Supporting excerpt from:
www.fas.org/nuke/guide/usa/airdef/an-tps-71.htm
AN/TPS-71 ROTHR (Relocatable Over-the-Horizon Radar)
...This unique radar system was originally designed to provide tactical warning to battle group commanders of air and surface threats at an extended range allowing time for responsive engagement. Two US Navy high-frequency (HF) over-the-horizon (OTH) radars known as ROTHR (Relocatable Over-the-Horizon Radar) are operated at Corpus Christi TX and Chesapeake VA, with coverage of the Caribbean Sea and portions of the Atlantic Ocean and the Gulf of Mexico.
The ROTHRs in Virginia and Texas are presently in full-time use for counter-narcotics surveillance, and a third is scheduled for installation in Puerto Rico in the near future.
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The ROTHR Virginia surveillance area covers more than 2.2 million square miles of the Caribbean extending north-south from southern Florida to the northern coast of South America and east-west from the western coast of Central America to the Lesser Antilles.
This coverage is achieved using 5-to-28-MHz radio waves that reflect from the ionosphere.
ROTHR is a land-based, high-frequency (HF) radar which can cover a 64-degree wedge-shaped area at ranges of 500 to 1,600 nautical miles. This extended range is achieved when transmitted HF energy is refracted by the ionosphere onto distant targets. The faint energy reflected back from these targets (backscatter) is detected by the radar receive antenna after returning along the refraction path. The surface of the earth and the targets in the area of interest reflect some of this energy back through the ionosphere to a separate receive site, where it is processed to generate target track information.
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Because the existing Over-the-Horizon Radar Systems were originally designed to perform a military mission and not a counterdrug mission, enhancements to provide a better capability to provide surveillance are being developed by the DoD Counterdrug Technology Development Program.
A series of adaptive waveforms have been developed to deal with the problem of Spread Doppler Clutter and its impact on Relocatable Over-the-Horizon Radar (ROTHR) systems for the ARPA sponsored Counterdrug Surveillance and Interdiction Program.
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