The STAG program was hiitiated at Eglin Air Force Base in 1990 under the sponsorship of the Armament Directorate of Wright Laboratory. The purpose of the STAG program is to investigate and support passive millimeter wave activities associated with the development of mart tactical gutonomous guidance systems. Development of the associated sensor technology focuses attention on a data collection/phenomenology effort to provide a firm foundation for sensor design and algorithm development. The center piece of the data collection/phenomenology effort is the mobile test bed referred to as MAPS, an acronym taken from Millimeter Wave Analysis of Passive Signatures. As shown in Figure 1, the MAPS will provide the data base of passive millimeter wave information concerning terrain, atmospheric and target signatures needed to support the critical development technologies associated with the STAG program. MAPS' low-bed trailer includes an equipment enclosure which is 8 ft. wide and 25 ft. long. Photographs ofMAPS are shown in Figures 2a through 2d.

A passive millimeter-wave camera capable of generating real time displays of the imaged scene, similar to video cameras, has been under development at TRW over the past two years. The camera operates at 89 GHz, has a 15 degree(s) X 10 degree(s) field-of-view, an aperture of 18' diameter, and displays the acquired image at a frame rate of 17 Hz. A major enabling technology is the focal plane array of direct detection MMIC receivers which guarantees reliable and low cost manufacturing of this camera, in addition to providing it with unique operational features. This work reports on progress achieved to date in the development and manufacturing of this new sensor technology.

Passive millimeter wave (mm-wave) imaging systems have attracted an increasing interest over the past years due to their superior poor weather performance compared with visible and infrared systems. In the UK the Defence Evaluation and Research Agency Malvern developed its first mm-wave radiometers in the late 1950s. These systems were bulky and had poor spatial resolution and low thermal sensitivity, but the considerable advances in semiconductor solid state devices have allowed the size and weight of imagers to be reduced. Advantage can also be taken of sophisticated on-line signal processing and of complex theoretical modeling and analysis. This paper examines the merits of the different operating frequencies in terms of atmospheric transmission vs. resolution and also discusses issues such as image processing. High quality images are presented to demonstrate the potential of this emerging technology.

The author will describe millimeter wave focal plane array (FPA) imagers developed primarily for concealed weapons detection (CWD) and through wall surveillance (TWS) applications. Both passive (radiometric) and active (radar) imagers will be described. The technology employed in these cameras is ideally suited to a wide range of other applications as well. Traditionally, passive millimeter wave images have been generated using scanned sensors of various types ranging from single elements to line arrays. A line scanner using FPA technology is being developed at Millimetrix for CWD and other applications. Scanning imagers, however, cannot meet the frame rate and sensitivity requirements for some applications. Certain CWD applications, in particular, require a passive, video rate (30 fps) imagers which we are also developing using a patented focal plane array technology we call Millivision^TM. Similarly, TWS applications demand an active, video rate imager which shares much of the same Millivision^TM FPA technology. Customers always need more resolution, more sensitivity, and a wider field-of-view all in the smallest possible package and at the lowest cost. To meet these difficult requirements, the Millivision^TM video rate imagers operate near 94 GHz and employ active optics and filled focal plane arrays, both of which will be briefly described. The optimally filled FPA is small angle scanned (dithered) electronically relative to the scene in a 4 X 4 matrix to achieve a 2 X 2 oversampling of the image. A multiplicative `super resolution' algorithm is then used to digitally enhance the spatial frequency resolution of the resulting image by a factor of approximately 2.

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.

Microwaves (MW) and Millimeter Waves (MMW) have the known benefit of penetrating the turbid Earth atmosphere under conditions where visible and infrared wave display rather limited optical depth. Radars represent conventional implementations of MMW sensor systems, while radiometers embody evolving, environmentally friendly, covert systems for civilian and military applications. This paper reviews the evolution of spaceborne passive MW and MMW sensor systems as primarily represented by Aerojet sensor products for atmospheric parameter sounding/imaging. The paper assesses current trends, and provides an outlook for the future of such sensor systems.

