Accelerated Aperture Synthesis from Free-flying Collectors

Radio frequency aperture synthesis from multiple free-flying collectors is traditionally dependent on highly accurate intra-constellation metrology and shared clocks. We demonstrate that relaxed precision of relative positioning and clocking can be acceptable through computational means in post processing. The work assumes a small cluster of RF collectors that can collect on an emitter at the same time, with position and time accuracy provided by commonly available aerospace GPS units. No cross links or inter-vehicle spacing is assumed. In lieu of accurate timing, correlation searches are performed at the ground station. A key part of the idea is an alignment stage following collection. This is composed of a Cross Ambiguity Function (CAF) search across all possible FDOA/TDOA shifts, and a phase alignment search. These time, frequency and phase corrections are applied to the data, bringing them into alignment with each other. Maintaining this alignment over a longer time span is the challenge, covered here in some detail. This paper covers our work to date. This includes on-orbit results, a simulation model of the coherence-through-computation mechanism, consisting of models of the geolocation process, Doppler correction over time, and transforming input signals to a common time and frequency reference. This paper extends our previous publication in the IEEE Aerospace conference[1] with several key advances, including parallized execution, interpolated TDOA/FDOA tracking, and multi-emitter tracking.