Passive radar for detecting aircraft without emitting
Guest blog by Vlad Lenive, Lead RF Systems Engineer at Plextek
The case for listening rather than shouting
Every conventional radar has the same vulnerability: it announces its presence. The moment a radar transmits, it becomes detectable, locatable, and, if the adversary is sophisticated enough, targetable. For surveillance in contested airspace, for monitoring without revealing the sensor’s position, and for defence against platforms built to evade active radar, this is a real operational constraint.
Passive bistatic radar can sidestep the problem. Instead of emitting its own signal, it listens for reflections of transmissions that are already in the environment: digital TV, FM radio, DAB, mobile signals and satellite downlinks are all candidates. The transmitter keeps broadcasting whatever it was broadcasting anyway; a receiver at a separate location picks up two versions of that signal (one via the direct path, and another via reflection from e.g. an aircraft) and compares them to extract target range and Doppler information, all without radiating a single watt.
Published research has shown that using a 10 kW digital terrestrial TV transmitter as a non-cooperative source, air targets can in principle be detected at ranges of 100 km or more, with range resolution of around 20 m after appropriate signal processing. The sensor is silent, inexpensive to operate and difficult to counter by jamming, because the transmitter belongs to the local broadcaster, not the radar operator.
The hard problem: direct path interference
The difficulty is that the receiver sits in the same electromagnetic environment as the direct broadcast, and hence the direct signal is typically enormous compared with the target return. In a representative scenario (100 km target range, 10 km transmitter-to-receiver baseline, direct path arriving through a sidelobe of the receive antenna), analysis shows the target-to-interference ratio at the receiver input can be as poor as −100 dB. Even after correlation processing against a long integration of the reference signal, the echo remains buried under the sidelobes of the direct path.
Adaptive antennas that steer a null towards the transmitter offer one mitigation, but achieving the required null depth (of the order of 70 dB for the scenario above) demands a static or very slowly rotating receiver. On a fast-rotating surveillance mount, practical null depth falls to around 30 to 40 dB, far short of what is needed. The gap between what null-steering delivers in a surveillance geometry and what the physics demands is the central challenge in passive radar.
How the gap gets closed
Making passive radar work for real-world surveillance requires stacking several independent suppression techniques so that each one contributes on top of the others.
Dynamic compensation in the receiver is the first layer. A dedicated reference antenna, pointed at the transmitter, captures a clean copy of the broadcast signal. A tracking canceller then uses that reference to actively subtract the direct path interference from the target channel. Because the main antenna is rotating, the amplitude and phase of the interference vary continuously, so the canceller must track them in real time through feedback loops for both amplitude and delay. In experimental work at the University of Birmingham, this approach demonstrated around 50 dB of suppression with a 30 Hz noise-equivalent bandwidth, a result that closely matched the supporting analytical model.
Antenna polarisation is the second layer. Most terrestrial broadcast transmitters use a fixed polarisation. Receive antennas that reject cross-polarised signals well on boresight often perform poorly in the sidelobes, which is exactly where the direct path usually arrives. Certain antenna topologies, such as the disk-on-rod antenna, maintain symmetric cross-polarisation rejection across their full pattern (around 25 dB off-boresight, with sidelobe and back radiation levels of 22 to 30 dB below copolar). This adds 10 to 15 dB of additional suppression without any added system complexity.
Layered on top of adaptive null-steering, these two mechanisms together can recover the 40 to 50 dB that fast-rotating surveillance geometry costs, bringing passive detection into the practical regime.
We can help
Our Principal RF Engineer, Vlad Lenive, has hands-on experience in passive bistatic radar. His earlier research into direct path interference suppression, including the dynamic compensation and cross-polarisation techniques described above, was carried out in collaboration with the University of Birmingham and published at the IEEE International Radar Conference. That understanding of the physics, signal processing, and antenna design involved in making passive radar work at operationally useful ranges now sits within the Plextek team.
We already list multi-static and passive radar as a key capability area and have undertaken R&D in this space in partnership with Cranfield University and L3Harris, culminating in a proof-of-concept multistatic radar system capable of using multiple types of signals of opportunity. Additionally, Plextek’s broader engineering expertise is directly relevant to developing passive radar systems.
Why this matters now
The illuminator landscape has expanded considerably since the early published work on DTV-based passive radar. LTE and 5G base stations are denser and more structured than any terrestrial broadcast network: DVB and DAB cover most of Europe, and Starlink and other LEO constellations represent an emerging class of space-based illuminators with global footprint and near-instantaneous revisit. From the point of view of a passive bistatic receiver, each of these represents free transmit power, with each signal having its own bandwidth and coverage geometry to be exploited.
There is growing defence interest in detecting low-observable and low-emission platforms, and passive bistatic sensing has moved into a genuine capability requirement. If the direct path interference problem can be solved, then a wealth of free illuminators become available for silent, persistent air surveillance.
We have the expertise to help organisations explore and develop passive sensing capabilities. If this is an area you are working in, let’s talk
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Authors
Vlad Lenive
Lead RF Systems Engineer, Plextek
