Autonomous loop switching: Interpreting and modifying the internal state of feedback tracking loops

Receiver tracking loops are implemented in software in modern space-borne radios. Software implementation allows loop designs to be modified in flight. Not only filter coefficients and gain, but also loop order and type can be modified. This flexibility enables new cognitive and autonomous capabilities. Loop designs can be optimized for each mission phase, and separate loops can be used for acquisition and tracking. Furthermore, the loops can be automatically adapted based on changing signal dynamics or SNR. However, if the loop has tracked away from its quiescent state, the loop will lose lock when the switch occurs unless the internal state of the loop is translated appropriately. This paper considers the internal state of feedback tracking loops. In particular, a physical interpretation of loop state is derived that enables translating the loop state from one design to another. Limitations to autonomous switching, including high-order signal states and noise, are described, and several examples are simulated. Practical applications for both near-earth and deep-space missions are discussed.