Passive Millimeter-wave Imaging (PMI) technology provides a powerful sensor capability for military and commercial imaging applications, during day or night, and in adverse weather. Recent advances in high-frequency antennas, MMW electronics, and high-speed signal processing, have brought real-time, high-contrast, high-resolution, wide-field PMI into the realm of technological feasibility. However, the substantial size, weight, and cost of previous PMI architectures have proved impractical for all but a few scientific implementations, creating a barrier to large- volume production. This reality has precluded PMI usage in several applications with demonstrable benefits, such as aircraft navigation and landing, radio-silent airborne surveillance/battle damage assessment, concealed weapons detection (CWD), or through-wall imaging. A new PMI architecture has been demonstrated which allows this wide- area, near-real-time staring capability with significant reductions in size, weight, and cost relative to previous designs. Specifics of this new PMI architecture will be presented along with a host of imaging data representing its current capability for airborne imaging, CWD, and through- wall imaging.

Passive microwave imaging with conventional linescanner systems is an extensively proofed technique with a long tradition and experience in civil and military application fields. During the last couple of years another promising technique, the aperture synthesis method, has become more of interest because of the principal possibility to generate 2D images without moving the aperture. In the first past of this paper, representative measurement results are shown from a 90 GHz linescanner system, cooled with liquid nitrogen, with a spatial and radiometric resolution of 1 degree(s) and 1.7 K for flight measurements, respectively. In the second part, a groundbased aperture synthesis radiometer imaging system at 37 GHz is described. Basically the system consists of a two-element interferometer with a variable baseline, which enables the complete sampling of the uv- plane sequentially. As a consequence this imaging equipment is only suited for the mapping of stationary targets. Experimental measurement results are demonstrated which were acquired in the near and far field with a spatial resolution of 0.6 degree(s) and a temperature resolution of about 1.5 K.

Millimeter-wave integrated circuits (MMICs) are briefly described and system configurations for millimeter-wave passive sensing utilizing MMIC's are reviewed in terms of the type of receiver (tuned RF or superheterodyne) and type of array (focal-plane or aperture). Data on the gain fluctuation which occurs in millimeter-wave field-effect transistors is presented along with an analysis of the effect of this fluctuation on radiometer sensitivity. Two MMIC remedies of this transistor gain fluctuation are then discussed. The first is the stabilization of the gain with a VHF pilot signal path built into a millimeter-wave MMIC low- noise amplifier; this MMIC has been designed, fabricated, and tested and system tests are expected later this year. The second remedy is a MMIC PIN diode switch which can be used as a Dicke load switch at the receiver input. The PIN switch, which also includes a thermal calibration method, can be chopped at a rapid rate (> 1 kHz) relative to the l/F spectrum of the transistor gain fluctuations so that sensitivity is not impaired. The layout and measured performance of a 80 to 100 GHz version of the switch is presented.

A passive millimeter wave (PMMW) camera capable of generating real time displays of the imaged scene, similar to video cameras, has been developed at TRW over the past two years. The camera operates at 89 GHz, has a 15 degree(s) X 10 degree(s) field-of-view, an aperture of 18' diameter, and displays the acquired image at a frame rate of 17 Hz. A major enabling technology is the focal plane array of direct detection MMIC receivers which enables this camera to be reliable and low cost, in addition to providing it with unique operational features. A state-of-the-art W-band passive millimeter wave FPA consisting of 1040 highly integrated direct detection MMIC pixels capable of generating real time images has been developed. We will discuss the assembly and test of high performing, high frequency GaAs MMIC chips in high volume and at low cost. This work reports on progress achieved to date in the development and manufacturing of this new sensor technology.

