (LEO). The driver behind the development of the PDC was the NigeriaSAT-2 mission commissioned in 2006 by the Nigerian National Space Research and Development Agency (NASDRA), which is due for launch in 2009. NigeriaSAT-2 is a high performance EO mission designed for a seven-and-a-half year lifetime to achieve 2.5 m imagery in a panchromatic waveband along with 5 m and 32 m imagery in four multi-spectral channels. The spacecraft will deliver high data throughput on an agile platform, whilst still maintaining high levels of pointing accuracy. All of this will be included in a 300 Kg satellite. The key requirements for the mission were to maximize the data capacity, whilst maintaining a very modest total system cost and minimizing the technical risks throughout the development phase. The majority of the spacecraft constituent parts, including the structure, the optical instruments and the PDC have all been developed by Surrey Satellite Technology Limited (SSTL) to satisfy the mission, so that the cost and schedule can be carefully controlled and be reliably repeated for future missions. Early design focused on making the system suitable for commercial operations, and consequently it was necessary to develop ways of maximizing the downlink data throughput without significantly affecting the mission cost. As a result of these requirements, an understanding of the necessary capability of the PDC was derived and then the requirement specification for each element of the PDC could be generated.