Vol. 9 No 6 December 2001
Millimetric (mm) Wave Imaging
Extension of imaging techniques from shorter wavelength of IR and visible to longer wavelength of mm wave requires substantial fundamental penalties in terms of size and spatial resolutions. In spite of the above cited limitations, passive imaging sensor has gained popularity due to its performance under adverse weather conditions, i.e., fog, dust and battlefield smoke. Besides, operating under stealth conditions the images generated have a high contrast, stable signature with respect to IR and active radar signature and provides easy interpretation of an image. The mm wave imaging through passive radiometric sensor is proving to be of great value for the covert surveillance of battlefield by sensing the man-made objects, and armoured vehicles via a remote platform. The all-weather capabilities of mm wave imaging sensor provides the necessary edge over the other technologies for use in stand-alone or as part of sensor suit for the imaging. The passive mm wave imaging system develops a picture by detecting non-coherent noise like radiant electromagnetic energy. In this, band emissivity varies greatly from nearly zero for metallic objects to nearly one for natural objects, like vegetation. Metallic objects appear very cold to a passive mm wave sensor due to their low emissivity (high reflectivity) relative to terrain and other non-metallic objects in the image. Because these metallic objects are almost totally reflective, so any type of countermeasure will have little effect on their detection. The key to passive mm wave imaging is the large temperature contrast in the mm wave spectral band due to large variation in emissivity value for the various target materials and terrain in an imaged scene. Temperature contrast of greater than 100 times the contrast of IR systems are common for the systems operating in the mm wave spectral band as any material at a temperature above absolute zero raDIATes in microwave, mm wave, and other spectral regions. All objects in a given scene of interest are either reflecting or raDIATing electromagnetic energy in a given spectral band.
DRDO has developed mm wave imaging sensors with single and multi-receiving elements for the surveillance. It can detect metallic vehicles up to a range of about 2 km. The single receiving element with optical aperture of 500 mm is capable of scanning a user-defined sector and collects reflected/raDIATed noise energy from the scene to be imaged. The sensor provides DC voltage levels from 0 volts to 5 volts corresponding to 100 K to 332 K which is digitised into 256 grey levels for generating the image. The sensor has a thermal sensitivity of 0.35 K for integration time of 20 ms with a spatial resolution of 7 milliradians. With these parameter the sensor is able to image a vehicle up to 1 km using change detection algorithms.
The sensor with single receiving element is a stand-alone system, mounted on a one ton vehicle, with minimal preparation time for operation. The system includes data acquisition system, image display algorithms, image processing algorithms for correction enhancement and for automatic change detection zooming, and also an algorithm in the image zone. To reduce the data acquisition time a second configuration with 16-feed lens antenna system has been integrated with 16-channel radiometric array. The channel balancing and frequency synchronous LO feed network at 94 GHz are the key techniques critical for image generation. The multi-feed multi- beam antenna system and high performance radiometric receiver are the critical technology elements in this system. A real-time image display, correction and enhancement algorithm has been incorporated in this imaging sensor. Direction field of view, frame rate and the size of the displayed image are all user selectable features through menu-driven software. Imagers with IFOV of 112 x 7 milliradians can be scanned in one plane for enhancing the field of view and is dithered after every scan for acquiring higher spatial frequency components of the scene. The reduction in image acquisition time by 16 is the prime advantage of the linear array imager.