Multi-Channel Radiometry in MMW Imaging Systems
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Multi-Channel Radiometry in MMW Imaging Systems
V.S.Ablyazov, V.V.Abramov, V.E.Lyubchenko, K.T.Mourzabulatov, S.Yu.Turigin Institute of Radioengineering & Electronics, Russian Academy of Sciences, Moscow, Russia ABSTRACT State of the art in development of millimeter wave radiometric imaging systems is analyzed and discussed. One-dimension array of 16 radiometers with mechanical scanning of the offset parabolic antenna is experimentally demonstrated that offers rms 0.03 K at time constant 1 s and instant field of view of 4×60 mrad. INTRODUCTION Detecting and identification of the objects in millimeter wave (MMW) range of electromagnetic spectrum becomes increasingly important for various applications: collision avoidance at the transport, discovering of the canceled weapons or contraband goods, medical imaging, plasma diagnostics and other areas. Due to the small wavelength of the testing signals the space resolution of MMW systems could be achieved as comparable with one for the optical systems. Small sizes of antennas and waveguides make possible the system to be performed as a portable in many cases. At the same time MMW imaging avoids the influence of such opaque obscurants as smoke, fog and atmospheric drops, which are so affecting to optical and infrared systems [1]. Radiometry is the most attractive for imaging as it employs only native electromagnetic emission of the subjects and avoids speckle (glint effects) arising when projecting irradiation is used in active radars. Meanwhile as the thermal emission is normally very weak the high sensitive receivers should be used to provide the appropriate contrast at the image of the scene been observed. The basic radiometric equation for the uncertainty of the temperature that can be measured is following:
δT = αTS /(∆f ⋅ τ ) 0.5 where α is a factor depending on the radiometer optics and scheme; TS is the system noise temperature; ∆f is a bandwidth of high-frequency (predetection) part of the receiver; τ is the integration time. It shows that ∆f and τ are the most important variable values depending on the receiver design. The bandwidth value ∆f is normally limited (1-3GHz), so the integration time is used to provide the appropriate receiver sensitivity. The systems employing one-pixel receivers with mechanical scanning [4,5] are too slow, so imaging arrays of Schottky-diode detectors or heterodyne receivers in some cases with low-noise transistor preamplifiers are used [2,3]. Two-dimension receiver arrays are quite complicated and expensive, so the optimized version seems to be
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1-D array that impose multi-channel (up to 20) radiometer with mechanical scanning, though the system with quasi-optical switches [6] are considered as promising. Signal processing is one of the most important parts of multi-channel radiometric system. It provides not only adequate distribution and intensity of the pixels at the monitor but also impose the “super resolution” [7]. EXPERIMENTAL The design of multi-channel radiometer performance was developed in this work at the freq
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