This paper addresses anomalies that occasionally appear in SAR images. The cause is assumed to be atmospheric inhomogeneities. The atmospheric effect is small and should be more noticeable at longer ranges. A first order model of the SAR image formation process is developed. This model predicts secondary effects in the image that are not directly observed. The secondary effects are attributed to coupling between the image and frequency (data) space.

Over the past ten years, Sandia has developed RF radar responsive tag systems and supporting technologies for various government agencies and industry partners. RF tags can function as RF transmitters or radar transponders that enable tagging, tracking, and location determination functions. Expertise in tag architecture, microwave and radar design, signal analysis and processing techniques, digital design, modeling and simulation, and testing have been directly applicable to these tag programs. In general, the radar responsive tag designs have emphasized low power, small package size, and the ability to be detected by the radar at long ranges. Recently, there has been an interest in using radar responsive tags for Blue Force tracking and Combat ID (CID). The main reason for this interest is to allow airborne surveillance radars to easily distinguish U.S. assets from those of opposing forces. A Blue Force tracking capability would add materially to situational awareness. Combat ID is also an issue, as evidenced by the fact that approximately one-quarter of all U.S. casualties in the Gulf War took the form of ground troops killed by friendly fire. Because the evolution of warfare in the intervening decade has made asymmetric warfare the norm rather than the exception, swarming engagements in which U.S. forces will be freely intermixed with opposing forces is a situation that must be anticipated. Increasing utilization of precision munitions can be expected to drive fires progressively closer to engaged allied troops at times when visual de-confliction is not an option. In view of these trends, it becomes increasingly important that U.S. ground forces have a widely proliferated all-weather radar responsive tag that communicates to all-weather surveillance. The purpose of this paper is to provide an overview of the recent, current, and future radar responsive research and development activities at Sandia National Laboratories that support both the Blue Force Tracking and Combat ID application. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Departments of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

A significant loss factor for Synthetic Aperture Radar (SAR) is the atmosphere, particularly under conditions of adverse weather. Dominating these losses are 1) atmospheric clear-air attenuation due to a) oxygen absorption, and b) water vapor absorption, 2) cloud liquid water absorption, and 3) attenuation due to rain. These losses are very nonlinear with respect to radar operating frequency, and their relative significance obviously depends on the nature of the weather being modeled. Attenuation is also very operating geometry dependent, and typically increases with range and shallower depression angles. The net effect is that while airborne radar operation at short ranges often favors higher frequencies owing to higher antenna gain for a given real aperture antenna, radar operation at longer ranges often favors lower frequencies in spite of this. In fact, at any particular range, an optimal SAR operating frequency that maximizes SNR exists for a given weather condition, transmitter power, and antenna aperture area. This paper discusses loss rates due to various loss contributors, and then quantifies expected atmospheric loss rates and their frequency dependence for typical adverse weather specifications for SAR systems. Furthermore, this paper describes airborne SAR operating geometries for which various radar frequency bands are optimal.

Sandia National Laboratories designs and builds Synthetic Aperture Radar (SAR) systems capable of forming high-quality exceptionally fine resolution images. Resolutions as fine as 4 inches (10 cm) in both slant range and azimuth are routinely formed in real time on board Sandia’s DeHavilland DHC-6 Twin Otter aircraft using a Ku-band SAR. Resolutions as fine as 6 inches (15 cm) in both slant range and azimuth are routinely formed using an X-band SAR. Careful system design allows high image quality as measured by nearly ideal Impulse Response (IPR) shapes, with typical Multiplicative Noise Ratios (MNR) of better than 20 dB, and a noise equivalent reflectivity usually better than -30 dB. Collection geometries routinely include squint angles 45 degrees both fore and aft of broadside, on either side of the aircraft. This paper offers a collection of high quality images representative of the output of Sandia’s testbed radar. High-quality fine-resolution images of a variety of target scenes will be displayed, with annotation describing relevant image parameters.

