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Studying Stealth
Air Force will soon begin operation of world's largest wide band
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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 bandwidth bistatic imaging and radar cross section (RCS) measurements of full-sized aircraft.
photo by David Asbell
The Bistatic Coherent
Measurement System (BICOMS), installed at an outdoor test facility at
Holloman Air Force Base in New Mexico, records measurements that are
essential to understanding the stealth characteristics of military
targets that use shaping as the primary approach to radar cross section
reduction.
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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).
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," explains 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. Then it was 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 says. "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 are 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
feet in length at typical ranges of 5,600 feet. On the fixed system, the
mobile dishes are duplicated, along with an additional 16-foot dish
provided for the 1 to 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 says.
"At a big range like this, efficiency is one of the big drivers,"
he explains. "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 says. "Bistatic calibration over wide bandwidths
(1 to 18 GHz and 34 to 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 says.
"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 to 18 GHz bandwidth," Lane explains. "Bistatic
measurements require linking the fixed and mobile radar systems with a
2-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 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.
For more information, contact Ted Lane, Sensors and Electromagnetic
Applications Lab, Georgia Tech Research Institute, Atlanta, GA
30332-0846. (Telephone: 770/528-7682);
(E-mail: ted.lane@gtri.gatech.edu)
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