Lightweight Distance and Relative Radial Velocity Estimation with a Passive RF Receiver

Abstract

Reliable positioning is essential for autonomous drones, yet global navigation satellite systems (GNSS) can become unreliable or unavailable. This work compares passive RF-based methods for estimating distance and relative radial velocity with a single unsynchronized receiver. Calibrated RSSI ranging and direct Doppler estimation are selected as lightweight approaches and evaluated using an RFSoC 4x2-based prototype with a continuous-wave signal. Under controlled indoor conditions, calibrated RSSI achieves a mean absolute percentage error of 6.3% over the tested positions, although antenna orientation changes RSSI by almost 7 dB at a fixed distance. Doppler estimation, with clock offset estimated from stationary measurements before and after motion, recovers the correct motion direction in all motion trials and achieves a mean absolute error of 0.33 m/s. The motion-trial RMSE and the stationary null-test standard deviation were both 0.43 m/s, suggesting that imperfect clock-offset compensation and estimator uncertainty account for a substantial part of the radial-velocity estimation error. The results show that both methods provide lightweight distance and radial-velocity features, but their accuracy is limited by propagation conditions and oscillator stability.