Illustrations of Over The Horizon Radars

DRDO’s Electronics & Radar Development Establishment (LRDE) has designed and is currently developing a prototype Over-the-Horizon Radar (OTHR). The radar is to be deployed on India’s Western Seaboard for surveillance/ detection in the Indian Ocean Region. The radar prototype is to have two different types of radar arrays – a log-periodic wire antenna (LPWA) array and a broadband monopole antenna array. The LPWA will probably perform the role of backscatter receiver to continuously determine the best transmission frequency. The monopole antenna array will consist of 32 elements. The initial prototype of this OTHR would be a Surface-Wave OTHR with a range of approximately 500 Km. This would be followed by a Sky-Wave OTHR with a detection range of about 2000 Km. These radars would have application in all three Services of the Armed Forces, wrote Brig Arvind Dhananjayan (Retd).

As is well known, most radars work on the principle of line-of-sight (LOS) propagation of Electromagnetic (EM) waves, with ranges limited by this principle to the visible horizon. While ground-based LOS radars can detect airborne platforms/vectors at relatively greater ranges, range to ground targets is limited by the visible horizon as radar signals travel with line-of-sight propagation.  As airborne technology for long-range engagement improved in accuracy/lethality/response times over the last millennium,  an urgent need was felt by armed forces to improve radar technology for provision of early-warning (EW) and enhanced situational awareness at far greater ranges. Apropos, this would necessitate detection ranges beyond the visible horizon and would therefore require to incorporate technology that could ‘see’ beyond LOS (BLOS). These operational requirements gave rise to the technology of the OTHR.

Among the milestones in development of OTHR technology, the erstwhile Soviet Union were the first to develop an OTH system in 1949 called Veyer. The first fully operational OTHR development was a joint US Air Force-British Air Force system known as Cobra Mist, constructed in the late 1960s/early 1970s. This OTHR could detect aircraft over the Western Soviet Union from its location in South-Eastern England. One of the first OTHR for detection of Soviet ballistic missile launches was the Sugar Tree OTHR built by the US in the 1960s. Reverting to the Soviet Union, their next known successful project was the Duga-1 OTHR, which was operational from 1976 to 1989 on the Black Sea Coast and formed a part of the Soviet Union’s EW radar network for Missile Defence. With the advent of airborne platforms and satellites with EW capabilities, the armed forces of the world lost interest in OTHR systems. OTHR technology has however seen a resurgence in recent times due to the rising requirements of maritime reconnaissance/surveillance and long-range/ low-cost situational awareness.