The essential technical aspects of Passive Millimeter Wave Imaging have been known for nearly a century, but widespread implementation did not occur until inexpensive millimeter wave amplifiers appeared in the 1990. This paper takes a look at the early work that has led to today's successful PMMW products, as well as some of the failed technologies that seem to be revisited over and over again.
Passive Millimeter Wave Systems often have low spatial resolution which limits the size of detectable objects.
However, there are some special cases in which objects that do not completely fill a pixel may be readily detected. The
geometry of wires and pipes make them particularly visible, although the contrast can vary rapidly with detector - target
geometry. In this paper a model is developed for the contrast of a reflective cylinder against an arbitrary background.
Experimental data is presented from a high clutter environment that shows sub pixel detection in practical
environments.
As Passive Millimeter Wave (PMMW) imagers mature, users have located them in more and more challenging locations. Because PMMW systems typically use natural sky coldness as a contrast source, indoor installations can become problematic. A number of semi-active illumination systems have been proposed, with various degrees of success. A problem many of them suffer from is reversed contrast - the illumination source is considerably hotter than the ambient scene, causing the cameras to act in unexpected ways. The relatively narrow extent of the illumination source is another problem, with speckle and glint often dominating the image. There are a number of unintended illumination sources in indoor locations, and all are poorly understood. This paper will examine several of them, as well as their polarametric properties, and discuss their effects on image quality.
Passive imaging for security and other applications has reached an important level of development. Ka and W band imaging systems are now commercial products, thanks to cheap and stable amplifiers. While deployment numbers are still modest, improvements to these systems will come from engineering and algorithm development, and not fundamental research. What research should focus on are system volume reduction and resolution improvements.
Fundamental research has several potential paths to solve these problems. Silicon Germanium CMOS can build acceptable millimeter wave amplifiers, and while SiGe noise figure is higher than GaAs, the ability to integrate RF and back end processing will push us closer to a CCD-like sensor. Antimony Arsenide features higher mobility than GaAs, with very low flicker noise and operation above 200 GHz which will reduce aperture size for equivalent resolution. More focus is needed in material characterization, particularly for clothing and common commercial materials.
Finally, Sparse Array technology may build flat, conformal structures with high resolution and relatively low detector count. This paper will briefly discuss the time line of past innovations, and explore the advantages and challenges of the new technologies that will drive this field forward.
KEYWORDS: Cameras, Extremely high frequency, Dielectrics, Systems modeling, Performance modeling, Black bodies, Metals, Sensors, 3D modeling, Data modeling
As semiconductor costs drop, Millimeterwave (MMW) imaging becomes more attractive to users of remote sensing and
security applications. Many cameras have been reported in the millimeter and even Terahertz bands, and early images
are always exciting. However, good system models are required to predict operational performance. All bodies above
Absolute Zero emit energy in accordance with Planck's law. However, the actual energy emitted depends not only on
the object temperature, but the emmisivity and surrounding temperature. Some materials have high emmisivity, such as
water or microwave absorber, and they emit primarily their own blackbody energy. Other materials, such as metals,
have very low emmisivity, and principally reflect the world around them. While there are no truly exotic physics
involved, getting models 'right' in some useful sense is a difficult and time consuming process. A simple ray trace
method with a non-polar environment / emmisivity models provides a basis to predict imager performance. An
unpolarized illumination source and an unpolarized detector are assumed to begin, which are then extended to include
polar effects. Inclusion of polar effects simply requires the complex dielectric constant, and the angle of incidence for
the ray being traced.
KEYWORDS: Radar, Synthetic aperture radar, Ka band, Signal to noise ratio, Unmanned aerial vehicles, Data conversion, Antennas, Missiles, Control systems, Oscillators
Growing interest in low cost unattended observations has lead Lockheed Martin Missiles and Fire Control to examine and demonstrate feasibility of a small inexpensive SAR/ISAR system at Ka band. By trading off state of the art performance for cost and volume, system performance for UAV applications is still adequate to provide important tactical information to battle field commanders in real time while reducing the exposure of war fighters to hostile fire. As RF and millimeterwave component become cheaper and more robust, system costs are expected to fall further. To demonstrate this concept, we have built a portable system with 500 Mhz instantaneous bandwidth, and have used to it gather SAR and ISAR data along with conventional HRR and LFM target data at Ka Band. Here we present sample data collected with our system along with supporting system performance supporting the use of such inexpensive systems in near term applications.
We built and tested a low-cost 8-by-8 millimeter-wave focal plane array using antenna-coupled micro-bolometers. The array consists of slot antennas coupled to nickel bolometers and was fabricated using optical lithography on high-resistivity silicon wafers. The measured noise equivalent temperature difference (NETD) of an individual element was 450 K. Simulation results corresponded with observed device performance. An improved design was then implemented using a square spiral antenna. We discuss the fabrication of this type of array element, include some modeling results, and present the methods and results of our measurements.
Millimeter wave cameras often have limited pixel count and image uniformity problems. While these may be expected to improve as technology advances, present simple image processing algorithms can provide significant improvement in users displays. Data collected with the existing NIJ camera is shown, along with uniformity corrections, and a straightforward correlation algorithm that provides a tracking dot on a weapon. This give a threat indication on what might at first appear to be a very marginal image stream. Suggestions for future work are also presented.
