Alpha Defense Exclusive

OTH Super Eyes to monitor Dragon’s movement in IOR

-Subodh Sharma and Aman Routray

In a big development, the premier radar development lab of DRDO is working on an “Over-the-Horizon” OTH radar system to keep a close eye on Chinese movement in Indian ocean region (IOR). The Indo-pacific is now turning out to be the most important part of the world today. For India, this part of the world is even more important as it is India’s Backyard.

The Indian Navy today calls itself as “Net Security Provider” in Indian ocean Region (IOR). The presence of Peoples liberation Army Navy in this region is increasing with every passing day and its certainly increase in future. Today keeping close vigil in this area is paramount requirement for the Indian Navy.


To monitor movement in high seas the countries often rely on satellite pictures, synthetic aperture radar of the maritime surveillance aircraft and drones, coastal surveillance radar units and ship borne radar systems. While aircraft-based surveillance, radar is impacted by the endurance of the aircraft, drones offer longer endurance. Though the need is to monitor the area 24X7 and therefore airborne surveillance radars are not a perfect fit in the requirement.

The Ground based radar system for the constant monitoring of the sea is comparatively better option when compared with airborne system, especially given to the endurance of the various platforms. Though, this approach suffers a major roadblock because of “Earth Horizon”. The effect of the earth horizon on performance of the radar is called as “radar horizon”. The radar horizon is a critical area of performance for object detection systems (detection radars) that is defined by the distance at which the radar beam rises enough above the Earth’s surface to make detection of a target at low level impossible.

Fig.1 Concept of Radar horizon

The costal surveillance radars or the shipborne radars can see a limited distance because of earth horizon. The need however is to monitor larger sea area. The solution that can help India to monitor this large sea area, without the need of costlier airborne sensor is “OTH – Over the Horizon Radar”.

What is Over-the-horizon radar (OTH)?

Over-the-horizon radar (OTH), also called as beyond the horizon (BTH), is a type of radar system with the capability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary land-based radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but these have generally been replaced by airborne early warning systems. OTH radars have recently been making a comeback because of improved computing and lesser operational cost when compared with airborne assets.

The OTH radars use two different techniques to see beyond that limit.

Shortwave systems

This type of OTH refract their signals off the ionosphere for very long-range detection. This is also called as skywave or “skip” propagation, in which shortwave radio waves are refracted off an ionized layer in the atmosphere, called as “ionosphere”.

The ionized part of the Earth’s atmosphere is known as the ionosphere. Ultraviolet light from the sun collides with atoms in this region knocking electrons loose. This creates ions, or atoms with missing electrons. This is what gives the Ionosphere its name and it is the free electrons that cause the reflection and absorption of radio waves.

The radio signals transmitted at an angle into the sky will be refracted towards the ground by the ionosphere, allowing them to return to earth beyond the horizon. A small amount of this signal will be scattered off desired targets back towards the sky, refracted off the ionosphere again, and return to the receiving antenna by the same path.

Fig.2 Shortwave System

Surface wave systems

Also called as “Ground wave” propagation type OTH. This type of the OTH system uses low frequency radio waves that, due to diffraction, follow the curvature of the Earth to reach beyond the horizon. These systems achieve detection ranges of the order of a hundred kilometres from small, conventional radar installations.

Fig.3 Surface Wave systems

Alpha Defense Exclusive Update

LRDE – Electronics & Radar Development Establishment is a working on a OTH prototype. The Defence Research and Development Organization’s Lab responsible for development of cutting-edge radar technology is working with its partners to realise an over the horizon radar prototype in coming 6 months. The system design is already complete and its now entering prototype realization stage.

The prototype radar will have two different type of radar arrays. The wire log-periodic antenna array and broadband monopole array. It is believed the log-periodic antenna array will be used to identify the best frequency to use. The “best frequency to use” depends on the current conditions of the atmosphere and the sunspot cycle. For these reasons, systems using skywaves typically employ real-time monitoring of the reception of backscattered signals to continuously adjust the frequency of the transmitted signal.

In the LRDE system this will be achieved with the help of log-periodic antenna array (Shown in the figure 4) and for the actual OTH operation broadband monopole array will be used. The prototype will have log-periodic antenna with 50-meter height, 100-meter length and 150×50 Meter size of ground mesh. Whereas the broadband monopole arrays will be in “Monopole doublet” configuration. A “Monopole doublet” has two monopole antennae placed right next to each other. The configuration that LRDE is working has 16 of these monopole doublets making it an array of 32 elements. (Example shown in the photograph 2)

Fig.4 Concept of Monopole doublet
Fig.5 Monopole Doublets arranged in an array


The LRDE system for now is a prototype which will enter prototype realisation place now. The implementation timelines are 6 weeks from the grant of the work order. However, the actual system will be developed after extensive trials and evaluation. The current system appears to be in range of 250-350 meters in length. However, the similar systems across the globe are around 2.5 to 3 Kilometres in range. This indicates, the system developed for now will be used as proof of concept and a larger system will be developed upon successful completion of this project.

