May 4, 1999 The U.S. Air Force will soon begin operation of an upgraded test facility believed to be the only one of its kind in the world able to conduct wide bandwith bistatic imaging and radar cross section (RCS) measurements of full-sized aircraft.
Bistatic measurements are essential to understanding the stealth characteristics of military targets that use shaping as the primary approach to radar cross section reduction (RCSR).
A technical article on the system has been published in the latest issue of the GTRI Journal of Technology.
The Bistatic Coherent Measurement System (BICOMS), installed at an outdoor test facility at Holloman Air Force Base in New Mexico, was designed by researchers at the Georgia Tech Research Institute (GTRI) for the U.S. Air Force 46th Test Group, Radar Target Scattering Division (RATSCAT).
"The Air Force has upgraded its existing fixed-site capability to a state-of-the-art system, and provided a mobile system that is almost identical to the fixed system," explained Ted L. Lane, GTRI principal research scientist and the project's principal investigator. "The mobile system can be moved around on the range, allowing the Air Force to do bistatic as well as monostatic tests."
The new mobile radar unit -- 40 feet tall, 66 feet long, 37 feet wide and weighing 90 tons -- was built at GTRI's research facility near Atlanta, disassembled and shipped to RATSCAT and reassembled there during the summer of 1998.
The two radar systems are coherent and linked together using fiber optics for bistatic measurements, but can also operate independently to provide high speed, simultaneous measurements of two separate targets under test. Operating together, the two systems can simultaneously produce bistatic and monostatic data from each radar unit.
"The throughput of the system will be an order of magnitude higher than it was," Lane said. "Automated features will help bring about a major reduction in setup time and test time. This will mean a significant reduction in cost for system operation."
GTRI researchers designed and supervised construction of the transportable unit, which is the world's largest mobile radar cross section measurement system. They also designed the equipment connecting the two systems, and the upgrades of the fixed unit. Johnson Controls Worldwide Services, Inc., will maintain and operate the range for the Air Force.
The fixed radar system can continuously sweep turntable-mounted targets from 1 GHz to 18 GHz and 34 to 36 GHz as they are rotated. The mobile system operates from 2 to 18 GHz, though its capabilities can be expanded because all mechanical systems in place to support Ka band.
On the mobile system, eight radar dishes ranging in diameter from 14 inches to 10 feet ensure uniform illumination of targets up to 80 ft in length at typical ranges of 5,600 ft. On the fixed system, the mobile dishes are duplicated, along with an additional 16-foot dish provided for the 1-2 GHz band.
On both systems, the antenna positions are computer controlled and provide an automatic peaking feature to reduce test setup time.
Beyond the expanded capabilities, the system includes automated calibration equipment that will improve its efficiency. A key part is a GTRI-designed field probe that provides detailed information in much less time than earlier systems. The automated field probe (AFP) is a coherent, one way amplitude probe that links the signals from either of the radar units to a transmitter located in the field probe via fiber optics.
"This is a very big improvement that allows us to quantify how uniform the electromagnetic field at the target plane is in real time," Lane said.
"At a big range like this, efficiency is one of the big drivers," he explained. "We expect an order of magnitude improvement in the system's efficiency, which means a reduction in the cost to the range user. In addition, improvements in accuracy and traceability are key issues particularly important in helping military agencies ensure that systems meet specifications and perform as needed."
Difficulties in calibration could affect the accuracy of the data -- and performance of the military systems. Calibration must be repeated during the day as normal heating and changes in the sun alter conditions of both the fiber optics and the test range.
"Monostatic radar cross section calibrations are relatively straightforward," Lane said. "Bistatic calibration over wide bandwidths (1-18 GHz and 34-36 GHz), wide bistatic angles, and especially for cross-polarization, is a real challenge. Monostatic testing can calibrate against known targets, but no standard exists for bistatic cross-polarization measurements in this test scenario. Since BICOMS is also a fully polarimetric system, this complicates the calibration. We have developed what we believe to be a simple extension of existing techniques to solve this problem and will be testing this approach during system validation," he said.
"Since the radars are wide bandwidth systems and the target is rotated during the measurements, both high resolution downrange and cross-range images are produced by each radar for both monostatic and bistatic conditions. This is in addition to total or absolute radar cross section data. Images can be generated with resolution on the order of 0.5 inches over the 2-18 GHz bandwidth," Lane explained. "Bistatic measurements require linking the fixed and mobile radar systems with a two-mile-long fiber optic cable, allowing synchronization of timing and control as well as phase locking necessary for gathering the coherent data."
At BICOMS, the mobile system can measure the scattered energy not only at different angles but also at varying distances from the aircraft under study. Because it can be moved very close to the targets, it can also measure near-field effects to understand how factors such as glint affect the radar signature. This is of particular interest for missile-aircraft engagements.
The project began in 1994 with a review of the Air Force's need for bistatic measurements at RATSCAT. Design and construction began in the summer of 1997 and involved five subcontractors. The mobile system was shipped to New Mexico in May 1998 and the fixed system began installation in June 1998. The upgraded facility is scheduled for operation in May 1999.
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