Passive imaging at millimeter wavelengths is a constant struggle for increased sensitivity and angular resolution. Aperture synthesis is a particularly attractive technique for attacking these problems since it offers a high resolution from a given total antennas area and greater flexibility in the positioning of the antenna elements. This in turn can lead to a greater total collecting area and hence greater sensitivity than might be achievable with a single scanned antenna and with the additional benefit of electronic scanning. The high loss of millimeter-wave transmission lines means that received signals must be frequency translated to a more suitable frequency prior to transport to correlators. Although down conversion enables transmission by coaxial cables, up conversion onto optical carriers enables very low-loss optical fibers to be used for transmission and electronically programmable delay lines. In this paper we describe proof-of-principle experiments that demonstrate the application of optical up-conversion in aperture synthesis and also the direct formation of an image on a conventional optical camera from millimeter-wave signals modulated onto an optical carrier.

A challenge in the development of multi-channel millimeter wave imaging radiometers is overcoming effects associated with the temperature dependence of receiver responsivity. In this paper, the stability of absolute radiation temperature measurements, made with direct and heterodyne detection radiometers, is investigated theoretically and experimentally. The agreement between theory and experiment is found to be good. Changes in measured radiation temperatures were found to be between 6 degree(s)K at 35 GHz and 145 degree(s)K at 220 GHz, for a one degree change in instrumental temperature. Suggestions are made, as to how the temperature stability of radiometers may be improved.

We have combined silicon micromachining technology with planar circuits to fabricated room-temperature niobium microbolometers for millimeter-wave detection. In this type of detector, a thin niobium film, with a dimension much smaller than the wavelength, is fabricated on a 1-micrometers thick Si₃N₄ membrane of square and cross geometries. The Nb film acts both as a radiation absorber and temperature sensor. Incident radiation is coupled into the microbolometer by a 0.37 (lambda) dipole antenna with a center frequency of 95 GHz and a 3-db bandwidth of 15%, which is impedance matched with the Nb film. The dipole antennas is placed inside a micromachined pyramidal cavity formed by anisotropically etched Si wafers. To increase the Gaussian beam coupling efficiency, a machined square or circular horn is placed in front of the micromachined section. Circular horns interface more easily with die-based manufacturing processes; therefore, we have developed simulation tools that allow us to model circular machined horns. We have fabricated both single element receivers and 3 X 3 focal-plane arrays using uncooled Nb microbolometers. An electrical NEP level of 8.3 X 10^-11 W/(root)Hz has been achieved for a single- element receiver. This NEP level is better than that of the commercial room-temperature pyroelectric millimeter-wave detectors. The frequency response of the microbolometer has a ln(1/f) dependence with frequency, and the roll-off frequency is approximately 35 kHz.

MMW Scanning Antenna remains one of the most challenging components in the imaging radar design. Electronically steered antennas require sophisticated fabrication and become prohibitively expensive if a large array is considered. Mechanically scanning antennas typically involve one or more hinged parts (lenses, mirrors or feeds). In operation they experience mechanical acceleration and forces that sharply limit scanning rate. We present test results of a recently developed MMW scanning antenna that is capable of providing a fast linear scan while requiring only continuous constant speed rotation. Two antennas applicable to the aircraft autonomous landing and automotive collision warning systems have been developed. They are characterized by simple design and low fabrication cost. Other antennas modifications based on the evanescent coupling principle are proposed to facilitate various radar functions.

Integration of plurality of radiometers into a panoramic receiving systems seems efficient for realization of `radiovision' (imaging) systems without scanning if small- sized sensors with good operating parameters are available. The use of Josephson Junctions (JJ) in self-pumping mode as one of various heterodyne detection methods is of principally new particular interest and trend, when we consider the properties such as the sensitivity, possibilities of frequency electronic tuning in big band, integrations with HEMT's and other nontraditional technical decisions, including the measuring of frequency of active signals on JJ. According to experimental and theoretical results it is possible to get the sensitivity not less than 0,01 K in 3 mm wave band at cooling temperature 20 - 30 K in passive imaging system at the base of matrix of JJ + HEMT's `pixels'.