A real-time airborne synthetic aperture radar requires accurate measurement of the antenna at high and low frequencies to produce well-focused images with minimal target location errors. Sandia National Laboratories achieves accurate motion measurement by updating a navigation solution derived from an inertial measurement unit with global positioning system measurements via a unique Kalman filter. An overview of the motion measurement design and implementation is presented for the Lynx and Rapid Terrain Visualization radars, as well as results from a variety of experiments.

Specification, verification, and maintenance of image quality over the lifecycle of an operational airborne SAR begin with the specification for the system itself. Verification of image quality-oriented specification compliance can be enhanced by including a specification requirement that a vendor provide appropriate imagery at the various phases of the system life cycle. The nature and content of the imagery appropriate for each stage of the process depends on the nature of the test, the economics of collection, and the availability of techniques to extract the desired information from the data. At the earliest lifecycle stages, Concept and Technology Development (CTD) and System Development and Demonstration (SDD), the test set could include simulated imagery to demonstrate the mathematical and engineering concepts being implemented thus allowing demonstration of compliance, in part, through simulation. For Initial Operational Test and Evaluation (IOT&E), imagery collected from precisely instrumented test ranges and targets of opportunity consisting of a priori or a posteriori ground-truthed cultural and natural features are of value to the analysis of product quality compliance. Regular monitoring of image quality is possible using operational imagery and automated metrics; more precise measurements can be performed with imagery of instrumented scenes, when available. A survey of image quality measurement techniques is presented along with a discussion of the challenges of managing an airborne SAR program with the scarce resources of time, money, and ground-truthed data. Recommendations are provided that should allow an improvement in the product quality specification and maintenance process with a minimal increase in resource demands on the customer, the vendor, the operational personnel, and the asset itself.

Classical work in the field of high-resolution radar often assumes that an echo signal is made of a number of components that can be decomposed via Fourier analysis. Adjacent components are said to be resolved in the frequency domain if the intensity between them drops at least 3 decibels. This working definition is an extension of Lord Rayleigh's criterion for optical resolution. The problem with this approach is that whereas Rayleigh's criterion assumes signal incoherence, thus allowing for the addition of power components, a high-resolution radar signal is often the coherent sum of sinusoids, which implies voltage addition. The purpose of this paper is to discuss the consequences of using Rayleigh's criterion in the analysis of radar signals. Specifically, computer simulations using a complex signal are analyzed via the periodogram as the relative phase between the two components of the signal is allowed to change. The net effect introduced by this phase variation is to reduce or increase the spacing and intensity between two adjacent spectral peaks. These changes are due to constructive or destructive interference of spectral cross terms that cannot be ignored when attempting to resolve frequency components from one another. For instance, the simulations show that when using the averaged periodogram, the intensity in-between two adjacent components is above the -3 decibel threshold for a phase range of 1.2π radians, although the standard resolution criterion of c/2β is satisfied. Similar results are obtained when using a number of windows that are known to control sidelobe levels. Thus, the use of Rayleigh's criterion to define the resolution of a high-resolution radar system is technically inconsistent and undermines our ability to perform quantitative comparisons of target profiles, Doppler profiles and range-Doppler images. In this light, the authors promote the adoption of alternative criteria for judging resolution gains based on the norm of the signal in the (spatial) frequency domain.

Ground moving target indication (GMTI) radars can detect slow-moving targets if their velocities are high enough to produce distinguishable Doppler frequencies. However, no reliable technique is currently available to detect targets that fall below the minimum detectable velocity (MDV) of GMTI radars. In synthetic aperture radar (SAR) images, detection of moving targets is difficult because of target smear due to motion, which could make low-RCS targets fall below stationary ground clutter. Several techniques for SAR imaging of moving targets have been discussed in the literature. These techniques require sufficient signal-to-clutter ratio (SCR) and adequate MDV for pre-detection. Other techniques require complex changes in hardware. Extracting the maximum information from SAR image data is possible using adaptive, model-based approaches. However, these approaches lead to computational complexity, which exceeds current processing power for more than a single object in an image. This combinatorial complexity is due to the need for having to consider a large number of combinations between multiple target models and the data, while estimating unknown parameters of the target models. We are developing a technique for detecting slow-moving targets in SAR images with low signal-to-clutter ratio, without minimal velocity requirements, and without combinatorial complexity. This paper briefly summarizes the difficulties related to current model-based detection algorithms. A new concept, dynamic logic, is introduced along with an algorithm suitable for the detection of very slow-moving targets in SAR images. This new mathematical technique is inspired by the analysis of biological systems, like the human brain, which combines conceptual understanding with emotional evaluation and overcomes the combinatorial complexity of model-based techniques.

