Onboard Doppler Compensation for Low-Rate Communications over Commercial Relay Satellites

NASA spacecraft performing scientific and exploration missions may increasingly communicate over commercial relay satellites, including those that operate at Ka-band. Ideally these spacecraft could use the same Ka-band terminal for low-rate telemetry/commanding. However, this use case presents an issue since most commercial relay systems were designed with terrestrial users in mind and cannot easily handle the Doppler effects of orbiting spacecraft. In this work we propose onboard Doppler compensation allowing spacecraft to use existing relay satellite infrastructure without modification. Specifically, we present a method for digital compensation based on a series of piecewise linear sweeps which minimizes resources used on the spacecraft’s radio. We perform orbital mechanics simulations to estimate the worst-case Doppler profiles experienced by spacecraft in several low-Earth orbits. We emulate these effects in a laboratory experiment and demonstrate the proposed technique will reduce Doppler effects by more than two orders of magnitude. The residual Doppler is small enough that modems used at commercial relay sites are able to receive error-free data even when operating at their lowest supported symbol rate.