Our ability to detect, track, and target hyper-, super- and subsonic missiles beyond the curvature of the Earth persistently is the most critical element of missile defense – without it we lose. No matter how powerful our terrestrial radars are, they all share one common limitation: the curvature of the Earth. Current shortwave radars emit radio waves that travel in straight lines, inherently limiting their detection range to objects that are stationed on or before the horizon. All of the missile threats to the United States mainland, Hawaii, Alaska and our territories will launch well over the horizon from multiple platforms in multiple domains, maneuvering below the range of our persistent current land based detection capabilities and overwhelming our point defenses. 

Over 30 years ago, when the threat of ballistic missile and Soviet bomber attacks on the continental United States was at its peak, an over-the-horizon (OTH) radar system was developed to provide for the detection of cruise and ballistic missiles, as well as ships and aircraft beyond what shortwave radars could nominally detect. As aerial, ground-based and space-based early warning systems became more popular with an increased focus on enhancing the range and flexibility of early warning systems, OTH radars fell out of favor with the United States due to their immobility and vulnerability to counterstrikes, and were adapted for use in maritime reconnaissance, drug enforcement and environmental measurements in many nations. France, Germany, Canada and the United States have transitioned OTH radars to focus on capturing information on drug traffickers and measuring ocean or wind currents, rather than for missile defense purposes.

There are two main types of OTH radars, OTH-SW or surface/ground wave radars, which use low transmission frequencies to bend around and follow the curvature of the Earth, and OTH-B or backscattering/skywave radars, which bounce radio waves off of the ionosphere at slightly higher frequencies to reach areas beyond the horizon. OTH radars are inherently immobile and restricted to flat, land-based installations. Furthermore, current OTH systems are massive, covering enough land to be seen from space.

Real world missile threats from beyond the horizon are significantly more proliferated, being used in combat today to a much greater degree than they were 30 years ago. The Houthi rebel forces in Yemen have boasted advanced ballistic missile and UAS capabilities, Iran has conducted attacks on United States bases in the region and recently released information on the “Kheiber Shekan” surface-to-surface missile with an estimated range of 1,450 km. In the Pacific, North Korea has conducted 11 missile tests over the past 40 days. Russia and China have led the world in the development and deployment of hypersonic, supersonic, and subsonic missile capabilities.

United States Northern Command NORTHCOM has illuminated absolute requirements to modernize their Command, Control, Communications, Computers for Intelligence, Surveillance and Reconnaissance (C4ISR) systems and capabilities. To defend the United States from over the horizon subsonic missiles, a set of four persistent over-the-horizon land-based radars facing North, West, East and South; providing a 360° view around the continental United States will be a fundamental component of protecting our population and nation as a whole. For the United States, land-based OTH radars are a credible answer to supplement multiple types of hyper-, super-, and subsonic missile threats to the mainland, including enemy stealth fighters, UAVs, sea vessels and numerous types of cruise or ballistic missiles launched from beyond the horizon. Australia, one of our closest allies, has already figured this out in the air and cruise missile defense of their entire nation, which is strikingly similar to the size of the interior of the United States. 

Australia, the leader in high frequency over-the-horizon radar technology, has developed the Jindalee HF-OTH radar and integrated it into the Jindalee Operational Radar Network (JORN), consisting of three OTH radars on the Northern and Western Australian coasts. The origins of JORN can be traced back to OTH experimenting in the United States and Australia shortly after the end of World War II. The United States cooperated with Australia on Project DUNDEE, a missile defense research project in 1997 and it is anticipated that the JORN will be essential tracking any launches from the Asian continent. The Jindalee boasts detection ranges of 1,000 to 3,000 km, but has been reported to have picked up Chinese missile launches over 5,500 km away. The JORN is currently undergoing the JORN Phase 6 redesign that will enhance its performance and extend JORN’s operational life past 2042. It has been reported to have the ability to detect stealth-equipped aircraft as well. 

There are other systems that can help tackle the problem of detecting launches over the horizon. Existing space based systems such as SIBRS and DSP, have played a vital role in early warning detection and tracking of missile launches. However, small, low heat, subsonic missiles can avoid detection from space-based radar, and future improvements to these subsonic missiles will lower the effectiveness of current space based radars. The Missile Defense Agency is set to tackle this issue by deploying the new Hypersonic and Ballistic Tracking Space Sensor (HBTSS) system which will be able to track hypersonic and ballistic missiles that travel in low orbit and that travel at a hypersonic speed. New early warning systems are also being equipped to aircraft such as the E-7A Wedgetail, the E-2D Advanced Hawkeye, and F-35A Lightning II which can patrol areas beyond the horizon of the continental United States. These aerial early warning systems provide the ability to traverse the horizon and physically extend the range of our detection capabilities, but have drawbacks in their ability to stay persistently airborne. With the existing space-based and aerial early warning systems the United States will not be able to effectively scan our horizons for all threats, and will need to adopt new OTH radar technology to address beyond the horizon subsonic threats. 

As it stands, the United States has budgeted to place a Tactical Mobile Over the Horizon radar (TACMOR) in the Pacific nation of Palau to increase its air domain awareness over the Pacific. However, the United States has continental OTH radars that are no longer in use for early warning detection or for any other missile defense purpose. The United States Navy currently operates two relocatable OTH radars (ROTHR), one currently at Chesapeake, VA and the other at Corpus Christi, TX. The other OTH radar used by the Air Force during the Cold War was designed to detect Soviet bombers, known as the AN/FPS-118 Over-The-Horizon-Backscatter (OTH-B). Both of these OTH radars are now being used for counter drug missions. As with other Cold War radars that were upgraded and reconfigured to support Homeland Ballistic Missile Defense (i.e. Cobra Dane), these OTH radars need to be recommissioned for missile defense. While upgrades will be needed, the promise OTH radars hold to detect and track subsonic missiles is paramount for defending the homeland. 

Our ally Australia is leading the way in OTH radar technology that should be emulated here in the United States. We have the capability right now to strengthen the homeland against low heat, subsonic missiles while we wait for advanced capabilities that are still in the testing and development phases. We know how to bring these OTH systems back online and upgrade them to fill in the radar coverage gap, and we should not delay a second more while our adversaries continue to expand their arsenals.