The problem of modulated noise first arose in Lord Rayleigh's investigations of acoustical backscatter off of rough sea surfaces. The same problem occurs in radar when the electromagnetic waveform takes an indirect return or transmit path to a scatterer (target) and then is received as a noise corrupted signal at the radar receiver. The effect is to produce modulated noise on the return signal. While most texts have a tendency to model the effect of noise as purely additive, it is more properly modeled as modulated noise. Recent work by the authors allow one to statistically characterize the effect of modulated noise on the received signal. This has some implications for phenomenology on being able to characterize radar backscatter from land, sea, weather as well as the implications for improved signal processing.

Scan-MUSIC algorithm, developed by the U.S. Army Research Laboratory (ARL), improves angular resolution for target detection with the use of a single rotatable radar scanning the angular region of interest. This algorithm has been adapted and extended from the MUSIC algorithm that has been used for a linear sensor array. Previously, it was shown that the SMUSIC algorithm and a Millimeter Wave radar can be used to resolve two closely spaced point targets that exhibited constructive interference, but not for the targets that exhibited destructive interference. Therefore, there were some limitations of the algorithm for the point targets. In this paper, the SMUSIC algorithm is applied to a problem of resolving real complex scatterer-type targets, which is more useful and of greater practical interest, particular for the future Army radar system. The paper presents results of the angular resolution of the targets, an M60 tank and an M113 Armored Personnel Carrier (APC), that are within the mainlobe of a Κ_α-band radar antenna. In particular, we applied the algorithm to resolve centroids of the targets that were placed within the beamwidth of the antenna. The collected coherent data using the stepped-frequency radar were compute magnitudely for the SMUSIC calculation. Even though there were significantly different signal returns for different orientations and offsets of the two targets, we resolved those two target centroids when they were as close as about 1/3 of the antenna beamwidth.

A COTS-based design for a monostatic position-adaptive radar concept is presented. The development and design effort is focused on a test experiment where a onboard radar-based instrumentation system allows a mini-UAV helicopter to hover back and forth in front of two large (side-by-side) “building-type” structures. Under this concept, the “smart” or “robotic” mini-UAV helicopter “position-adaptively” converges to a location between the two “building-type” structures in order to interrogate an object-of-interest that may be located between these “building-type” structures. Design issues with regard to major sub-systems and interfaces between these sub-systems are discussed. Applications for this type of system include intelligence gathering from indoor and outdoor urban environments and underground facilities via deployment a tier of position-adaptive mini-UAV’s.

As the Army moves toward more lightly armored Future Combat System (FCS) vehicles, enemy personnel will present an increasing threat to U.S. soldiers. In particular, they face a very real threat from adversaries using shoulder-launched, rocket propelled grenade (RPG). The Army Research Laboratory has utilized its Aberdeen Proving Ground (APG) turntable facility to collect very high resolution, fully polarimetric Ka band radar data at low depression angles of a man holding an RPG. In this paper, we examine the resulting low resolution and high resolution range profiles; and based on the observed radar cross section (RCS) value, we attempt to determine the utility of Ka band radar for detecting enemy personnel carrying RPG launchers.

