- Buyers Guide
Military Microwaves Supplement
Cold War to Counter Drug
Ellen Ferraro and Drew Ganter
A small twin-engine airplane flies low over the water to avoid radar detection. At an undisclosed Caribbean airfield, two men waiting by a truck signal that it is all clear to land. The plane lands and the men quickly begin to offload its cargo. To their surprise, they are met by government agents. All three men are taken into custody; the plane and its cargo, 10 bales of cocaine and a briefcase of money, are seized without incident.
"What happened?" the pilot thought to himself. "Everything was perfect tonight." Little did he know that from 1000 miles away an advanced early-warning, over-the-horizon radar was watching his entire trip and relaying his position to government agents who were moving to intercept him.
Swords to Badges
Scenarios such as this one are an important part of the new war...the war on drugs. The role of defense is changing and the US now focuses much of its energy and resources onto deterring the flood of small planes, boats and trucks carrying millions of dollars worth of illegal substances across its borders. One system, the relocatable over-the-horizon radar (ROTHR), has been able to change its focus as well and become a key sensor in supporting this new mission.
Originally, ROTHR was developed for the US Navy to provide wide area ocean surveillance for the nation's fleet defense from both sea and airborne threats. The first engineering-development system was tested in Virginia and delivered to the Navy's Fleet Surveillance Support Command in 1988. The system then was transported to Amchitka, one of the Aleutian Islands located 1300 km off the coast of Alaska, for operational evaluation. The tactical military radar scanned the skies from Japan to the eastern USSR looking for high speed bombers. ROTHR performed its surveillance duties successfully and the Navy intended to install 12 of these systems around the world to provide the fleet with an early warning of Soviet attack.
With the collapse of the Soviet Union and the decline in defense spending, it was unlikely that such a deployment of ROTHR systems ever would occur. However, while testing the first production system, the radar operators noticed that ROTHR was not only tracking large commercial aircraft crossing the Caribbean, but also small twin- and single-engine planes. On May 1, 1989, ROTHR provided the joint Coast Guard/Customs center in Miami with critical detection information that led to an involved chase and the seizure of an aircraft carrying drugs to the Bahamas. This drug bust was the first one attributable to a long-range radar system. At approximately the same time, Congress assigned the Department of Defense with the role of counter-drug detection and monitoring and, as a result, the Pentagon decided to leave the ROTHR system at its test site in Virginia to search for drug traffic in the Caribbean.
Since that time, the ROTHR Virginia system has been operating as part of the national counter-drug strategy in conjunction with the ship and airborne assets of the Navy, Customs, Coast Guard and local law enforcement agencies. A second ROTHR counter-drug surveillance site was installed in Texas in 1995. Together, these systems monitor 8.5 million square kilometers of air and sea space from Mexico to Columbia to the islands of Trinidad and Tobago, as shown in Figure 1 . More than 60,000 flights in this region are tracked each year. Most of these flights involve innocent private or commercial aircraft, but a small percentage are carrying narcotics toward the borders of the US. When the air route of a plane raises the suspicions of ROTHR operators, information on the target is passed to the Joint Interagency Task Force in Key West, Florida via a secure communications link. This agency, which is commanded by a US Coast Guard admiral, is the regional dispatcher of the counter-drug effort in the Caribbean. Since 1992, information obtained by ROTHR has contributed to the seizure of over 50,000 kilograms of narcotics and tens of millions of dollars in cash and resources.
No Place to Hide
Conventional microwave radars (300 MHz to 30 GHz), similar to those in use by the Federal Aviation Administration and the military, are line-of-sight sensors. That is, they require a direct path between the radar's antenna and the target in order to be effective. The radar transmits a signal that is reflected by the target back to the radar's antenna where it is received and processed. This inability to see beyond the horizon limits the amount of area that conventional radars can cover to approximately 35,000 to 350,000 square kilometers. As a result, an aircraft may evade detection by flying low to the ground, hiding behind the horizon or mountains, or simply flying around the radar's coverage area, as shown in Figure 2 .
ROTHR operates in the high frequency (HF) band (2 to 30 MHz). The system bounces its signal off of the ionosphere, a complex layer of charged particles at a height of 100 to 450 km above the earth. While microwave signals penetrate the ionosphere and continue into space, HF signals are refracted through the ionosphere back down toward the target where they are reflected, or backscattered, to the radar antenna via the ionosphere once again. This process allows ROTHR to detect targets well beyond the horizon from above, making it difficult for targets to mask themselves by flying low or behind land masses, as shown in Figure 3 . When this capability is combined with the sheer size of the ROTHR coverage area, drug traffickers are given no place to hide regardless of the tactics they employ.
