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A new concept of small, mobile, virtually undetectable radars is quietly gaining traction in the air surveillance community. Passive radar systems, which emit no signals, could complement conventional radar coverage in a number of important applications. As the technology reaches maturity, we take a look at the key benefits and assess the real-world potential of this future generation of sensors.


Radars detect objects by sending out electromagnetic signals and listening to the echoes that bounce back. So a conventional radar (short for RAdio Detection And Ranging) requires both a transmitter and a receiver.

What's different about a passive radar is that it doesn't have its own transmitter but uses some of the many existing sources of electromagnetic energy, such as low-frequency broadcast transmitters, and simply listens to the reflected signals. The Multistatic Silent Primary Radar (MSPR) developed by Thales Air Systems, for example, uses FM radio transmitters as the energy source. Passive detection techniques have been used since the Second World War, when operators donned headsets to listen for incoming aircraft. But with the computing power and sophisticated algorithms available today, passive radar is a viable technology that offers military commanders a reliable way to extract meaningful information from the background noise.

Passive radar systems offer a number of key benefits. First, they are hard to detect by conventional means. Electronic sensors cannot pick them up because they do not transmit their own signals. They have no dedicated transmitters generating heat, so they cannot be detected by their thermal signatures. And although the broadcast antennas are visible to the naked eye, they are generally small and quite difficult to spot. This high level of discretion is a major advantage in air surveillance, because potentially hostile or non-cooperative aircraft have no way of knowing that they are being watched.

The second major benefit of passive radars is that they are relatively easy to set up. They do not operate in their own frequency band so there is no need to request frequency allocations before using them.


Major nations are already extensively equipped with conventional radar systems, but surveillance coverage is incomplete because of terrain features like mountains and valleys — and simply because the Earth is round. Passive radar offers an alternative to the gap-filler radars often needed to complement conventional air surveillance sensors. And a 3D air picture generated by a multistatic passive radars can provide the same kind of information as a conventional radar system. By combining conventional radars with passive radars, users have access to a complete air picture without needing to worry about which sensors are being used.

Small UAVs now pose as a significant threat to both military installations and industrial facilities. Passive radar could maintain surveillance of the airspace immediately surrounding a strategic site, and has the potential to enhance security and provide effective protection from these new threats.

Early warning
The earlier a threat is detected, the better the chances of neutralising it successfully. The Direction Finding System (DFS) developed by Thales Communications & Security relies on technology similar in some respects to passive radar. Deployed concurrently with a passive radar system, the DFS can detect an aircraft at a range of up to 200 km — early enough to observe its behaviour and determine whether it is a threat or not.
The DFS works by listening to the aircraft's radio frequency transmissions (GPS, radio communications with the ground, etc.) to determine its 2D map coordinates and flight path and observe its behaviour for long enough to decide whether it constitutes a threat. The passive radar takes over when the aircraft is within a range of 50 km and determines its position in three dimensions. On average, that still leaves about 15 minutes to react if need be. Industrial operators have shown considerable interest in the potential of this type of solution for protecting their strategic sites.

Another major benefit of passive radar is its ability to detect low-observable targets such as stealth aircraft, which are designed to be invisible to conventional radar sensors. Passive radar systems sift through a host of signals of opportunity, such as low-frequency FM radio broadcasts, and determine how they are blocked or altered by having to pass through or around objects. A stealth aircraft therefore has exactly the same the probability of being detected as a regular aircraft, making the technology of particular interest to military planners.

Timescale for deployment

Despite the enormous potential of this technology, no passive radar systems have been deployed to date. Most air forces are already extensively equipped with conventional systems, and they like to be in full control of the transmit and receive chain of each of their radars. Using signals of opportunity that are not part of their own systems would be a major revolution, and there are few signs that such a cultural shift will take place any time soon.

The civil aviation industry is examining the potential for adopting this technology as a less expensive alternative to the conventional radars used for approach and landing. Here again, given the exacting safety and dependability requirements of the civil air transport sector, signals of opportunity are not deemed sufficiently reliable. However, studies are underway as part of the SESAR programme to evaluate the potential of passive radar systems using dedicated radio transmitters. Depending on the findings of these studies, the future role of passive radar in civil airspace surveillance should be clearer by about 2020.

Meanwhile, the passive radar community is continuing to develop the technology further, confident that it will eventually find its place on the market. Thales Air Systems has an R&D team of about 10 people working in this field, and all the other major players in the defence and aerospace industry are investing in the technology. The low cost of the hardware involved also makes passive radar a particularly attractive field for academic researchers, who are developing sophisticated processing algorithms to improve passive radar performance as well as investigating the potential of new digital broadcasting signals.