When Serbia shot down U.S. Air Force F-117 during the Kosovo war in 1999, skeptics of stealth claimed vindication. However, that success was due to a combination of poor mission planning, smart air-defense operators exploiting both radar and ELINT sensors, some vulnerability in the first-generation platform–and pure luck. Low-observable technology has moved on, and the F-22, F-35 and the latest UCAVs are stealthier than the F-117.

However, counter-stealth technology has also been further developed, including passive coherent location (PCL) techniques. In the late 1990s, Lockheed Martin’s Silent Sentry was the first such complete system to be publicized, and others followed, in both the West and the East. Now EADS Cassidian (Hall 2a, Stand A253) has gone public with a system it calls Passive Radar, which it began developing in 2006. It is a system that also has useful applications in air-traffic control, according to Cassidian. “The principle of passive radar has been known for a long time,” said Elmar Compans, head of sensors and electronic warfare at Cassidian. “However, we have now integrated the latest capabilities of digital receiver and signal-processing technology to significantly enhance range and detection accuracy by monitoring various emitters at the same time.” Frank Bernhardt, project manager airborne and space radars for Cassidian, added: “I’m very proud of what our engineers have accomplished; we don’t know of any other system that has fused emissions from three bands.”

PCL is the capture and analysis of the radiation that is reflected from an aircraft as it flies through the fields of various transmitters, such as analog or digital radio broadcasters, television stations and mobile phone stations. By networking multiple PCL receivers, coverage of a wide area can be achieved. Further, PCL offers detection capacity in areas of radar shadow, such as mountainous terrain. It is capable of locating extremely low- and slow-flying objects. And, ironically, a PCL system is itself stealthy, because it generates no electronic emissions.

Cassidian’s first experimental passive radar system in 2008 covered only FM transmissions, but by 2011 the company had extended coverage to digital radio (DAB) and digital terrestrial television (DVB-T) transmissions. The system of antennas, a mast, processors and operator displays has been packaged to fit in a commercial van. It has already been evaluated by the German Office of Defense Technology and Procurement (BWB), and will be available for sale in 2015, said Cassidian.

Bernhardt is bullish about the system’s ability to detect low-observable aircraft. “Our multistatic architecture is a big counter-stealth advantage. And stealth aircraft are coated against high frequencies, whereas we operate from 100 MHz upward,” he said.

The Cassidian Passive Radar has a high detection-update rate (every 0.5 seconds); covers 360 degrees; has 3-D (meaning that it includes altitude) capability to about 40,000 feet; and demonstrates robust track continuity, especially during high-speed maneuvering, claims the company. “We can detect low-altitude targets better than an active radar, because of the low frequencies that we use,” added Bernhardt.

The system can exploit up to eight FM transmitters at a time, to give a location accuracy of 500 meters (1,650 feet) at the maximum range of 200 kilometers (125 miles). Only one DAB and one DVB-T frequency can be exploited at one time, but an accuracy of 10 meters (33 feet) can be achieved, albeit at shorter maximum range of 50 kilometers (30 miles). But as noted previously, the Cassidian system can fuse data from all three bands.

The air-traffic-control applications could be used for gap-filling for temporary security during high-profile events, such as G8 summits, or where radar transmission frequencies are in short supply, or could pose a radiation hazard to bystanders.

“We could place a system on wind-turbine farms, that could automatically switch on anti-collision lights when aircraft or flocks of birds approach,” suggested Bernhardt. He revealed plans to incorporate a mode-S or mode-5 emissions tracker into the system within the next year or so. The system could also find applications in ground and maritime surveillance, detecting small boats and protecting harbors and coastlines.

Cassidian plans to offer the passive radar system in various options that will be customer-driven, it said. These will include fixed, re-locatable and fully mobile options and single-to-multi locations. A choice of frequency bands will be offered, although Bernhardt admitted that a three-sensor system would be expensive.

In Europe, Thales Raytheon Systems also developed a PCL product a few years ago. However, a marketing manager at the company told AIN that it was not currently being offered. He said that the technology was certainly promising, for both defense and ATC applications, but that more research and development was needed.