Although bouncing a radar signal off the ionosphere sounds simple, it is not. The ionosphere is a complex zone of the earth's atmosphere that consists of three primary layers: E, F1 and F2. The density, height and thickness of these layers change significantly with geographic location, time of day, season and sunspot cycle (which varies over several years). Therefore, two radars are needed: one to search for targets and one to monitor the ionospheric environment. ROTHR's backscatter radar component is responsible for performing all of the target-detection functions.
The complicated task of monitoring the environment is divided among three components, as shown in Figure 4 . The vertical sounder, shown in Figure 5 , measures the height and density of the layers directly overhead, while the downrange backscatter sounder, shown in Figure 6 , measures the backscattered energy vs. operating frequency over the entire radar coverage area. The third component, the spectrum monitor, is shown in Figure 7 and provides continuous information about which frequencies are in use by other systems, such as the Coast Guard or ham radio operators. Together, these components determine the radar operating parameters for optimum target detection by the backscatter radar and provide the information necessary to accurately determine a detected target's position over the ground.
ROTHR is a bistatic radar, meaning it has separate transmit and receive sites. Figure 8 shows a simplified system block diagram. Figure 9 shows the ROTHR transmit and receive sites. All of the system's functions are divided between these two sites, which may be located over 100 km apart. The transmit site generates the high power waveforms used by the backscatter radar and the vertical and downrange sounders. Phased-array antennas steer the transmitted radar and sounder beams downrange electronically while the vertical sounder antenna directs its energy straight up toward the ionosphere. Because of the long transmitted wavelengths, the radar antenna stands as high as 60 m and consists of separate high and low band arrays, which are selected depending on the frequency in use. Each array is sized optimally to provide a narrow beam of energy to be used for target detection. The downrange backscatter sounder array consists of two elements, also 60 m high. These elements are spaced to provide a single beam of energy wide enough to span the entire ROTHR coverage area for environmental soundings throughout the entire operating frequency band.
The receive site collects the backscattered energy and performs the necessary environmental measurements and target-detection tasks. The receive antenna array comprises 372 dipole pairs referred to as twin whip endfire receive pairs (TWERP) and measures over 2.5 km in length. This extremely long antenna array is required to provide the necessary azimuth resolution
to detect targets several thousand kilometers away. These elements are shared among the backscatter radar, downrange backscatter sounder and spectrum monitor functions. In addition, a separate vertical sounder antenna is used to receive the overhead ionospheric soundings. Two advanced signal processors process the received data digitally in range, azimuth and Doppler shift. One signal processor performs all of the target-detection tasks while the second performs the environmental measurements. This information then is sent to the OCC where the sounder measurements are used to model the ionosphere and the detected radar targets are associated to form aircraft or ship tracks for operator display.
The OCC is the nerve center of the system where all of these complex radar functions are tied together. This center may be located remotely from the transmit and receive sites or co-located with the receive site. For example, the control center site for both the Texas and Virginia ROTHR systems is located in Virginia. Tasking orders from external agencies are received by the OCC, which then schedules all of the radar operations necessary to perform the required mission. At least 16 operators control and maintain the ROTHR systems from the control center 24 hours a day, seven days a week. Large color displays allow these operators to monitor all the targets in the radar coverage area. The location, course and speed of each target help an operator determine whether a target is a suspected drug trafficker. This information then is communicated to the tactical commanders in Key West. Figure 10 shows a high resolution display skirting along the mountains in southern Haiti and then up the border between Haiti and the Dominican Republic. The target track is shown in red.
On the Horizon
The future looks bright for ROTHR, and there are many improvements on the horizon that will secure its position as an integral part of the national counter-drug strategy. A community of scientists and engineers from government agencies, institutions and companies around the world are working together to improve and enhance the capabilities of ROTHR as a counter-drug system.
The planes that traffic drugs are often small and slow moving, making it a challenge to maintain track on them. Advanced tracking techniques are being implemented to improve track quality and consistency for a wide variety of these difficult targets. As the drug traffickers learn that they cannot hide in the air, they may turn to the sea. However, the ROTHR system already has shown that it is capable of detecting other forms of traffic, such as small boats and shipping vessels, and new detection and tracking techniques are being developed to enhance this capability.
