The ability to obtain millimeter wave images under a variety of environmental conditions, such as rain, snow, fog, smoke, dust, etc., has numerous DoD as well as commercial applications. The demonstrated ability to look through doors, walls and clothing has recently extended potential millimeter wave applications to contraband detection and surveillance within buildings. Though the phenomenology supports the generation of high quality millimeter wave images, present-day frame time capabilities limit the use of millimeter wave cameras. Several solutions to frame time reduction are currently being investigated within government and industry. Two popular approaches include: (1) Electronic scanning focal plane arrays (FPA); (2) Mechanical raster scanning of a single antenna beam. One significant difference between the two approaches noted above is the number of receiving channels required. This is important because camera cost is driven by the number of receiver channels used in a camera, as well as the added complexities associated with inter-channel gain stability. There are a number of applications that do not require a motion picture capability. Images obtained sequentially at a nominal rate of one per second would satisfy the needs of a wide range of applications. It is evident, however, that the motion picture quality of a starring FPA may ultimately reduce the market for one-second cameras. In the interim, the one-second camera fills an important need. The goal of the Radiometric One Second Camera (ROSCAM) investigation is to demonstrate a practical millimeter-wave imaging (MMWI) camera, with a frame time of approximately one second. The approach combines a high-speed mechanical raster scanning antenna system with a single-channel radiometric receiving system. For baseline comparison, it is assumed that the scene is comprised of 1,000 pixels, each sampled for one millisecond, to generate a single frame in one second. The ROSCAM is based on combining a state-of-the-art radiometric receiver with a high-speed mechanical antenna scanning mechanism. One purpose of the initial measurement program described here, was to determine the ability of an existing high-speed raster scanning antenna to meet ROSCAM antenna requirements, specifically, a Field of View (FOV) consisting of 1,000 pixels scanned in a frame time of one second. A by- product of this investigation was the determination of the number of radiometer channels needed to generate a motion picture with a similar FOV. This paper includes: (1) Description of the ROSCAM Breadboard; (2) ROSCAM Performance Capabilities; (3) Measurement Results; (4) Conclusions.
A passive millimeter-wave (PMMW) camera capable of generating a real time display of the imaged scene, similar to video cameras, has been developed at TRW and is undergoing field testing. The camera operates at 89 GHz, acquiring images at a frame rate of 17 Hz. This work reports on the video imaging generated by the camera. This research is carried out under the Passive Millimeter-Wave Camera Consortium, a cost-shared program between the Defense Advanced Research Programs Agency and an industrial consortium that includes Honeywell, McDonnell Douglas and TRW. It is managed for the Department of Defense by NASA-LaRC.
The U.S. Navy Coastal Systems Station (CSS) is currently executing a program to develop a small, lightweight, low power passive millimeter wave imager. The end user will be Naval Special Operations Forces (SOF). The program began by conducting a feasibility assessment of the potential Passive Millimeter Wave (PMMW) technology that would meet the Naval Special Warfare (NSW) mission requirements. A performance analysis was conducted to compare the capabilities of the various PMMW imager technologies. Finally, a technology development road map is under development, which will include all recommendations for hardware development and image processing. Other DoD and industrial programs are being monitored for leveraging potential to insure the imager program will use the latest technology available. As a result of a technology survey, CSS decided to leverage their development funds with Eglin Air Force Base to develop an antenna-coupled microbolometer. This paper will discuss the program plans, and the potential applications of PMMW technology to Naval Special Warfare.
This paper discusses the issues that need to be addressed in the design and development of real time millimeter wave (MMW) imaging systems. The advantage offered by MMW imagers under adverse weather conditions are considered as are their thermal and spatial resolution. It is shown that it is difficult to use fully integrated focal plane arrays of receivers in large aperture high performance systems since the focal plane is so large. Instead scanned systems are considered to be preferable. Mechanical and electronic scanning techniques are described as is the use of image processing techniques which may increase the sharpness of an image by up to a factor of four. The paper introduces a new figure of merit for image quality which can be used to compare imaging systems.
