Half-duplex relative navigation and flight autonomy for distributed spacecraft systems

Distributed spacecraft systems concepts have been developed to leverage the inherent advantage of multiple, redundant sensing assets, including expanded capability, improved robustness, and graceful functional degradation. Both military and civilian missions have been advanced, with near-term technology demonstration efforts designed to serve as pathfinders to fully capable systems to meet future challenges. Distributing capability among multiple platforms, however, results in a fundamental increase in the complexity of coordinating and operating space systems due to delays in state knowledge and the need to integrate individual spacecraft autonomy within the broader system context. At the same time distributed spacecraft systems typically require added capabilities relative to monolithic spacecraft designs, such as command and control architectures that support diverse communication channels for functions such as crosslink communication and relative navigation measurements. The importance of these additional capabilities is particularly evident in relative navigation functionality, which in many systems may dominate absolute orbit determination requirements. This work describes a technique to address the fundamental need for relative navigation among distributed space assets that focuses on a minimalist hardware implementation that is suited for microsatellites, rovers, and other potential physically limited systems. Test results are provided from experiments implemented on The Johns Hopkins University Applied Physics Laboratory's crosslink transceiver (CLT) operating as a crosslink communication and navigation system in a time-division multiple access modes. To address the coordination and operation of a distributed spacecraft system under conditions that require capabilities such as regular relative navigation and communication, a flight autonomy architecture is defined that specifically addresses the complexities of controlling multiple, distributed assets. This architecture is based on the use of model-based programming and discrete event systems that use explicit logic models of spacecraft hardware and software components coupled with high-level control strategy specifications.