U.S. Building Advanced Over-The-Horizon Radar On Palau

Progress is being made on the deployment of a new long-range over-the-horizon radar system for the U.S. Air Force that will be stationed on the Pacific island of Palau. The news comes as the Pentagon’s push to drastically beef up both its offensive and defensive capabilities in the Western Pacific gains steam. 

Called the Tactical Mobile Over-the-Horizon Radar, or TACMOR, the sensor station will be installed on the increasingly strategic island of Palau to enhance air and maritime domain situational awareness for U.S. and allied forces in the region. The Department of Defense announced on Dec. 28 that it had awarded a $118.4 million contract to Gilbane Federal for the construction of reinforced concrete pads and foundations in support of the installation of TACMOR equipment on the island. The contracting activity for what’s being called the TACMOR infrastructure project is the Naval Facilities Engineering Systems Command Pacific, and work is expected to be completed by June 2026. 

The Republic of Palau on a map of the Pacific. Credit: Encyclopaedia Britannica/Universal Images Group via Getty Images

Readily available information about TACMOR outside of Defense Department documents is slim at this time, so The War Zone has reached out to the U.S. Indo-Pacific Command (INDOPACOM) for more about the development effort. The outfit has since responded but mainly with the details from the contract announcement, so we have followed up with additional questions and have yet to hear back.

For context, a traditional ‘sky wave’ or ‘backscatter’ over-the-horizon radar (OTHR) usually consists of a large array of antennas spread out over an area, and the transmission and receiver sites are usually geographically displaced. OTHRs possess the ability to detect targets at extreme ranges beyond what is known as the ‘radar horizon.’ Commonly, radars are limited by line of sight due to the curvature of the earth in relation to the altitude they are operating at. OTHRs overcome this critical limitation, but with some major drawbacks.

OTHRs were commonly deployed throughout the Cold War, especially as part of early warning ecosystems, but started to become less relevant for some applications after the war ended. More recently, these large static systems have been coming back into style thanks to major geopolitical shifts, advancements in computing power, and the value proposition they provide.

While they offer much lower fidelity than many of their line-of-sight counterparts in most cases, they make up for it in huge range, hundreds of miles to thousands of miles (1,500 miles is common, for instance, for powerful military sky wave/backscatter OTHR systems) and a large coverage area. The sky wave type of OTHRs achieve this by bouncing their radio waves off the ionosphere to a coverage area where they hit targets and return the same way to the receiving station. The angle at which the radio waves hit the ionosphere dictates the range, and that angle is limited to a specific envelope or the waves won’t properly bounce back toward earth. As such, placing the radar in the right place in relation to the targeted surveillance area or areas, to begin with, is key.

How OTHR works: A powerful shortwave signal from a large transmitting antenna (left) reaches a target beyond the horizon by refracting off the ionosphere, and the echo signal from the target (right) returns to the receiving antenna by the same route. Credit: Charly Whisky/Wikimedia Commons

Another type of OTHR is the kind that leverages the ‘surface wave’ concept, which uses the conductive saltwater of the ocean and a radio wave’s propensity to bend around curves under the ‘Norton Principal’ to propagate a radar wave over the horizon. This is often referred to as a ‘ground wave.’ These systems have a far less range than sky wave-type OTHRs, but they also don’t have a huge blind spot closer in due to not relying on bouncing off the ionosphere. With a range of a couple of hundred miles or so, they can still be very useful in tracking ships, low-flying aircraft, and cruise missiles far below the radar horizon. Pairing sky wave OTHR with surface wave OTHR can provide a more complete ‘layered’ picture of the situation nearby and far away from a given radar installation area. Palau may be getting both, or at least one system for closer-in monitoring in addition to this longer-range system.

A rendering of surface wave radar versus a traditional radar. (Radartutorial.eu)

As noted earlier, these systems are not nearly as precise or resolving as many of their line-of-sight counterparts, and their static nature makes them vulnerable to attack. Atmospheric and space weather conditions also play a big role in their effectiveness from moment to moment. Still, they can offer wide-area situational awareness and critical sensor cueing and early warning, which would allow other higher-resolution sensors without such a long reach or large view to investigate a target further. With networking and data-fusion, this can happen near seamlessly. They can also spot stealthy targets that are optimized to evade detection by fire control and traditional line-of-sight search radars operating on higher frequencies. All of these characteristics can be extremely valuable when monitoring an area like, say the South China Sea (roughly 1,000 miles from Palau) or, when it comes to surface wave types, the low-altitude and surface approaches to the island.

While not traditionally capable of independently generating ‘engagement quality’ radar tracks, with high-speed computer processing and artificial intelligence-enabled software, an OTHR’s lower-fidelity data can prove to be very revealing and actionable, much more so than in the past. China knows this very well, as it too is leveraging this technology to a large degree in the same theater of operations and especially in the South China Sea.

A modern OTHR on Palau will be able to support space-based and terrestrial-based sensor and weapon systems for the potential cueing and early warning of incoming hypersonic weapons, cruise missiles, ballistic missiles, enemy aircraft, and ships. Most of all, OTHR allows for persistent monitoring of specific areas that would otherwise require many types of radar systems forward deployed over a huge area on the ground, in the air, and at sea at any given time, which may not even be possible.

According to the Air Force’s budget justification documents for Fiscal Year 2023, TACMOR will be a quarter of the size of traditional OTHR and will be stationed in the Republic of Palau in the hopes of enhancing the situational awareness of activity in the air and on the ocean.

