A direction-finding system built entirely from commercial off-the-shelf hardware can achieve bearing accuracy to ±3 degrees across a 100-metre operating range, packaged within a single waterproof Peli case. That result, produced using an Ettus USRP2 software-defined radio with a four-element dipole array and a suite of subspace-based algorithms combined with Kalman filtering, directly challenges the assumption that precision RF direction finding requires bespoke, expensive equipment. For MOD and defence programme teams at the feasibility stage, it is a result worth examining.
What Is Phased Interferometry and Why Does It Suit SWaP-Constrained Platforms?
Phased interferometry is a direction-finding technique that determines a signal’s angle of arrival by measuring the phase difference between signals received at spatially separated antennas. Because it operates on phase rather
than signal amplitude, it requires no large or complex receiver chain, and it performs well on low-bandwidth signals.
This makes it a natural candidate for platforms where size, weight, and power are hard constraints.
In a linear array, each antenna receives an incoming wavefront at a slightly different time, producing a phase offset
that is directly proportional to the angle of arrival. The governing relationship is:
θ = sin−1(λ φ / 2πd)
where φ is the measured phase difference, d is the antenna spacing, and λ is the wavelength. Solving for θ
accurately in practice, however, depends on both the quality of the phase measurement and the algorithm used to
extract it from noisy real-world signal data.
Which Algorithms Deliver Reliable Bearing Accuracy from COTS Hardware?
Not all DF algorithms perform equally well when hardware is constrained. The four approaches evaluated were
MUSIC (MUltiple SIgnal Classification), Averaged MUSIC, Root-MUSIC, and ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques). All are subspace methods that decompose the signal covariance matrix into signal and noise subspaces, enabling bearing estimation
without requiring a high-cost wideband receiver.
Root-MUSIC proved particularly effective. Rather than scanning a spatial spectrum, it treats the problem as a polynomial whose roots correspond directly to the phase shifts of the arriving signals. This avoids the angular resolution limits of conventional spectral scanning and is computationally efficient enough to run on modest
processing hardware.
All three subspace algorithms were combined with a Kalman filter, which refined bearing estimates recursively as
successive signal bursts arrived. The combination produced consistent bearing accuracy to ±3 degrees across the full angular test range, under real-world outdoor conditions at 100 metres.
Mathtech Consultants has applied this combination of interferometric geometry, subspace signal processing,
and Kalman-based estimation to direction finding challenges in defence and aerospace contexts. Contact us to
discuss whether this approach is appropriate for your programme.
What Does a SWaP-Compliant COTS DF System Look Like in the Field?
The trial system used the Ettus USRP2, an SDR platform supporting modular RF front ends, fitted with a WBX
daughter board covering 50 MHz to 2.2 GHz. Four dipole antennas were arranged in a linear array on an orientable
turntable. The full hardware, excluding signal generators, was housed inside a single Peli case: waterproof, portable, and deployable without specialist infrastructure.
To resolve four antenna inputs through the USRP2’s two available channels, a heterodyning arrangement used two signal generators tuned to offset frequencies, producing heterodyned pairs at 56.25 kHz and 156.25 kHz. These were separated in software using FFTs. Phase differences were preserved through the mixing process, allowing the algorithm suite to operate on correctly phased data without hardware modification.
The trial setup required no bespoke receiver hardware. It required rigorous algorithm selection, careful signal chain
design, and a methodical approach to characterising the noise subspace.
Does ±3-Degree Accuracy Meet Operational Requirements?
That depends on the application. For initial bearing acquisition, target cueing, or platform orientation in a congested RF environment, ±3 degrees is operationally useful. It is consistent with what purpose-built DF equipment achieves at considerably greater cost and weight. The trial result does not suggest that COTS hardware is a substitute for
every precision DF application. It demonstrates that for a defined class of problems, particularly those where SWaP
and procurement cost are binding constraints, the accuracy gap between bespoke and COTS systems is far smaller than is commonly assumed.
The wider implication for procurement teams is that feasibility assessment should be driven by demonstrated
algorithmic performance on representative hardware, not by assumptions about what commercial-grade
equipment can deliver.
If your programme requires rigorous DF feasibility analysis or bespoke algorithm development within SWaP
constraints, contact Dr. Tim Wren at Mathtech Consultants to discuss your challenge directly.
Download the White Paper
The trial conditions, hardware architecture, mixing schematic, and algorithm comparison that underpin this post are documented in full in the Mathtech white paper: Low Cost ‘Size Weight and Power’ Direction Finding via Interferometry. It is the appropriate starting point for any programme team assessing whether this approach is transferable to their own platform constraints.