Significance of OTH Radar

This over the horizon radar system can be used for monitoring and tracking the ship movement in a large area. The Indian ocean region is vast and keeping constant vigil with limited airborne system is not just challenging but a costly affair. Establishing an OTH site will give Indian forces eyes that can peek over the horizon for a long distance.

Similar Systems across the globe

Various countries have been working on different type of systems from as late as 1960s.This solution has again attained popularity because of improved electronics and power sources. The system today is much more feasible than it was 60 years ago:

  1. United States of America:  The United states of America has various system and both US airforce and US Navy used them for long time. Today most of the US system are either in warm storage or in cold storage.
  2. Russia: In soviet era, USSR has various system used extensively during the cold war. These systems were either dismantled or abandoned. However, in early 2014, the Russians announced a new system, called Container, that was to see over 3000 km.
  3. Australia : A more recent addition is the Jindalee Operational Radar Network developed by the Australian Department of Defence in 1998 and completed in 2000. It is operated by No. 1 Radar Surveillance Unit of the Royal Australian Air Force. Jindalee is a multistatic radar (multiple-receiver) system using OTH-B, allowing it to have both long range as well as anti-stealth capabilities. It has an official range of 3,000 kilometres (1,900 mi), but in 1997 the prototype was able to detect missile launches by China over 5,500 kilometres (3,400 mi) distant.
  4. Canada : The research on OTH in Canada started way back in 1984. Canada’s installed two SWR503 HFSWR systems at Cape Race and Cape Bonavista, Newfoundland. The sites underwent a technology evaluation in 2000 and were subsequently upgraded and operationally evaluated in 2002.
    In 2007 operation of the Canadian systems was halted due to concerns over the potential for harmful interference with primary spectrum users. In 2010 the unique capability of HFSWR to provide low cost surveillance of the EEZ resulted in a re-evaluation of the technology and subsequent development of a 3rd Generation, (3rd Gen) HFSWR system based on the principle of sense-and-adapt technology.
    In June 2019, MAEROSPACE was granted a global license to design,, manufacture, and internationally market the Canadian HFSWR System and its derivatives.
  5. France : The French developed an OTH radar called NOSTRADAMUS during the 1990. NOSTRADAMUS is probably second most famous OTH system out there after Australian Jindalee.
    In March 1999, the OTH radar NOSTRADAMUS was said to have detected two Northrop B2 Spirit flying to Kosovo. It entered service for the French army in 2005, while it was still in development.
    Launched officially in 2009, the French STRADIVARIUS research project developed a new over-the-horizon radar (High Frequency Surface Wave Radar – HFSWR) capable of monitoring maritime traffic up to 200 nautical miles offshore. A demonstration site is operational since January 2015 on the French Mediterranean coast to showcase the 24/7 capabilities of the system that is now offered for sale by DIGINEXT.
  6. China: Several OTH-B and OTH-SW radars are reportedly in operation in China.
  7. Brazil: The OTH 0100 Radar is capable of monitoring vessels beyond 200 MN (370 KM) away from shore, exceeding the direct line of sight of conventional radars.
  8. Iran is also working on an over-the-horizon radar system called as Sepehr. It is reported, this radar system has a range of 3000KM, and Iran currently uses this to detect and locate the US Carrier Battle Group.

Once operational, India will be the 10th country to have an operational Over-the-horizon radar system. This radar system will allow Indian Navy to monitor the ship movement in a large area in real time. Operationally this will enhance the surveillance and monitoring capability of Indian Navy significantly.


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By Alpha Defense

Alpha Defense initially a solo venture but now a defense group by people from various demographics of India covering defense news and updates. We believe in unbiased analysis of every subject in hand. Our mission is to provide simplfiied defense information to the public.

5 thoughts on “OTH Super Eyes to monitor Dragon’s movement in IOR”
  1. Great initiative by the DRDO and Government Of India in terms of Aatmanirbhar Bharat to monitor our shores from Chinese aggression.
    Any expected timeline regarding the system to enter prototype testing phase and then officially being deployed and will this system also be CAPABLE OF DETECTING ENEMY STEALTH TARGETS.

    1. Official deployment largely depends on testing performance.
      The timeline for realization is 6 weeks and if we believe the work order is out in a couple of weeks then we are looking at timeline of 4-6 months…

  2. Is this something like what the USSR used during the heights of the Cold War against American stealth birds?

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