Accurate Attitude Determination is Critical to Air Force's Tethered Aerostat Radar System.

Business Editors & Aerospace/Defense Writers

SANTA CLARA, Calif.--(BUSINESS WIRE)--June 10, 2002

Thales Navigation's Ashtech ADU2 GPS Receivers Ensure

Accurate Real-Time Data for Federal Drug Interdiction Efforts

In America's campaign to stem the flow of illegal drugs into the country, there are few weapons as powerful and cost-effective as the Air Force's Tethered Aerostat Radar System (TARS), a series of blimp-like, radar-equipped balloons strung along the southern U.S. border from California to Puerto Rico.

The system's primary mission is to support the federal drug interdiction program by detecting aircraft that enter U.S. air space across the southern border and reporting that data in real time to appropriate agencies.

Since TARS requires accurate attitude determination to ensure proper operation, each aerostat has been equipped with an Ashtech ADU2 GPS receiver from Thales Navigation to track its attitude. Thales Navigation is a subsidiary of Thales, a leading professional electronics company headquartered in Paris, France, with activities in aerospace, defense and information technology and services.

The TARS network was established in 1984 by the U.S. Customs Service and is now part of the Department of Defense, with the Air Force as executive agent. From its fixed sites, TARS provides almost continuous surveillance of the nation's southern coastal and inland border.

The radar system is operated and maintained for the Air Force by Lockheed Martin Technology Services. The company's Naval Electronics and Surveillance Systems-Radar Systems was recently awarded a contract to build L-88(V)3 radars to replace current L-88A radars. Each L-88(V)3 radar system will have a new Ashtech ADU3 GPS receiver.

Turning to GPS for Attitude Determination

Accurate attitude determination is essential to TARS, so the aerostat-borne radar can provide precise data. Before adopting GPS for TARS attitude determination, Lockheed Martin used magnetic compass augmented gyros.

"We chose the Ashtech ADU2 GPS receiver from Thales Navigation because it has no moving parts," explained James Houyouse, manager of engineering at Lockheed Martin in Melbourne, Fla. "It's accurate, cost-effective and reliable."

Lockheed Martin began to consider replacing gyros with GPS in the mid-1990s when it was developing a next-generation radar. It looked at ways to leverage new technology and found that GPS offered improved accuracy, cost and weight. "We need heading data 24/7 because it's a real-time system," Houyouse said. After integrating the ADU2 with a fiberoptic gyro to achieve a 100Hz update rate, the company was able to achieve 99.9 percent reliability with the new system.

Delivering Accurate and Reliable GPS

Measuring 420,000 cubic feet, about twice the volume of a Goodyear blimp, a TARS aerostat is a balloon that can rise to 15,000 feet while tethered by cable to a ground-based mooring system. The hull consists of two chambers separated by a gas-tight fabric partition. The upper chamber is filled with helium and the lower chamber is a pressurized air compartment, called a ballonet. The radar is attached to an aerostat's belly inside a windscreen compartment, a pressurized envelope that protects the radar from wind and other hazards. A payload support structure, or ring, is at the bottom of the aerostat, linked to the ballonet. The GPS antenna is mounted on the inside of the aerostat and attached to the payload ring to achieve sufficient rigidity for accurate azimuth data.

"The Ashtech ADU2 gives us about 0.10 degree of azimuth accuracy," Houyouse said. "That exceeds our requirement of 0.25 degree of accuracy. For an application requiring attitude determination, like the TARS, the number of satellites tracked by the receiver is critical. We need to track at least eight satellites to provide a working solution nearly 100 percent of the time. The Ashtech receivers give us that."

With the receivers installed and operational, the government sent evaluators to collect real-time data and test the new system. Using an aircraft with a differential GPS (DGPS) measuring instrument, the test recorded the plane's position data provided by both the onboard DGPS and the TARS radar, and then compared the data for accuracy. The Ashtech receivers passed the test.

Upgrading to Next-Generation GPS Technology

Lockheed Martin is committed to upgrading TARS with the latest available technology to improve the performance, accuracy and reliability of the system. It will use the next-generation ADU3 Ashtech receiver in future systems. The Ashtech ADU3 uses proven fourth-generation technology delivering high accuracy in a rugged system.

Ashtech ADU3 is an integrated real-time three-dimensional GPS position and attitude determination unit with onboard SBAS (Satellite-Based Augmentation Systems), WAAS, EGNOS and MTSAT support, as well as a DGPS beacon receiver. It provides precise heading, pitch and roll, and three-dimensional position and velocities for static and dynamic platforms. The ADU3 offers enhanced positioning accuracy and satellite tracking in both differential and autonomous modes. It features the new Ashtech DG16 receiver technology, allowing for SBAS and beacon differential positioning and positioning accuracy to the sub-meter level.

Using the latest radar technology from Lockheed-Martin and the latest Ashtech GPS receiver technology from Thales Navigation, TARS remains one of the government's most effective counter-narcotics detection and monitoring assets.

About Thales Navigation

Thales Navigation is a leading developer and manufacturer of positioning, navigation, communications and guidance equipment with operations throughout the U.S. and Europe. Thales Navigation markets its Magellan brand GPS solutions in the consumer electronics, recreation, and automotive markets, and its Ashtech GPS and GNSS professional products in the survey, GIS/Mapping, and OEM markets. The company, a subsidiary of Thales, headquartered in Paris, is based in Santa Clara, Calif.