A laser system designed to identify and track chemical or biological attacks
on the Pentagon is providing a glimpse into the future of long-range threat
detection equipment.
Perched on top of a raised elevator shaft on the roof of the Pentagon’s
Navy annex, one kilometer from the main building, a pencil-thin laser beam constantly
sweeps the air. The light waves bounce off particles, which are analyzed based
on changes in the frequency of the returning waves.
The light detection and ranging (LIDAR) system, called “WindTracer,”
was developed by CLR Photonics. It is used internationally in airports and atmospheric
research laboratories.
Across the highway, CLR technicians ensconced in a hotel room near the Pentagon
examine real-time displays of wind direction and speed. Another monitor displays
a shimmering white mist—the usual pattern of background dust, pollution
and water normally found in the D.C. air. The goal is to create computer models
to determine the direction a toxic plume could drift if released near the Defense
Department’s headquarters.
LIDAR is one of many sensors contributing to the wind-modeling program. More
standard weather forecasting equipment is also being used, including battery-powered
wind sensors mounted on light poles and buildings, a 100-foot tower mounted
with sensors planted on Arlington Cemetery property, and a 30- by 10-foot blimp
to measure the winds at various heights.
The LIDAR system is also on the lookout for anomalous clouds that could indicate
the presence of a biological weapon or other toxin. Scientists can detect a
release, chart wind speed and direction, and calibrate the size and likely movement
of a possible threat. The $1 million WindTracer will remain permanently on the
annex roof.
“LIDAR can do initial detection and tracking, but it needs other systems
to verify what [substance] it’s looking at,” said CLR business unit
manager Jerry Pelk, one of the on-site supervisors of the project. “There’s
a lot of research going on in that area, but the technology is not there yet.”
During a May exercise, researchers released a non-toxic, colorless gas over
a three-day period, simulating how chemical or biological agents would flow
around and into the Pentagon. The scientists used sulfur hexafluoride, commonly
used in airflow testing, for the test release. The Food and Drug Administration
considers the gas safe enough to permit its injection into the human body.
When an agent is released, the laser picks out the difference and highlights
the moving cloud on a display, which resembles a Doppler radar screen. The anomalous
cloud appearing bright red against the haze of background aerosols. “As
it turns out, the backscatter is pretty evenly distributed,” Pelk notes.
Models of the air patterns at various times—such as increased pollution
from rush hour—are factored into the LIDAR picture. Data quality software
filters out hard targets, such as birds or low-flying airplanes.
The system’s main limitation is the weather. A steady rain will reduce
to about half its normal 8 to 10 km range. “That’s our Achilles
heel,” Pelk acknowledges. However, rainy conditions are the least preferable
for aerosolized weapon attacks, minimizing the gap in the system’s capability.
Toxic gas attacks, however, would be indistinguishable from naturally occurring
gases, but such agents typically creep across the ground instead of dispersing
into the air as a biological attack could do.
The Defense Department has employed LIDAR technology for many years. A WindTracer
unit is in permanent use at Dugway Proving Grounds to test the integrity of
point detection systems. By releasing simulants and tracking the plume with
LIDAR, researchers can tell when the senc= j¥pce them, PelaT‡id.
The LIDAR is used in conjunction with point detectors, which can be activated
after the anomaly has been identified. However, the future of long-range systems
may include identifying dangerous plumes using laser/light systems.
“If you use two wavelengths in the UV spectrum scanning foreign chemicals,
one will get absorbed, the other won’t,” Pelk said. “By comparing,
you can make an estimate if that chemical is in the atmosphere.”
Short-range LIDAR detection systems combine the traditional laser scans with
ultraviolet light. The UV can differentiate between biological and non-biological
material. False positives include pollen, molds and some agricultural pesticides,
but since different biological agents fluoresce at different waves, identification
becomes possible.
Another technology in development at CLR Photonics’ parent company, Coherent
Technologies Inc. is a database of agent “signatures” for use in
a system designed to recognize anything that reflects the IR light back in a
familiar way.
“In the near term, this could operate in a closed loop, and operate around
a fence-line around a building,” Pelk said. “It would be looking
for something to cross that fence line.”