Phase-Interferometry Based Direction-of-Arrival Estimation Using Coherent Software Defined Radios

Abstract

This paper presents an experimental direction-ofarrival (DoA) system using two coherent NI USRP-2901 softwaredefined radios synchronized by a GPS-disciplined OctoClock, with one radio transmitting a narrowband tone and the other receiving on a two-element patch antenna array. Circular statistics on fifteen six-minute recordings from $-70^{\circ}$ to $+70^{\circ}$ show resultant vector length $R \geq 0.95$ for 14 of 15 angles, confirming that GPS disciplining removes oscillator drift and that residual variation comes from indoor multipath, which is often severe rather than modest. In live operation, classical interferometry reaches 2.32° mean absolute error at broadside, degrading at wider angles consistent with the arcsine sensitivity $1 / \cos \theta$. A secondary feed-forward estimator on the same calibrated phase underperforms classical interferometry during live streaming for identifiable reasons in its feature pipeline. The study reports both the strengths and the practical limitations of coherent low-cost SDR platforms for real-time DoA estimation.