A position-adaptive radar system concept is presented for purposes of interrogating difficult and obscured targets via the application of low-altitude smart or robotic-type UAV platforms. Under this concept, a high-altitude radiating platform is denoted as a HUAV and a low-altitude “position-adaptive” platform is denoted as a LUAV. The system concept is described by two modes. In Mode-1, real-time onboard LUAV computation of a phase parameter denoted as “signal differential path length” allows the LUAV to position-adaptively isolate a “signal leakage point”, for example, between two buildings. After the LUAV position-adaptively converges to an optimum location, the system enters Mode-2. Under this Mode-2 concept, a technique denoted as “exploitation of leakage signals via path trajectory diversity” (E-LS-PTD) is developed. This technique is based on modulating scattering centers on embedded objects by implementing a fast trajectory on the HUAV while the LUAV is hovering in front of an “obscuration channel.” Analytical results include sample outputs from an initial set numerical electromagnetic simulations.

The operations of tractor-trailers, mixed with other vehicles on US interstates, has resulted in a number of major tractor-trailer overturn events which often cause traffic to stop for hours along a major traffic corridor because the wreckage physically blocks the corridor. In some cases, the blockage remains for a day or more if toxic substances are being carried by the tractor-trailer. Mixed vehicle use of the interstates coupled with the fact that some tractor-trailers with high center of gravity loads are driven too fast ensure that the overturn events experienced to date will continue. In a previous SPIE paper, the Georgia Tech Research Institute researchers demonstrated that an X-band radar (10.5 GHz) could detect the oscillation frequency of tractor-trailer trucks within 1,000 feet of the radar. This radar was used to control a vibration damper on a bridge to provide longer bridge life. Previous studies conducted by GTRI show that tractor-trailer trucks reflect several orders of magnitude more energy than passenger vehicles, vans and other smaller trucks. In many cases, the large tractor-trailer truck radar signature alone could be used to identify the trucks from the other traffic. However, since non-ranging homodyne radar was being used, it was not known if smaller radar cross section targets that were closer to the radar than the tractor-trailer trucks would have a larger radar signature. This paper examines the issues associated with identifying tractor-trailer truck radar signatures from the radar signatures of other vehicles within the antenna beam of a non-ranging homodyne radar.

It is becoming more important for the designer of radar (and other military sensing) systems to be able to provide military commanders and procurement decision makers with a concept of how a new system can enhance warfighting capability. Showing enhanced sensor performance is no longer sufficient to sell a new system. In order to better understand issues relating to sensor employment, we develop a top-level functional architecture of the kill chain for Air-to-Ground targeting. A companion paper constructs an executable model in the form of a Colored Petri Net (CPN) from the architecture. The focus on architecture that we present here aligns well with the new Department of Defense guidance, which requires new acquisition programs to be structured around system architectures. This should provide a common reference system for communication among warfighters, planners, and technologists. The translation to an executable model should allow identification of technology insertion points.

Air-to-Ground targeting in a kill chain requires timely information flow and coordination of a large number of complex and disparate systems and events that are distributed in space and time. Modeling and simulation of such complex systems poses a considerable challenge to the system developers. Colored Petri Nets (CPN) provide a well-established graphically-oriented simulation tool with the capability to incorporate design and performance specifications and operational requirements of complex discrete-event systems for verification and validation under a variety of input stimuli. In this paper, we present the results of a preliminary study of implementing rudimentary, yet realistic, kill chain modules for Air-to-Ground combat using CPN tools. We have developed top-level functional kill chain modules incorporating its primary functions. It is expected that the modularity of the CPN-based simulation framework will enable us to incorporate further details and breadth in system complexities in order to study real world kill-chain simulation environment as an integral part of DOD's C4ISR architecture.

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.

Land spatial statistical characteristics obtained experimentally and from the topographical maps are analyzed. It is shown that the probability density function of backscattered signal is determined by the temporal fluctuations and the spatial fluctuations because of scattered areas replacement. The experimental spatial statistical characteristics of micro-scale roughness (rms height, the slope distributions, and autocorrelation functions) are presented. It was shown that for mathematical description of surface for different fields it is better to use the fractal process. The spatial statistics of land backscattering for surfaces without vegetation is determined by the distributions of surface local slopes. The roughness height and slope distributions for different terrain types are presented that are obtained from electronic topographical maps. The method of clutter map development is analyzed, it is shown its great dependence on radar and target heights.