The results of a series of microwave measurements were made to confirm the relationship between measured brightness temperatures (Tb) and the Fresnel equations as a function of local incidence angle, polarization and frequency. The Phillips Laboratory's microwave transmission code RADTRAN allows the input of surface emissivity or reflectivity and the thermometric temperature of the surface to account for surface radiation properties. The experimental verification was carried out using the Millimeter Wave Analysis of Passive Signatures (MAPS) system. MAPS consists of a spectral camera mounted on an scissors lift tower with elevation/azimuth positioning and operating at frequencies of 35, 60 and 95 GHz, with both vertical and horizontal polarization.

The military use of millimeter wave radiometers has been studied since the 1960's. It is only recently that advances in the technology have made passive millimeter wave (PMMW) systems practical. It is well established that metal targets will have a large contrast ratio versus the background in the millimeter wave (MMW) regime and that atmospheric propagation through clouds, fog and light rain is possible. The limitations have been the noise figures of the detectors, the size of the systems, and the cost of the systems. Through the advent of millimeter wave monolithic integrated circuits technology, MMW devices are becoming smaller, more sensitive, and less expensive. In addition many efforts are currently under way to develop PMMW array imaging devices. This renewed interest has likewise brought forth the need for passive millimeter wave system modeling capabilities. To fill this need, Nichols Research Corporation has developed for Eglin AFB a physics-based image synthesis code, capable of modeling the dominant effects in the MMW regime. This code has been developed to support the development of the next generation of PMMW seeker systems. This paper will describe the phenomenology of PMMW signatures, the Irma software, validation of the Irma models and the application of the models to both Air Force and Navy problems.

As passive millimeter wave sensor technology matures, algorithms which are tailored to exploit the benefits of this technology are being developed. The expedient development of such algorithms requires an understanding of not only the gross phenomenology, but also specific quirks and limitations inherent in sensors and the data gathering methodology specific to this regime. This level of understanding is approached as the technology matures and increasing amounts of data become available for analysis. The Armament Directorate of Wright Laboratory, WL/MN, has spearheaded the advancement of passive millimeter-wave technology in algorithm development tools and modeling capability as well as sensor development. A passive MMW channel is available within WL/MNs popular multi-channel modeling program Irma, and a sample passive MMW algorithm is incorporated into the Modular Algorithm Concept Evaluation Tool, an algorithm development and evaluation system. The Millimeter Wave Analysis of Passive Signatures system provides excellent data collection capability in the 35, 60, and 95 GHz MMW bands. This paper exploits these assets for the study of the PMMW signature of a High Mobility Multi- Purpose Wheeled Vehicle in the three bands mentioned, and the effect of camouflage upon this signature and autonomous target recognition algorithm performance.

In the modeling of the radiance from a buried object, two principle models have been used--the incoherent model and the coherent model. The difference between these two models is in the method of combining radiation from the same source but after traversing different path lengths, due to partial reflections at the ground-air interface, to the receiver. Coherent addition of the contributions gives rise to a signal which oscillates, as a function of the burying depth, about that which would arise from incoherent addition of the contributions. The degree of coherency of the contributions is shown to be a function of the receiver bandwidth, burying depth, and surface roughness. Criteria for determining the degree of coherence are given. Experiments supporting the theory are cited and examples are given.

In order to predict the performance of a passive millimeter wave sensor under a variety of weather, terrain and sensor operational conditions, TRW has developed the Advanced Radiometric Millimeter-Wave Scene Simulation (ARMSS) code. This code provides a comprehensive, end-to-end scene simulation capability based on rigorous, `first-principle' physics models of the passive millimeter wave phenomenology and sensor characteristics. The ARMSS code has been extensively benchmarked against both data in the literature and a wide array of millimeter-wave-field-imaging data. The code has been used in support of numerous passive millimeter wave technology programs for interpreting millimeter wave data, establishing scene signatures, performing mission analyses, and developing system requirements for the design of millimeter wave sensor systems. In this paper, we will present details of the ARMSS code and describe its current use in defining system requirements for the passive millimeter wave camera being developed under the Passive Millimeter Wave Camera Consortium led by TRW.

