21-02-2010, 09:38 AM
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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
Introduction
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
Imaging Principles
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.
ADVANTAGES
Non-harmful to humans
Rapid surveillance
Permits accurate 3D images
Low cost
Applications
Aircraft landing and guidance
Low-visibility navigation and situational awareness
Reconnaissance and surveillance
Oil-spill detection
Imaging of people and concealed weapons detection
Search and rescue and more¦
Oil spill detection
¢ Oil patches varies the effective emissivity of those areas through changes in thickness
¢ The observed temperature can be used to infer the thickness of the oil
Conclusion
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.
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
Introduction
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
Imaging Principles
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.
ADVANTAGES
Non-harmful to humans
Rapid surveillance
Permits accurate 3D images
Low cost
Applications
Aircraft landing and guidance
Low-visibility navigation and situational awareness
Reconnaissance and surveillance
Oil-spill detection
Imaging of people and concealed weapons detection
Search and rescue and more¦
Oil spill detection
¢ Oil patches varies the effective emissivity of those areas through changes in thickness
¢ The observed temperature can be used to infer the thickness of the oil
Conclusion
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.