Because of the look-down nature of the ROTHR system, a large portion of the received signal is in the form of ground or ocean clutter. These returns are fairly stationary and normally occupy a small portion of the Doppler spectrum, allowing targets to be detected by the Doppler shift their movement causes. This same principle is used in military airborne early-warning systems to detect targets. However, the ionosphere also presents many unique challenges to this type of target detection. Polar auroras and equatorial instabilities in the ionosphere induce Doppler shifts in the stationary clutter returns at ranges beyond the normal ROTHR coverage area. These returns enter the ROTHR coverage area through ambiguities inherent in processing a radar signal and fold into regions where slower-moving targets may be found. Meteors entering the earth's atmosphere leave trails of ionized gas that are detected by the ROTHR receivers. Due to their extremely short duration, these returns are often spread completely across the Doppler spectrum, masking actual targets. While the Doppler shifts from the auroras, meteors and equatorial instabilities are a form of unwanted clutter, other environmental effects (such as lightning) add large noise impulses to the received signal. Nearby thunderstorms are not always the lone culprit of this impulsive noise because ROTHR's receivers may feel the effects of lightning strikes from several thousand kilometers away.
All of these conditions can make detecting a small, slow-moving aircraft very difficult. ROTHR's capabilities are being improved constantly to mitigate undesired environmental effects and enhance target detection. New signal processing methods have been implemented to prevent the equatorial ionosphere's ambiguous clutter returns from folding into the ROTHR coverage area. Impulsive noise-excision techniques being incorporated into the system currently remove the effects of lightning from the received signal.
Over-the-horizon radars were once thought of as crude instruments providing only rough estimates of target location. ROTHR has proven this generalization to be untrue and continual emphasis is placed on improving the radar's accuracy. New algorithms to model the ionosphere and convert target positions from slant coordinates relative to the radar-to-ground position in latitude and longitude are being studied. These improvements, together with a newer, more powerful computer architecture and new high resolution displays, enable ROTHR operators to watch closely for targets that may be attempting to avoid detection.
These enhancements and the possibility of additional radar sites to provide improved coverage over South America and Mexico illustrate ROTHR's capabilities as a highly effective counter-drug system. ROTHR has the unique, unprecedented ability to watch over 8.5 million square kilometers of space 24 hours a day. However, no one system does it all, and the over-the-horizon community is working together with a large, dedicated counter-drug team to limit the amount of drugs entering the US illegally. This war on drugs is an ongoing series of battles where it is difficult to distinguish victory from defeat. Countless hours of effort from many agencies often are required for a single drug interdiction to take place. With new technologies and capabilities, the counter-drug effort is able to deter more drug traffickers more effectively and efficiently. In the first nine months alone, the new ROTHR system in Texas stopped over $45 M worth of drugs from entering the US. The result is fewer drugs on the streets of America and countless numbers of lives saved.
The authors would like to thank the many scientists and engineers who have contributed to the design, development and success of the ROTHR system, especially Ralph Jennett and George Thome of Raytheon Co. They would also like to thank the government agencies, including NRL, NRaD and Rome Laboratories, that have funded and contributed to the development of the many new enhancements and capabilities for ROTHR. Finally, the authors would like to thank James Bucknam and Eli Brookner of Raytheon Co., Commander Robert Hillery of the US Navy and Joe Thomason of Naval Research Laboratory for reviewing this article.
Ellen Ferraro received her PhD degree in electrical and computer engineering from the University of Massachusetts at Amherst in 1994 under a NASA graduate student research fellowship. In 1994, she joined the Systems Design Laboratory at Raytheon Co. where she has been involved in the analysis of scattering and propagation over the ocean. More recently, she has investigated spread clutter mitigation techniques for enhanced small-target detection with ROTHR as well as the use of expert system technology to aid in ROTHR's counter-drug mission. Ferraro is a member of the IEEE and an officer of the Boston Section of the Society of Women Engineers.
Drew Ganter received his BSEE from Norwich University in 1989 and is presently completing the requirements for an MSEE from Old Dominion University. His graduate research includes investigating the use of cooperative aircraft transponders to enhance the positional accuracy of the Navy's ROTHR. Ganter joined Raytheon Co. in July 1989 and has been involved with installation and acceptance testing of both operating ROTHR sites. Recently, he has been investigating signal processing techniques to mitigate the effects of spread clutter and lighting upon the detection of small aircraft to enhance ROTHR's performance in its counter-drug mission. Ganter is a member of the Aerospace & Electronic Systems and Signal Processing societies of the IEEE.
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