In the paper a novel passive submillimeter imaging system is presented. The operation is based on antenna-coupled microbolometer technology operating at room temperature. The demonstration system uses single pixel detector placed in the focus of a 60 cm diameter aluminum parabolic reflector. The primary mirror is tilted about its axis to scan an image at the target plane. The design bandwidth of bolometer spiral antenna is nominally from 300 micrometer (1 THz) upwards. The electrical noise equivalent power (NEP) is exceptionally low 1.5 (DOT) 10-11 W/(root)Hz. Results presented in the paper indicate that the sensor technology is a possible solution in the future to produce large format imaging focal plane arrays for passive terahertz imaging.
Recent advances in monolithic semiconductor technology at W- band (75 to 110 GHz) have enabled implementation of compact radiometer front-ends for imaging applications. This paper describes a downconversion approach using Gallium Arsenide (GaAs) monolithic microwave/millimeter-wave integrated circuits (MMIC) to build modules for a 32-element receiver array. The MMIC downconverter module contains low noise amplifiers (LNA), microstrip bandpass filter, Schottky diode mixer and IF amplifiers. In conjunction with the local oscillator (LO), each downconverter serves as a superheterodyne receiver channel in the imaging array. The W- band array has 32 downconverter modules and 8 LO distribution modules which distribute LO power from a Gunn diode oscillator to each downconverter. The LO distribution module incorporates varactor phase shifters with LO drive amplifiers for phase adjustments of plus or minus 180 degrees to match the phase output from each receiver channel. The downconverter modules of the 32-element array demonstrated greater than 40 dB RF-to- IF gain and an average noise figure of 5.7 dB over a 10 GHz bandwidth centered at 94 GHz. Uniformity of the MMIC devices allows gain tracking within plus or minus 2.5 dB and phase tracking within plus or minus 18 degrees between the 32 receive channels. For Dicke radiometer operation, a PIN diode switch MMIC has been inserted in front of LNA in the downconverter module. Noise figure and gain results for the PIN switch front-end will be presented.
A state-of-the-art W-Band passive millimeter wave focal plane array (FPA) consisting of 1040 highly integrated direct detection pixel has been designed, developed, assembled and tested. The FPA has been integrated into a passive millimeter wave video camera and has generated real time images. Each pixel is a highly integrated MMIC chip receiver. The MMIC chip is a wide band, high gain, low noise, 0.1 micrometer InGaAs HEMT amplifier with an integrated switch and Schottky barrier diode detector. The FPA uses a brick architecture. Each brick or module consists of 4 MMIC chips or pixels and lay side-by- side on the card. Many cards are stacked to create the array of pixels. In the next generation FPA, the 1 X 4 modules and cards have been dramatically simplified with 50% less assembly time. In addition, the module and card still require no tuning and minimal test time. Thus a significant cost reduction in the FPA is expected over the first generation FPA without sacrificing performance. To further reduce cost and improve performance, new MMIC chips are being designed.
The long-term potential of terrestrial passive millimeter-wave imaging is contingent upon demonstrating real-time imaging with a system that can be conveniently integrated into a ship, land vehicle or aircraft. For small imagers with modest resolution, this can be readily achieved using fully-staring focal plane arrays, but as in the infrared, the high cost per pixel means that scanning (preferably electronic) of a smaller number of detectors, across the image is attractive. Aperture synthesis using sparse and filled arrays of antennas offers high sensitivity and resolution from a small number of antennas that can be conformal to vehicle shape, but requires complicated beam-forming technologies. Alternatively, electronic scanning can be accomplished by electronic modulation of a filled antenna. This discusses approaches and technology requirements for achieving electronic beam- steering.
Lack of reliable fire warning and detection systems for urban/wildland interface, large area industrial facilities and transportation systems result each year in a loss of millions of dollars worth of property; it also endangers lives. Typical optical fire detection sensor do not work well under frequency encountered adverse atmospheric conditions and, in addition, are incapable of covering sizable areas. WaveBand has recently developed hardware to study the feasibility of fire detection using a millimeter wave (MMW) scanning radiometer. It has proven the advantages of remote fire detection even under adverse weather conditions and through fire-generated smoke, better immunity to false alarms than optical sensors, and larger area of coverage. Despite using a wavelength that is much longer than that of visible light, the MMW sensor can accurate pinpoint the location of a developing fire.
Multielement radiometric matrix of sensors of millimeter-wave range was developed for radio-imaging in the range. Use of the matrix allows to transfer from mechanical scanning of observed scenes to electronic one and significantly decrease time of observation.