Recently considerable efforts are put in development of Ground Penetrating Radar (GPR) systems for detection and identification of the buried artifacts and structures. The GPR performance is associated with the properties of the local soil and buried targets as well as the implementation of its hardware and software. When the aim in conventional GPR systems is detection of objects, some applications require identification of objects. It is necessary a new generation of GPR hardware for the identification of objects. This new hardware should perform highly accurate measurements of the scattered field. In this paper, the development of GPR system for the identification of buried small objects is presented. System covers broad bandwidth more than 2 GHz in order to achieve sufficient down-range resolution. GPR is operated lower frequencies because of technological restrictions of pulse generator. The performance of GPR for buried object detection and identification depends significantly on the ability of the antenna to radiate impulses into the ground without distortion. GPR antennas are operated close to the ground for efficiently coupling the energy into the ground. However this causes antenna characteristics to change as the ground condition changes. This characteristics instability makes it difficult to reduce the antenna clutter in the post processing. A novel broad band GPR antenna was developed over a wide frequency range to improve the stability. This new design was improved from dielectric loaded horn antenna.

Sandia National Laboratories designs and builds Synthetic Aperture Radar (SAR) systems capable of forming high-quality exceptionally fine resolution real-time images. Resolutions as fine as 4 inches (10 cm) in both slant range and azimuth are routinely formed in real time on board Sandia’s DeHavilland DHC-6 Twin Otter aircraft using a Ku-band SAR. Resolutions as fine as 6 inches (15 cm) in both slant range and azimuth are routinely formed using an X-band SAR. Careful system design allows high image quality as measured by nearly ideal Impulse Response (IPR) shapes, with typical Multiplicative Noise Ratios (MNR) of better than 20 dB, and a noise equivalent reflectivity usually better than -30 dB. Collection geometries routinely include squint angles 45 degrees both fore and aft of broadside, on either side of the aircraft. This paper offers a collection of high quality images representative of the output of Sandia’s testbed radar. High-quality fine-resolution images of a variety of target scenes will be displayed, with annotation describing relevant image parameters. This paper is the second of a set of two portfolios.

Passive indoors imaging of weapons concealed under clothing poses a formidable challenge for millimeter-wave imagers due to the sub-picowatt signal levels present in the scene. Moreover, video-rate imaging requires a large number of pixels, which leads to a very complex and expensive front end for the imager. To meet the concealed weapons detection challenge, our approach uses a low cost pulsed-noise source as an illuminator and an array of room-temperature antenna-coupled microbolometers as the detectors. The reflected millimeter-wave power is detected by the bolometers, gated, integrated and amplified by audio-frequency amplifiers, and after digitization, displayed in real time on a PC display. We present recently acquired videos obtained with the 120-element array, and comprehensively describe the performance characteristics of the array in terms of sensitivity, optical efficiency, uniformity and spatial resolution. Our results show that active imaging with antenna-coupled microbolometers can yield imagery comparable to that obtained with systems using MMIC amplifiers but with a cost per pixel that is orders of magnitude lower.

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.

This paper describes a high performance opto-mechanically scanned mm-wave imager intended to monitor the ground movement of aircraft in adverse weather conditions. It employs two counter-rotating mirrors that are tilted about their axes of rotation. They simulate the linear scan of a single high speed, large aperture flapping mirror. When used with a linear receiver array they can produce a two-dimensional scan of the scene at TV rates. In the present application they were used with a single receiver and a large flapping mirror to produce a two-dimensional scan of the scene of ±10° vertically and 60deg; horizontally. One of the rotating mirrors had a concave surface and acted as the focusing element in the imager. The two mirrors were driven from a single servo motor using timing belts and toothed pulleys. The flapping mirror was slaved to the motion of the rotating discs using an electronic cam. The single channel 94GHz receiver consisted of an InP LNA followed by a down-converter and a detector. The video output passed to an A/D converter and was displayed on a conventional PC. This system has virtually 100% transmission and can be used at any waveband.

