Investigating latency in PnP GN&C architectures

This paper examines the nature of PnP avionics latencies and their effects on a spacecraft attitude control system via simulation and hardware-in-the-loop experiments. Linear analyses for a single-axis control system simulated in the MATLAB-SIMULINK environment with delays were performed to characterize different delay types and determine stability margins. These included effects of static and variable delays in the forward and feed-back paths. The analytical model was also used to establish a baseline system and validate the testbed system´s nominal behavior. Testing was done in an AFRL industry partner PnP testbed. Several test cases were developed with various levels of network and component processing loads designed to introduce delays in the system. Initial baseline testing indicated that system latencies were quasi-static in nature meaning delays were effectively constant compared to loop dynamics. Both time domain and frequency domain analyses were performed to populate a test matrix for various load conditions ranging from the baseline with no additional loads to maximally loaded cases. Frequency domain results varied little from the baseline cases to the cases with heavy loads with respect to phase lag; however, the heavily loaded cases did exhibit more noise. Time domain analysis of the maximal loading cases showed maximum sample time jitter of 1.5 times the nominal sample time with predominant jitter about one sample. The major effects of the maximally loaded cases presented as some signal degradation but all cases remained stable. The main conclusion was that latencies caused by maximal loading were on the order of the sample period and did not significantly affect control system performance for the point design under consideration.