Successful demonstration of a video rate 94 GHz camera for concealed weapons detection has lead to interest in commercialization of this system. Besides the basic physics of object detection, there are many details a practical system needs to consider for the transition from experiment to product. We describe improvements in the RF modules, back end electronics, and user interface, along with additional video images captured with the improved system. RF module improvements include better low noise amplifiers and RF switches, along with a two-temperature calibration method resulting in a noticeable improvement in image quality. Analog electronics now have a more rapid offset correction, better stability, and better dynamic range. The user interface will now permit image fusion and other user- friendly features. Indoor experiments with active illumination are reported.
Successful demonstration of a video rate 94 GHz camera for concealed weapons detection has lead to interest in commercialization of this system. Besides the basic physics of object detection, there are many details a practical system needs to consider for the transition from experiment to product. We describe improvements in the RF modules, back end electronics, and user interface, along with additional video images captured with the improved system. RF module improvements include better low noise amplifiers and RF switches, along with a two-temperature calibration method resulting in a noticeable improvement in image quality. Analog electronics now have a more rapid offset correction, better stability, and better dynamic range. The user interface will now permit image fusion and other user- friendly features. Indoor experiments with active illumination are reported.
W-band radiometers have been shown to be effective in detecting metallic and non-metallic weapons concealed under heavy garments in both indoor and outdoor situations. With a real time-time MMW camera, we now may display subjects with weapons in both indoor and outdoor situations. The system consists of a linear array of W-band detectors in a 12 inch Cassegrain system with a mechanical scanner along with support and display hardware.
W-band radiometer have been shown to be effective in detecting metallic and non-metallic weapons concealed under heavy garments in both indoor and outdoor situations. We now have a system providing near real time-time display and weapon detection in a man portable demonstration system. The system consists of 34 W-Band detectors in a 12 inch Cassegrain system with a mechanical scanner. The system also includes a long wave infrared camera, which aids the operator in selecting targets.
Lockheed Martin and Intelligent Machine Technology are working to build a demonstration system for the Department of Justice and the Air Force Research Laboratory that can be extended to take advantage of rapidly moving semiconductor improvements. Our current radiometer modules use a PIN switch for the Dicke calibration function followed by multiple low noise amplifier stages, a Schottky detector and a video amplifier. Sensitivity of such modules is primarily a function of the insertion loss and noise figure of the front end MMIC circuits. Processing improvements at Lockheed Martin and refined chip design will result in a 3 dB improvement in the effective noise figure within the next year. Imaging is also greatly improved by achieving greater effective bandwidth and higher operating frequency. Whereas present modules operate from 80 to 90 GHz, MMIC improvements will provide for operation up to 140 GHz with a doubling of the bandwidth in the near term. Receiver operation up to 540 GHz has also been demonstrated at other labs.
Rapidly improving technology is transitioning current laboratory images into reality with fielded systems expected within two years. W-band radiometers have been shown to be effective in detecting metallic and nonmetallic weapons concealed under heavy garments in both outdoor and indoor situations. Systems that are in development will provide real-time display and weapon detection in controlled situations. The incorporation of technology improvements that have already been demonstrated will double the sensitivity and provide affordable systems with rapid scanning for general use. Lockheed Martin and Intelligent Machine Technology are working to build a demonstration system for the Department of Justice and the Air Force Research Laboratory that can be extended to take advantage of rapidly moving semiconductor improvements. Our current radiometer modules use a PIN switch for the Dicke calibration function followed by multiple low noise amplifier stages, a Schottky detector and a video amplifier. Sensitivity of such modules if primarily a function of the insertion loss and noise figure of the front end MMEC circuits. Processing improvements at Lockheed Martin and refined chip design will result in a 3-db improvement in the effective noise figure within the next year. Imaging is also greatly improved by achieving greater effective bandwidth and higher operating frequency. Whereas present modules operate from 80 to 90 G Hz, MMEC improvements will provide for operation up to 140G Hz with a doubling of the bandwidth in the near term. Receiver operation up to 540G Hz has also been demonstrated at other labs.
Concealed weapons pose a significant threat to military and civilian personnel protecting secured facilities and in low intensity conflicts. Passive millimeter wave and highly sensitive infrared sensors can detect these weapons. Parallel processors and a sensor fusion algorithm developed for engagement of military targets promise to solve this problem.
KEYWORDS: Extremely high frequency, Radiometry, Imaging systems, Signal to noise ratio, Sensors, Fiber optic gyroscopes, Passive millimeter wave imaging, Target detection, Receivers, Visibility through fog
Passive millimeter wave (MMW) imaging provides a breakthrough capability for driver vision enhancement to counter the blinding effects of inclement weather. This type of sensor images in a manner analogous to an infrared or visible camera, but receives its energy from the MMW portion of the electromagnetic spectrum. Technology has progressed to the point where MMW radiometric systems offer advantages to a number of vision applications. We report on our developmental 94 GHz radiometric testbed, and the eventual technological evolutions that will help MMW radiometers and radars meet military and commercial market needs.
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