Coastal Systems Station under the sponsorship of the Office of Naval Research (ONR) is exploring the use of a Passive Millimeter Wave (PMMW) sensor for Naval Special Warfare applications. The technology of passive millimeter wave imaging will provide the capability to do concealed object detection, zero visibility navigation, and clandestine passive markers. Nichols Research Corporation and Coastal Systems Station (CSS) have conducted a feasibility assessment of PMMW sensors to meet NSwt NSW mission requirements. The assessment began with a mission requirements analysis which will provided the measure of effectiveness for the performance evaluations. Following the requirement analysis, a technology survey was conducted to determine the state of the art of PMMW imaging. Performance analysis were conducted to compare the capabilities of the PMMW imager to the measures of effectiveness. Data collections using existing PMMW imaging devices, developed by Eglin AFB, were conducted to validate the modeling results and provide empirical system performance, and phenomenology data. This paper will present results from the PMMW data collection effort.
The feasibility of using passive centimeter and millimeter wave emission to detect landmines is investigated. The work concentrates on the modeling and measurement of the radiation temperatures of metal and plastic plates. These plates are considered both surface mounted and buried under thin layers of soil. Experimental measurements have been made at frequencies of 3 GHz, 10 GHz, 35 GHz and 94 GHz. A two interface model, which uses the dielectric constants of the media, is used to predict the radiation temperatures of the plates. Conclusions are made about the radiation bandwidths in which the various plates, surface mounted and buried, can be detected.
In recent years there has been considerable advancement in millimeter and submillimeter wave radiometry applied to atmospheric remote sensing. Contributing to this advancement are the measurements made by the Millimeter-wave Imaging Radiometer (MIR) and subsequent analysis of the data. The MIR is an imaging radiometer designed to fly aboard the National Aeronautics and Space Administration's (NASA) ER-2 high altitude aircraft with nine frequency channels between 89 and 325 GHz. Since its maiden flight in May 1992, the MIR has flown over 80 sorties comprising more than 300 flight hours. In addition, the instrument has participated in several ground-based experiments yielding hundreds of hours of data. The MIR data have been used for comparing atmospheric water vapor profiles derived from different measuring techniques. Images of the atmosphere reveal these frequencies are sensitive to the presence of liquid and ice clouds; analysis of the data suggests that characteristics of the clouds may be extracted from the data. The instrument has been used to support calibration and validation studies of satellite sensors. Analysis indicates ground-based radiometric observations near the 183 GHz water vapor absorption line may be useful for obtaining accurate ground-based measurements of low values (less than 1.0 cm) of precipital water vapor. This paper describes the application and utility of millimeter and sub-millimeter wave radiometric imaging for atmospheric research in the context of accomplishments attained with the MIR.
For most radiometers, periodic calibration is essential to ensure sensor measurement accuracy. This paper describes the application of a unique active microwave circuit that simulates known radiated thermal temperatures over a stepped range of less than 105 K to over 300 K. The device, when connected or switched to the input of a receiver, can be incrementally stepped via a bias voltage from the coldest to the warmest temperature. It offers the potential to replace more complex and expensive external and internal calibration techniques. In addition to calibration, the device also serves to test the receiver linearity and measure receiver noise figure. The paper will identify significant advances in the area of modeling, design, development, and test of the stepped 'cold/warm' noise source. Several prototype FET circuits, using 150 and 300 micrometer InP devices, have been built and demonstrated at K-band frequencies of 18 - 22 GHz. Work is in progress to build and test the next device at 37 GHz. Tests have been conducted at Raytheon Systems, the University of South Florida (USF), and the National Institute of Science and Technology (NIST). The results illustrate good agreement between simulation and measurements. Block diagrams are included to show the method of calibrating radiometers, testing receiver linearity, and measuring receiver noise figure.
