In-Orbit Space Situational Awareness Using Doppler Frequency Shift

With near-Earth space becoming increasingly congested, the threat of satellite collisions escalates. Existing collision avoidance methods heavily rely on ground-based radar measurements to determine the ephemeris of space objects and dictate maneuvering commands. This article introduces a method that proposes estimating the miss distance between two spacecraft solely through Doppler frequency shifts of an active beacon, thereby enhancing in-orbit situational awareness and autonomy. Our methodology commences by establishing the relative geometric model of two arbitrary orbits and subsequently formulating an optimization framework. Accordingly, a spacecraft should be capable of deducing the optimal geometrical parameters that minimize the variance between measured Doppler shifts and calculated ones, which subsequently allow the estimation of the miss distance. To validate our model, extensive simulations are conducted across diverse scenarios to evaluate the accuracy of estimating the miss distance. Furthermore, we perform rigorous real-life Doppler measurements using a ground station that tracks low Earth orbit satellites, aiming to ascertain the attainable precision in Doppler estimation.