How can TV signals be used as radar?

Last week you might have seen the story about our project looking at whether standard TV transmissions could one day be used as a potential replacement for conventional radar.

I’m delighted with the amount of interest the project has generated, but thought there was scope to give a deeper explanation of the technology behind the headlines.

The work has been part of Project PROVE – or Passive Radar Operational Viability Evaluation – and the technique we were testing is called Multi-Static Primary Surveillance Radar, but before I explain about MSPSR, it is worth understanding how we currently perform non-cooperative radar surveillance.

Non-cooperative detection of aircraft is currently performed by traditional Primary Surveillance Radars (PSRs). These systems, without request or stimulus, transmit pulses of radio frequency (RF) energy in the direction the rotating antenna is facing. If an object is on the path of the pulses, a proportion of energy is reflected back. The aircraft’s position is then derived by combining the time for the RF energy to hit the aircraft and bounce back, along with the angle the antenna was facing at the time of transmission.

How Primary Radar works

For these traditional PSRs, the transmitter and receiver are collocated, using the same antenna to transmit and receive.  If we go back in time to some of the very earliest radar experiments in the 1930s, the transmitter and receiver were separated, often by tens or hundreds of metres. These were known as bistatic radars, which are the principal building blocks of an MSPSR.

Operating similarly to PSRs, a transmitter sends out a signal that is then detected by the receiver and known as the reference.  If the same transmission strikes an aircraft, a proportion of the energy is then reflected towards the receiver. The receiver then has two pieces of information; the arrival time of the reference signal (baseline) and the indirect signal that has bounced off the aircraft (known as bistatic range). This enables the system to estimate the location of the aircraft somewhere on an oval around the transmitter-receiver baseline, known as an oval of Cassini.  However, at this stage the position of the aircraft is still ambiguous.

With one receiver you know the aircraft is somewhere on the oval.

To resolve this ambiguity, two and preferably three more baselines are required, with each producing an oval of Cassini somewhere on which the aircraft is located. We can then process the data from each of the baselines and look for common results. Where three or more baselines correlate we can declare that an aircraft has been detected.

Multiple receivers then allows you to point point the aircraft where the signals correlate.

What I’ve described is for a single transmitter and spatially distributed receivers, however collocated receivers and multiple transmitters are just as valid a set-up for an MSPSR, as are hybrid architectures and there is no perceived performance difference between them. The key benefit of MSPSR over conventional radar is the potential reduced operating costs, especially where the system exploits non-conventional third party transmissions such as the TV signals we’ve been using.

It could also be a potentially more efficient use of the radio spectrum than conventional radar, while we also think MSPSRs experience less interference from wind turbines, although the results were not entirely conclusive.

There is still a long way to go before this could ever replace conventional primary radar, but it’s been an exciting project to work on and I’m looking forward to seeing where the research goes over the next few years.