A mm-wave imager has been developed for medical applications and will be reported here. It uses a single-channel mechanically scanned heterodyne radiometer and diffraction-limited focussing optics to achieve high resolution images of subcutaneous body temperature. The particular design constraints imposed by close-range operation (tens of centimetres) will be discussed. The instrument uses a novel, patented method for regular calibration of both the thermal and spatial response of the imager. Results obtained from healthy volunteers will also be presented.

It has been demonstrated that passive MMW imagers can be used to detect obstacles through the fog, such as treelines and hillsides, which might be encountered in the path of a low-flying aircraft. However, the brightness temperature contrast between the horizon sky and the obstacle can often be quite small in foggy conditions, on the order of 5 K or less. Reliable detection of this contrast without image processing requires a passive MMW imager with a Δ-T_min of about 0.2 K, which is quite challenging for existing 30-Hz imagers. While improvements in passive MMW imagers continue, it is useful to look at image analysis techniques that have the potential to improve obstacle detection by increasing the amount of information extracted from each image frame. In this paper we look at the ways that texture can be used to extract more information from the imagery. By merging textural information with the brightness temperature contrast information, there is the potential to enhance the detection of objects within the scene. The data used for the analysis presented here is 93-GHz, passive imagery of a deciduous treeline scene and a concrete building scene. The data were taken from the roof of a 4-story building to simulate the view of a low-flying aircraft. The data were collected over many months with an ARL-built Stokes-vector radiometer. This radiometer is a single-beam system that raster scans over a scene to collect a calibrated 93-GHz image. Texture measurement results for image segment samples, including autocorrelation and spatial edgeness, are presented in this work. Also presented are the effects of applying a modified Sobel edge detection technique to imagery with the least detectable obstacles.

An antenna-coupled metal-oxide-metal (MOM) diode for dual-band Infrared (IR)-millimeter wave (MMW) detection is presented. Electron-beam lithography and conventional sputtering techniques were used to fabricate a Ni-NiO-Ni diode coupled to an Infrared slot antenna at 28 THz and a coplanar waveguide (CPW)-fed MMW twin slot antenna at 94 GHz; simultaneous dual-band detection was tested and verified.

A 35GHz imager designed for Security Scanning has been previously demonstrated. That imager was based on a folded conical scan technology and was constructed from low cost materials such as expanded polystyrene and printed circuit board. In conjunction with an illumination chamber it was used to collect indoor imagery of people with weapons and contraband hidden under their clothing. That imager had a spot size of 20mm and covered a field of view of 20 x 10 degrees that partially covered the body of an adult from knees to shoulders. A new variant of this imager has been designed and constructed. It has a field of view of 36 x 18 degrees and is capable of covering the whole body of an adult. This was achieved by increasing the number of direct detection receivers from the 32 used in the previous design to 58, and by implementing an improved optical design. The optics consist of a front grid, a polarisation device which converts linear to circular polarisation and a rotating scanner. This new design uses high-density expanded polystyrene as a correcting element on the back of the front grid. This gives an added degree of freedom that allows the optical design to be diffraction limited over a very wide field of view. Obscuration by the receivers and associated components is minimised by integrating the post detection electronics at the receiver array.

Trex Enterprises has developed a second-generation passive millimeter-wave imaging system for detection of concealed weapons and explosives at standoff ranges. Passive millimeter-wave sensors form an image from naturally emitted blackbody radiation in the millimeter-wave portion of the electromagnetic spectrum. Radiation at this wavelength passes through most types of clothing, allowing the user to acquire an image of any articles on a suspect’s person that differ significantly from the human body in their reflectivity or radiometric temperature at millimeter-wave wavelengths. Trex Enterprises previously demonstrated a first-generation concealed weapon detection system with the ability to detect handguns and knives under heavy clothing at a range of 27’. The second-generation imager, while similar in concept, has an improved field-of-view and a much reduced size and weight. The imager is to be put through a battery of tests by both Trex Enterprises and the National Institute Of Justice to determine its ability to detect both metallic and non-metallic knives and handguns as well as various types of explosive devices. The tests will be conducted indoors and outdoors at various ranges.

