Architectures for pointing a payload hosted on a GEO spacecraft

One general process for developing a space mission is to design and build a custom spacecraft to host one or more payloads (e.g., instruments and/or communications hardware). Where a spacecraft is already designed, there can be room for an additional payload for a secondary mission. Substantial cost savings can be realized for the secondary mission if the primary spacecraft bus can provide a platform that meets all interface and pointing requirements for the secondary mission. A number of instruments have been hosted as secondary payloads on a spacecraft in Geostationary Earth Orbit (GEO), with several different pointing control architectures. This paper describes two pointing control architectures as they apply to a hosted payload with its own steering mirror and line of sight (LOS) controller that helps to achieve tight pointing stability for Earth observations. One architecture has technical and programmatic advantages compared to the other. Programmatic differences between architectures are discussed in qualitative terms, highlighting issues related to testing and verification of pointing requirements early enough in the development to allow for modification of the secondary payload LOS controller design. For technical comparisons between architectures, strawman requirements for pointing and geolocation are introduced as metrics. The effects of host spacecraft attitude and orbit determination uncertainties are discussed using notional geolocation error allocations. Pointing stability and performance issues related to sensor type, placement, and data latency are illustrated using a representative model of vehicle flexible body dynamics, over a range of model parameters. Results show that there is a risk of pointing performance degradation and even instability due to poor gyro placement and/or gyro data latency. This has implications regarding hosted payload controller bandwidth and achievable disturbance rejection. One pointing architecture does well in mitigating the risk while maximizing performance. That architecture shows promise in achieving reasonable geolocation accuracy.