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.
PASSIVE MILLI METER WAVE IMAGING
Passive Millimeter Wave imaging is a method of formation of images through passive detection of naturally occurring millimeter wave radiation from a scene
All natural objects whose temperatures are above absolute zero emit PMMW radiation.
Millimeter-waves are much more effective (Lower attenuation) than IR in poor weather conditions such as fog, clouds, snow etc..
The amount of radiation emitted in the millimeter-wave range is 10 times smaller than the amount emitted in the infrared range.
However, current MMW receivers have at least 10 times better noise performance than IR detectors and the temperature contrast recovers the remaining 10 .
Electromagnetic radiation windows occur at 35 GHz, 94 GHz, 140 GHz and 220 GHz.
In the electromagnetic spectrum the MMW Region ranges from 10^10-10^12 Hz.
This makes millimeter-wave imaging comparable in performance with current IR systems
Millimeter wave imaging is of two types:
ACTIVE MILLIMETER IMAGING
PASSIVE MILLIMETER WAVE IMAGING
How PMMW imaging is done
Effective radiometric temperature =
Te = Ts + Tsc.
Surface scattered temp = Tsc=pTi.
Surface brightness temp = Ts=eTo
Images can be formed by mapping Te as a function of position in the scene.eg: for metal Ts=0 & Tsc=pTi
Factors that affect radiometric temperature are:
Emissions from scene constituents
Reflections of down welling sky radiations by the scene
Upwelling atmospheric emission between scene and observer
Propagation of electromagnetic energy from scene to the observer.
Detects millimeter waves from the scene
Measures radiation from the scene and form the required images
Uses radiometers which are calibrated receivers of thermal radiation
Here bolometer type Dick radiometer is used.
The incident radiation is focused on to a detector element and it causes a rise in temperature.
A single focused receiver can be raster across the scene to form the image. Systems have also been designed and built to scan an array of focused receivers using mechanical methods.
Array will be faster scanned to receive all frequencies.
These frequencies are successively sent to the demodulator and to the image processor.
Various gray codes are assigned to different frequencies at image processor.
After one complete scanning we get a complete frame. Scanning Rate is very fast.
Non-harmful to humans
Permits accurate 3D images
Aircraft landing and guidance
Low-visibility navigation and situational awareness
Reconnaissance and surveillance
Imaging of people and concealed weapons detection
Search and rescue and moreÂ¦
Oil spill detection
Â¢ Oil patches varies the effective emissivity of those areas through changes in thickness
Â¢ The observed temperature can be used to infer the thickness of the oil
Make life safer
Provide the military with a key advantage under low-visibility conditions
Enhanced vision landing capability for commercial airlines
Low-visibility conditions as a threat to safety or a hindrance to travel will be overcome.