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Definition
Passive millimeter-wave (PMMW) imaging is a method of forming images through
the passive detection of naturally occurring millimeter-wave radiation from a
scene. Although such imaging has been performed for decades (or more, if one includes
microwave radiometric imaging), new sensor technology in the millimeter-wave regime
has enabled the generation of PMMW imaging at video rates and has renewed interest
in this area. This interest is, in part, driven by the ability to form images
during the day or night; in clear weather or in low-visibility conditions, such
as haze, fog, clouds, smoke, or sandstorms; and even through clothing. This ability
to see under conditions of low visibility that would ordinarily blind visible
or infrared (IR) sensors has the potential to transform the way low-visibility
conditions are dealt with. For the military, low visibility can become an asset
rather than a liability. In
the commercial realm, fog-bound airports could be eliminated as a cause for flight
delays or diversions. For security concerns, imaging of concealed weapons could
be accomplished in a nonintrusive manner with PMMW imaging. Like IR and visible
sensors, a camera based on PMMW sensors generates easily interpretable imagery
in a fully covert manner; no discernible radiation is emitted, unlike radar and
lidar. However, like radar PMMW sensors provide penetrability through a variety
of low-visibility conditions (moderate/heavy rainfall is an exception). In addition,
the underlying phenomenology that governs the formation of PMMW images leads to
two important features. First, the signature of metallic objects is very different
from natural and other backgrounds. Second,
the clutter variability is much less in PMMW images than in other sensor images.
Both of these characteristics lead to much easier automated target detection with
fewer false alarms. The wide range of military imaging missions that would benefit
from an imaging capability through low-visibility conditions, coupled with its
inherent covertness, includes surveillance, precision targeting, navigation, aircraft
landing, refueling in clouds, search and rescue, metal detection in a cluttered
environment, and harbor navigation/surveillance in fog. Similarly, a number of
civilian missions would benefit, such as commercial aircraft landing aid in fog,
airport operations in fog, harbor surveillance, highway traffic monitoring in
fog, and concealed weapons detection in airports and other locations. This article
introduces the concept of PMMW imaging, describes the phenomenology that defines
its performance, explains the technology advances that have made these systems
a reality, and presents some of the missions in which these sensors can be used.
Overview
of millimeter wave radiometry:
The
regime of the electromagnetic spectrum where it is possible for humans to see
is that part where the sun's radiance peaks (at about 6,000 K): the visible regime.
In that regime, the human eye responds to different wavelengths of scattered light
by seeing different colors. In the absence of sunlight, however, the natural emissions
from Earth objects (at about 300 K) are concentrated in the IR regime. Advances
in IR-sensor technology in the last 30 years have produced detectors sensitive
in that frequency regime, making night vision possible. The exploitation of the
millimeter-wave regime (defined to lie between 30 and 300 GHz, with corresponding
wavelengths between 10 and 1 mm) follows as a natural progression in the quest
to expand our vision. The great advantage of millimeter-wave radiation is that
it can be used not only in day and night conditions, but also in fog and other
poor visibility conditions that normally limit the "seeing" ability
of both visual and IR sensors.
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