We present our results on the application of image processing techniques for passive millimeter-wave imaging and discuss possible future trends. Passive millimeter-wave imaging is useful in poor weather such as in fog and cloud. Its spatial resolution, however, can be restricted due to the diffraction limit of the front aperture. Its resolution may be increased using super-resolution techniques but often at the expense of processing time. Linear methods may be implemented in real time but non-linear methods which are required to restore missing spatial frequencies are usually more time consuming. In the present paper we describe fast super-resolution techniques which are potentially capable of being applied in real time. Associated issues such as reducing the influence of noise and improving recognition capability will be discussed. Various techniques have been used to enhance passive millimeter wave images giving excellent results and providing a significant quantifiable increase in spatial resolution. Examples of applying these techniques to imagery will be given.
Method of optimization is considered to improve resolution and noise properties of millimeter-wave radio vision system. Conditions of optimal measurements are realized by coincidence of bandwidths both of signals and device.
To recover spatial information from bandlimited images using maximum likelihood (ML) and constrained least squares techniques it is necessary that the image plane be oversampled. Specifically, oversampling allows the blur component induced by spatial integration of the signal over the finite size of the detector element(s) to be reduced. However, if oversampling in the image plane is achieved with a fixed array, the field of view (FOV) is proportionately reduced. Conversely, if the FOV is to be preserved then proportionately more samples are required implying the requirement for additional detector elements. An effective solution to obtaining oversampling in the image plane and subsequently preserving the FOV, is to use either controlled or uncontrolled microscanning. There are a number of methods to achieve microscanning including translation of the sensor array in the image plane and exploitation of airframe jitter. Three unique sixteen-times-Nyquist oversampled passive millimeter wave (PMMW) images; a point source, an extended source, and an M48 tank were carefully obtained. Both ML and constrained least squares (CLS) algorithms were used for restoration of spatial information in the images. Restoration of known extended source object functions (contained in the extended source image) resulted in resolution gains of 1.47 and 3.43 using the CLS and ML methods respectively, as measured by increase in effective aperture.
Iterative image restoration algorithms developed using a maximum likelihood (ML) estimation framework are attaining considerable significance in recent times for super-resolution processing of passive millimeter wave (PMMW) images. In this paper we offer a processor requirements analysis for implementing these algorithms, which provides assurance on the feasibility of their implementation using commercially available microprocessors, even for applications where processing time may be of critical importance. Two optimized versions of these algorithms, one developed by augmenting each iterative estimation step with a post-filtering operation and the other developed by incorporating a background-detail separation approach in the estimation process, are developed which provide superior resolution enhancement performance while simultaneously suppressing noise-induced and ringing artifacts in the restored images. Results of processing data acquired from a 95 GHz 1 foot diameter aperture radiometer are included to demonstrate that these algorithms offer significant superresolution capabilities for processing PMMW imagery.
Passive millimeter wave (PMMW) imaging systems have many important applications, both military and commercial. However, due to the longer wavelength, resolution is limited when compared with shorter wavelength imaging systems of comparable aperture size. One approach to this problem is super resolution. Over sampling in the focal plane supports super resolution techniques that utilize maximum likelihood and constrained least squares methods, while preserving the field of view. In order to test super resolution algorithms with real image data, a versatile PMMW test bed constructed for the Air Force Smart Tactical Autonomous Guidance (STAG) program was utilized to obtain 16x over sampled images of three test targets. First, a Gunn diode oscillator (GDO) source was imaged in order to accurately measure the point spread function (PSF) of the imaging system. A special source pattern was then imaged to measure the system's response to differently oriented step functions, and to determine the system time constant. An M48 tank was then imaged as an example of a real world military target. In each case the experimental images were obtained by mechanically scanning a single TRF receiver module in a two-dimensional raster pattern in the focal plane of a 94 GHz, f/i, two element, refractive, telecentric imaging system. Imagery was obtained at horizontal scan velocities ranging from 1.27 cmlsec to 12.7 cmlsec with 1270 or 1 344 samples per horizontal scan line, and a horizontal sample spacing of 0. 1 mm. For comparison, Nyquist sampling was achieved with a 64x84 image size and 1 .6 mm sample spacing. In order to achieve the large amount of over sampling, and be able to handle the resulting large quantities of image data, hardware and software modifications had to be made to the existing STAG test bed. These changes will be reported along with the sampling details and results for all three targets. Keywords: Passive Millimeter Waves, Millimeter Wave Imaging, Radiometry, Smart Tactical Autonomous Guidance, Super resolution, Over Sampling, Image Enhancement, Nyquist Sampling