2015-11-04 By Guy Martin
European defense giant Airbus Defence and Space is developing a passive radar system that will soon be available to customers.
Active radars have been around for decades but they have their disadvantages, according to Frank Bernhardt: Head of Program: Passive Radar at Airbus Defence and Space Electronics. By emitting a signal, active radars give away their positions and although there are various ways in which they can try and avoid detection, this is a significant drawback. Other limitations are emitting radiation that can interfere with other signals or can violate health and safety rules (such as emitting in a built-up area).
The idea of passive radar technology has been around since the 1930s, with Robert Watson-Watt performing experiments in 1935. However, it was not until some 20 years ago that technology (especially computer power) matured enough for passive radar to become viable.
The first passive radar systems were bistatic – in other words, the transmitter and receiver were in different places. Modern passive radar uses any transmitter of opportunity to detect disturbances and reflections in electromagnetic signals in order to determine the position of an object. Bernhardt said that FM radio signals and digital video broadcasting signals are the main passive radar sensors as they are strong emitters good for air surveillance.
Bernhardt said that the resurgence of passive radar has come about because of the availability of advanced processing technology that requires huge computing power. Another driver is the fact that there are a lot more emitters out there, providing many more signals to work with – if there are no electromagnetic signals in the atmosphere, passive radar obviously cannot work.
Bernhardt said that a passive radar system needs several transmitters and receivers to detect targets. The current Airbus system uses FM, digital audio and digital video transmissions to provide 360-degree coverage and three dimensional tracking. Tracks are updated every one to two seconds as the radar processes incoming signals. Range, using FM, is up to 250 km (for a commercial aircraft size target) with 500 meters accuracy while using digital audio and video broadcasting gives a range of up to 30 km with 10 meter accuracy. The passive radar software is sophisticated enough to detect helicopter and propeller blades, giving an indication of what type of aircraft has been detected.
Some of the many benefits to passive radar include jam resistance (it would require enormous power to jam all transmitters in an area), covert operation, silent operation, low power consumption, safety (no emissions, allowing it to be used in built up areas like suburbs and harbors), low cost (no moving parts or transmitter) and no frequency interference.
Another benefit of passive radar is that it can detect stealthy aircraft. It uses low (less than 1 GHz) frequencies, which stealth aircraft aren’t designed to absorb. Although Airbus hasn’t done trials with stealth aircraft, Bernhardt said that he believes passive radar could be promising in this regard although this was not the main reason for developing the technology.
Although passive radar has many defence applications, it has also been promoted for air traffic control as it can be used in built-up areas and has a relatively low cost. Airbus Defence and Space has been exploring the use of passive radar for air traffic control with Germany and the United Kingdom.
The Airbus passive radar mounts passive radar antennas on a 14 metre folding mast fitted to a Mercedes Sprinter vehicle. The radar is able to detect large and small aircraft out to hundreds of kilometers, giving their position, speed and height.
Passive radar is still a relatively early technology which Lothar Belz, Airbus Senior Manager Media Relations, believes will only mature in about five years’ time. At present it is not accurate enough for things like fire control. Nevertheless, Bernhardt said that “We are forming passive radar into a viable product.”
Airbus has been heavily involved in the development of passive radar since 2011/12 but certification, qualification and so on still has to be done, with viable systems only making it onto the marketplace in several years’ time. In the meantime, quite a number of potential customers have expressed interest in the technology, including NATO countries.
As Airbus Defence and Space is still developing the technology, it has various improvements in the works, such as exploring the exploitation of satellite transmitters (such as digital video broadcasting for handheld devices) and LTE and other waveforms such as cellphone signals. Bernhardt predicts that 20 years into the future, passive radars will be mounted on aircraft and be able to move around on the ground.
Last month, the French Air Force said the first flight of a passive airborne radar took place at Air Base 701 at Salon-de-Provence, in south-eastern France on 16 October. Since 2013, the French air force Academy, Ecole de l’Air, has carried out passive radar research. The project is a joint effort of the Centre de Recherche de l’Ecole de l’Air (CReA), the National Office for Aerospace Research (ONERA), and the SONDRA laboratory, a joint venture between ONERA and the Central-Supélec school.
The experimental aircraft used for these tests was a Busard motoglider belonging to ONERA, fitted with a complete receiver system that was specially developed for the project, and which in particular can be integrated into a pod carried by the aircraft. The French Air Force said that data integrity was validated and data is currently being processed by the project team to further develop processing methods especially tailored to the airborne environment.
As passive radar (also known as passive coherent location, passive bistatic radar, piggyback radar and covert radar) is such as promising field, it has attracted a number of companies like Thales (with its Aulos), Selex (with its Homeland Alerter 100) and Elta. In the 1990s Lockheed Martin developed its Silent Sentry system that used FM radio towers as a source of radiation.
South Africa is one of a few countries in the world to have successfully developed a passive radar system. The University of Cape Town, in association with Peralex Electronics and CSIR Defence, Peace, Safety and Security (DPSS) developed a prototype system that can be deployed in the field. During testing, three receiver sites were established in the Western Cape and the system was used to detect airliners at ranges of 150 km. Testing was also done at the Paardefontein range outside Pretoria, where a light aircraft (Cessna 172) was successfully detected and tracked.
Guy Martin visited the Airbus Defence and Space Electronics facility in Ulm, Germany, as a guest of the company.