European companies, especially in the East, are continuing to refine passive ground-based technologies with the potential to detect stealth aircraft. The best known of these is the detection and correlation of emissions from aircraft–such as from radars, radar altimeters and other navigation devices–using ESM/ELINT techniques, sometimes known as passive emitter tracking (PET).

Various techniques are used to calculate the position of the target, such as triangulation, interferometry and TDOA (time difference of arrival). In recent years, detection has become possible by capturing and analyzing the signal fluctuations caused by an aircraft as it flies through the fields of civilian radio, television and mobile phone transmitters. The latter technique uses Doppler-shift processing together with bi-static range and angle-of-arrival measurements. It is usually known as passive coherent location (PCL), but is sometimes referred to as “passive radar.” The accuracy of both PET and PCL systems has improved progressively thanks to the increase in processing power.

Czech company ERA has inherited and refined the PET and TDOA technology that was developed in the former Czechoslovakia and exported from there to East Germany, North Korea, Syria and the Soviet Union. The PRP-1 Kopac system of 1963 was followed in turn by the KRTP-81 Ramona in 1979 and the KRTP-86 Tamara in 1987 (later upgraded to KRTP-91 version).

In 2009, ERA created the VERA-NG passive emitter tracking system with a lighter, more compact antenna unit. Last May, the NATO Communication and Information Agency (NCIA) signed a contract with ERA for two VERA-NG mobile systems, after previous demonstrations during NATO exercises.

ERA’s latest PCL technology demonstrator is called Silent Guard. This design exploits commercial terrestrial radio broadcasting signals in the 88- to 108-MHz band (up to eight FM channels per sensor). The system has 360-degree coverage in azimuth. ERA claims a range of up to 150 km depending on antenna type and target RCS.

Silent Guard operates in a completely covert mode, and has a redundant architecture. The system has very low power consumption because there are no rotating parts in the antenna unit. For this reason, the maintenance cost is also very low.

Pole Position

Polish company PIT has also developed the work done on both ESM/ELINT and TDOA systems in that country during the Warsaw Pact era. The company offers the SRS-25S Gunica system, which operates from 0.5 to 18.0 GHz.

Also in Poland, The Institute of Electronic Systems of the Warsaw University of Technology in Poland developed and tested a PCL technology demonstrator called PaRaDe (Passive Radar Demonstrator) in 2009-12. The PaRaDe technology demonstrator can do real-time processing of FM signals and offline processing of DVB-T, GSM and Wi-Fi signals. The development team claimed a tracking range of 750 km. PaRaDe uses a three-meter diameter antenna array, which is located on an eight-meter high mast.

Another Polish company, Bumar Elektronika, is working on a totally new design for a mobile passive surveillance system, which will combine PCL and TDOA technologies in one chassis. That will be the first system of this kind in the world.

One of the best-known passive systems is the Kolchuga, built by Ukrainian company Topaz. It is a long-range mobile system that receives and triangulates ESM/ELINT data. It can detect and identify pulse and continuous wave emitters from 135 MHz all the way to 18 GHz. Detection range is up to 600 km, and tracking range up to 200 km. In the standard version, three antenna-carrying vehicles are deployed in line, and can cover a 120-degree arc. The newest Kolchuga-3K system adds a fourth vehicle and array to provide 36-degree coverage with a detection range of 450 km. Both versions can simultaneously track up to 200 targets.

Turning to emitting radar systems, the former Soviet Union and Warsaw Pact countries also specialized in VHF-band systems, which are more likely to detect stealth aircraft because of their larger wavelength. VHF radar development continues today in the Belarus, the Czech Republic, Poland, Russia and Ukraine, and includes the use of active electronically scanned arrays (AESAs).