“TACMOR will be operated by the United States and located in the Republic of Palau, with the agreement of the government of Palau,” FY2023 Air Force budget documents read. “Development, test and evaluation, and acquisition of the system and associated components will provide warfighters with the capability to close gaps in surveillance coverage in key regions of the Pacific area of interest to the United States and our Allies.”

People watch a television broadcast reporting the North Korean missile launch at the Seoul Railway Station on July 4, 2017 in Seoul, South Korea. North Korea had fired an unidentified ballistic missile from a location near the North’s border with China. Credit: Photo by Chung Sung-Jun/Getty Images

The overall TACMOR initiative kicked off in July 2017 as a previously funded Joint Capability Technology Demonstration (JCTD) with the Air Force. JCTDs are meant to examine and assess the military utility of new technology and determine its system integrity as well as where it will best fit among operations, and TACMOR has since completed this process and transitioned to a program of record with the service.

Air Force budget documents from 2018 detailed that TACMOR infrastructure support officially began in 2017, as well. The facilities being constructed to accommodate TACMOR equipment on Palau under this project include “electrical utilities, reinforced concrete pads, and foundations, tie downs for equipment, water and wastewater, access roads, paved parking and turnaround areas, two levels of security fencing, and extensive site work,” read the Air Force literature.

Other TACMOR efforts funded under the program will include what the Air Force is describing in FY2023 budget documents as a remote, unattended transmit site consisting of ten elements and high-power amplifiers transmitting high-frequency sky wave OTHR waveforms. A separate receive site will also be remote and unattended and will be made up of 128 dual-monopole antenna elements receiving the over-the-horizon reflected energy from the transmit site. This receive site will additionally feature “secure facilities for signal processing of the received data and real-time target extraction information.”

The U.S. Navy’s Relocatable Over-the-Horizon Radar to show what the transmit and receive sites for TACMOR could shape up to look like. Credit: U.S. Navy

The necessary communications infrastructure for TACMOR will also be funded under this program. While specifics weren’t included in the budget documents, the Air Force did say that this feature will “enable the data flow from the transmit and receive sites to an off-site operations control center.” This center will then be responsible for planning and executing the Combatant Command’s missions in the region by disseminating the real-time targeting and tracking information gathered by TACMOR to U.S. and allied forces.

In the new year, the Air Force hopes to continue the development and installation of TACMOR’s communications infrastructure, flesh out the material needed to begin the Initial Operational Test and Evaluation (IOT&E) process, and ultimately finalize the requirements for specific contractor deliverables in support of IOT&E. Then, the Air Force says it will competitively award a single indefinite-delivery, indefinite-quantity contract with multiple task orders.

The emergence of TACMOR only underscores the Pentagon’s increased importance placed on providing a persistent U.S. military presence in the Indo-Pacific as of late. Palau itself is becoming especially strategic as the United States focuses on preparing for a potential high-end future conflict in the area against Chinese forces, but also on addressing the threats posed by the missile arsenals of adversaries like North Korea

A screencap from an official Chinese People’s Liberation Army video showing the launch of a DF-15 short-range ballistic missile, purportedly from exercises in August. Credit: PLA

Palau is an independent country with an established history with the United States under the 1994 Compacts of Free Association (COFA) agreement, which governs the military relationship between the two nations. The U.S. military has been making significant use of Palau’s islands for various military exercises over the years including this year’s Valiant Shield 2022

The Valiant Shield exercise saw the first-ever launch of a Patriot surface-to-air missile system in Palau — a system that could be vital to protecting TACMOR during a conflict. The Air Force also used this opportunity to experiment with A-10 Warthogs and explore the ways that the aircraft could operate from distributed locations, like Palau’s Roman Tmetuchl International Airport. Angaur Airfield on Palau’s island of Angaur has also been deemed suitable for C-130 aircraft to operate from as an alternative to the country’s airport. 

One of the 23rd Wing’s A-10Cs at Roman Tmetuchl International Airport on Palau. Another Warthog is seen behind to the left. Credit: USAF

Activities on and around Guam, Tinian, and other U.S. islands in the region have also been conducted. These islands have all become hotspots for U.S. military infrastructure expansion in recent history. Guam specifically is a U.S. territory and the Missile Defense Agency (MDA), the Army, and the Navy have together been working on new air and missile defense capabilities on the island, which can be read about in detail in this past War Zone piece

In fact, the MDA just awarded Lockheed Martin a contract worth $528 million that expands upon the company’s existing Aegis Weapon System to implement integrated air and missile defense capabilities into an Aegis Guam System. This capability will allow U.S. forces to engage ballistic threats with SM-3 interceptors at various points in the missiles’ flight paths, though other interceptors like the SM-6 and the forthcoming Glide Phase Interceptor could eventually be integrated into Aegis Guam as well. 

A THAAD battery has also existed on Guam for years, and this would likely be integrated with the Aegis system to provide an additional layer of ballistic missile defense. Other missiles could be integrated and deployed to counter the cruise missile threat to the island, too. There are, however, geographical factors that go into building facilities like Aegis Guam and TACMOR on these relatively small and mountainous islands.

Regardless, even though the station has yet to be completed, knowing that the development of TACMOR for Palau is progressing strongly communicates the growing shift in the U.S. military’s priorities in the Pacific. 

Contact the author: Emma@thewarzone.com and Tyler@thedrive.com