COTS technology enables passive ground-based radar

By Wilson P. Dizard III

A darkened DC-9 cargo jet skims at low altitude across the Gulf of Mexico during a night flight north. In the cargo bay, hundreds of neatly wrapped bricks of cocaine lie stacked on freight skids, forming tens of millions of dollars of contraband. In the cockpit the pilot and co-pilot, their faces illuminated by the glow from the instrument panel, check multiple radar warning receivers. They assure the supercargo that the Drug Enforcement Administration`s electronic defenses are inactive or evaded, and the smuggling flight can continue. The pilot lights a cigar as dawn breaks over the Gulf and the DC-9 enters U.S. airspace.

Without warning, two Gulfstream executive jets emblazoned with U.S. DEA markings flank the smuggler`s craft and waggle their wings, signaling the DC-9 to land. But wait. The DC-9`s radar detectors were clear. How did those DEA pilots find the smuggler aircraft, or even know it was there?

The DEA aircraft, had help from a passive ground-based radar system, which used the reflected signals from two country music radio stations and a PBS TV affiliate broadcasting "Sesame Street" to an early morning audience. In Detroit, three days after the drug interdiction, the price of a vial of crack cocaine increases substantially.

GAITHERSBURG, Md. Commercial off-the-shelf (COTS) signal processing hardware and COTS visualization software form essential parts of the Silent Sentry surveillance system, a passive radar product family that Lockheed Martin Mission Systems of Gaithersburg, Md., recently unveiled.

The Silent Sentry System uses commercial TV and radio signals that broadcast in the 40 to 100 MHz frequency range. Silent Sentry detects airplanes, helicopters, cruise missiles, ballistic missiles, re-entry vehicles, and rockets in real time with high accuracy, Lockheed Martin officials say.

Silent Sentry also can detect and track threats as small as remotely piloted vehicles and drones, Lockheed Martin officials say. As a passive system, Silent Sentry transmits no radio frequency "signature" that could attract enemy anti-radiation missiles or trigger countermeasures. "This quality gives it a superior level of security for surveillance operations, and is also an environmentally friendly solution," according to a Lockheed Martin statement.

"Television and radio transmissions blanket the Earth and extend into the far edges of space," says Terry Drabant, president of Lockheed Martin Mission Systems. "They give Silent Sentry broad application for a wide range of surveillance tasks, and unlimited access to broadcast signals for accurate, precise detection of targets."

The Silent Sentry system first was conceived 15 years ago at IBM, says Lorraine Martin, director of command and control technology programs for Lockheed Martin Mission Systems. Subsequently, the Silent Sentry research project transferred to Loral and thence to Lockheed Martin in a series of corporate mergers.

"The first and foremost enabling technology for this system is enhanced signal processing power," Martin says. "We`re using Silicon Graphics XL processors. Another enabling technology is that the processors have come down in price. Previously, we were using [processors] with top-of-the-line capabilities."

Complete Silent Sentry systems range in cost from $3 to $5 million. Because they use COTS technology, users can scale the systems for two, four, or more of the high-capacity Silicon Graphics parallel processors. "We don`t know of any other system doing the job the same way," Martin says. "When we compare it to tracking and scanning radars, its costs compare very well."

She added, "The next enabling technology is that Lockheed Martin has a distinguishing capability in our investment in the algorithms to process the signals in real time. That`s where our proprietary data begins." Martin declined to say what type of signal processing semiconductors or algorithms the Silent Sentry system uses. "We`ve got something special here, and we don`t want to give it away."

Lockheed Martin officials call their technology Passive Coherent Location, and say it uses signals of near-resonant, continuous-wave transmitters such as commercial radio and TV stations. Because radio and TV signals travel close to the ground, Silent Sentry "provides superior illumination of low-flying threats," according to a Lockheed Martin statement.

Silent Sentry is a multi-static illuminator surveillance system, which can use one, two, three, or more commercial broadcast signals to determine precise three-dimensional target trajectories. The system`s signal processors can update more than 200 target tracks eight times per second.

The entire system fits in a footprint of 27 square feet. TV and radio transmissions operate at sufficiently low frequencies so as to avoid adverse influence from the weather, Lockheed Martin officials say.

Because the Silent Sentry system does not emit RF signals, its power requirements are limited: it uses 20 amp, 200-to-240-volt AC power. The system is configured with a high-speed tape data system for target tracking archival, and uses Edge Product Family visualization software from Autometric Inc. of Springfield, Va.

Unlike scanning radars, Silent Sentry provides continuous, "staring" coverage of airspace. The Silent Sentry system output is a three-dimensional tactical display combining digital terrain, imagery, and maps. Some Silent Sentry systems can mount in buildings and fixed structures, while others go in trucks or shelters for rapid relocation. The system is not yet ready to go aboard aircraft.

"We`re in the process of working with vendor organizations on packaging requirements," Martin says. "We expect to get dramatic size reductions in coming years, and ride the curve of [hardware improvements] to increase processing power. Our current system is ground-based. Many technologies go from ground, to air, to space."

The Silent Sentry system works best when it can use reflected signals from three transmitters. However, it can provide a significant amount of information based on the reflected signal from only one emitter, Martin says.

"With one transmitter, you get a two-dimensional track, cross-range and down-range, but not altitude," she says. "The resolution and accuracy are not as good. It gets to be very respectable as you get more transmitters."

Lockheed Martin can provide "cooperating transmitters," which users can locate some distance from the Silent Sentry system to work in tandem with existing commercial radio or TV broadcast signals to help illuminate targets.

"We`ve tested the system against a wide array of targets under varying conditions," Martin says. "The [testing] conditions are classified or sensitive to the customers. The customers have asked to remain confidential. Our target market is the U.S. [government], both civil and military. We will only sell to government-approved customers."

The Silent Sentry system has a range of better than 90 miles. "It`s a wider range than some surveillance systems," Martin says. "Altitude is always the biggest challenge ... It`s an area we expect to improve." Clouds and birds have no effect on the system`s performance. Lockheed Martin declined to comment on the effect of chaff and similar radar-spoofing devices.

Martin added, "Right now, we are projecting delivery of the system six months from receipt of an order. We see a wide variety of applications. It works in concert with other systems in a `system of systems` perspective. It notches up the surveillance capabilities of civil and military users."

The Silent Sentry uses a phased array antenna with digital beam forming, says Sam Crow, Silent Sentry product manager. It can scan an azimuth of 90 degrees to 360 degrees. Producing the receiving antenna are engineers at Lockheed Martin Federal Systems in Owego, N.Y.

Silent Sentry developers also have used Yagi antennas, similar to TV antennas, as receive antennas. "We`re able to leverage off the entertainment industry, that makes a lot of antennas [for signals in the Silent Sentry bandwidth]," Martin says.


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