The Air Force program, Smart Tactical Autonomous Guidance (STAG), has as its central concept, the use of passive millimeter wave imagery (PMMW) to enable an autonomous vehicle to perform its own smart guidance and attack. The algorithms on board the vehicle use image flow to derive the necessary range information for obtaining real time navigation updates. The results of a natural-imagery feasibility program will be reported, aimed at validating the STAG approach. PMMW imagery will be taken from land- based vantage points that mimic the geometry of an airborne, down looking, sensor. Essentially, hardware-in-the-loop simulation will be performed, where PMMW data will be real, and the loop extends over many miles of outdoor terrain. The only departure from an actual mission will be that it is not real-time. All imagery will be gathered using frame times consistent with existing camera capabilities. Image flow and other data processing will be done off-line. The key ingredients will be the sequences of imagery and the computer processing of that imagery. The means for accomplishing both have been developed under the STAG program. The camera to be used operates at W-band and consists of an f/1, refractive, telecentric, image forming system with a 30 cm diameter input aperture. Image flow involves a model whose parameters are determined via automated pixel tracking from frame to frame. Passive range maps are then generated, and navigation is accomplished through the subsequent correlation of these maps with reference elevation maps. Automatic target recognition is also addressed.

The mathematical description of the rise of radio-brightness contrasts (RBC) of terrains with the chaotic roughnesses and local dielectric anomalies is offered with account of scattered thermal radiation of atmosphere. The analysis of influence of humidity, porosity, as well as terrain roughnesses on radio-brightness contrasts was carried out. In natural conditions at wavelength of 8 (DOT) 10^-3 m the measurements of RBC--wet/dry ground, dense/loose soil, metal/ground were performed. The comparison between experimental data and theory have shown its good agreement, that testifies about adequacy of offered mathematical description. The technique and results of stochastic synthesis of radio-thermal images of the wood, sandy bank, river and island from aerial photos and corresponding trace radiometric measurements are submitted. The reliability of synthesis is confirmed by correlation overlapping of the synthesizing radio-image and fragment of radiometric profile, which was not used in the synthesis.

This paper describes a fast non-linear algorithm which is capable of enhancing passive mm-wave images beyond their diffraction limited resolution from a knowledge of the imager point spread function. It is similar to the error energy reduction method of Gerchberg but may be applied to a general scene with no well defined spatial extent of known zero intensity. It was developed as a near real time algorithm that could be implemented in imager hardware.

The need for superresolution processing of images in multispectral seeker environments for facilitating smart munition guidance is being increasingly recognized, particularly when the sensor suite includes Millimeter-Wave (MMW) sensors with rather poor inherent resolution capabilities. Despite the technological breakthroughs being made in advanced radiometer designs, the inherent problems associated with diffraction limited imaging impose limitations on the resolution of acquired imagery thus necessitating efficient post-processing to achieve resolution improvements needed for reliable target detection, classification and aimpoint selection. Quantitative results from a recent project directed to superresolution processing of passive MMW images obtained from a 95 GHZ 1-foot diameter aperture radiometer are presented in this paper. The spectral extrapolation performance resulting from the implementation of an iterative Maximum Likelihood restoration algorithm is demonstrated and the robustness of the algorithm that facilities a blind implementation useful in scenarios characterized by an incomplete knowledge of sensor point spread function is highlighted.

In passive imaging, the spatial information acquired is strictly bandlimited. Because of this limitation, a number of postprocessing strategies have been proposed to accomplish a measure of superresolution. These strategies incorporate prior information about the image to improve resolution. We show that unless this information is shift- variant, it is unable to contribute to any superresolution. Shift-variant information about the image can be shown to be equivalent to forcing a correlation among the basis images that represent the image. We show that accomplishing superresolution from this correlation is very difficult and has fundamental limitations. Finally, we discuss the potential gains available from using prior information and propose an acquisition strategy that in some cases could improve the potential for superresolution.

A method of passive millimeter-wave imaging with super- resolution using a phased array antenna system has been developed. The enhancement of the image resolution has been achieved by using several mathematical methods including the new method of reduction invented by the Russian mathematician Pytiev.