Los Alamos National Laboratory (LANL) and AeroAstro have recently investigated the feasibility of space-based passive interferometric millimeter wave imaging (PIMI). The goal of this study is to explore a new capability that can offer day/night, all-weather, passive imaging with a 1-meter resolution, by means of millimetric interferometry via a small constellation of microsatellites. According to our preliminary study, a system with four LEO satellites operating at multiple frequency channels within 95-150 GHz is capable of providing an imagery of 1-m spatial resolution. The corresponding temperature sensitivity is estimated to be ~20°K, enough to distinguish most artifacts from a variety of backgrounds. To achieve the stated resolution and sensitivity with only four satellites, we make use of ten frequency channels to synthesize ten effective baselines between any pair of satellites. In addition, the satellites will “stare” at a common target area off the track direction for about 2 minutes while they pass over the area. This type of observation will introduce much improved spatial frequency coverage due to the relative rotation of the baseline vectors. It also improves the imagery SNR with a longer viewing time, as compared to a downward looking system. To the target, the side-looking observation also has the advantage of near constant incident (zenith) angle. The satellites are required to perform a formation flight but a rigid formation is not necessary. Simultaneous interferometric measurement of GPS signals, together with inter-satellites ranging will allow us to monitor the baseline length and direction to an adequate accuracy. A tradeoff study has also been conducted between the system performance and the technology availability, i.e., the current state-of-the-art technologies for space-borne antenna, millimeter-wave receiver, high-speed digitizer, inter-satellites data communication, and so forth.

Passive millimeter-wave imaging has excellent all weather capability but requires large apertures to give adequate spatial resolution. Linear restoration can enhance the resolution by a factor of two, while under favorable conditions non-linear restoration can enhance it by a factor of four. The amount of enhancement possible is generally limited by the amount of noise present in the original observed image. Preprocessing can reduce the effect of this noise and the image may be selectively restored. The high spatial frequency content of an image exists largely at edges and sharp features and these may be restored using non-linear restoration techniques. The smoother background between these features contains fewer high frequencies and needs less restoration. Adaptive non-linear restoration techniques have been investigated whereby the amount of restoration applied to an image is a function of the first and second derivative of the image intensity. In many non-linear restoration techniques the amount of high spatial frequency content introduced into the restored image is uncontrolled. This problem has been overcome through the use of the Lorentzian algorithm, which imposes a statistical constraint on the distribution of gradients within the restored image. Recently attempts have been made to explain why the distribution of gradients within an image is Lorentzian in terms of randomly distributed gradients of random size. Images are presented to demonstrate the effectiveness of these methods.

The spatial resolution of millimeter wave (MMW), submillimeter wave (SMMW) and infrared (IR) quasi-optical imagers based on the usage of focal plane antenna arrays (FPAA) is limited by several common factors. Analytical expressions for the point spread functions (PSF) of FPAA imagers are derived for both coherent and spatially incoherent imaging. The possibility of developing advanced super-resolution imaging algorithms using PSFs determined to a high accuracy is discussed.

In this paper we present the overview of problems and difficulties, which are common in passive radio vision. The central part of the article is dedicated to mathematical resolution enhancement methods - superresolution. We consider several algorithms and discuss benefits and drawbacks they have. Performance of the algorithms is compared using a set of test images - both simulated and real-life. The last ones are taken using 3-mm and 8-mm passive radio vision systems. Such advanced questions as subpixeling technique and artifact suppression using the wavelet transform are also reflected in the paper. The illustrations are included in order to demonstrate significant improvement of the resolution along with artifact suppression achieved on real-life observed images.

The antenna scanning modes and the image displaying methods used in a millimeter wave radiometric imaging system have great influence on its imaging quality. In order to obtain high quality millimeter wave radiometric images, intensive studies are needed. This paper describes the antenna scanning mode and the image displaying method of 8mm Ground Radiometric Imaging System (GRIS) and compares two different image displaying methods. The experimental results prove the rotating whole antenna concial scanning mode and unequal size element arc displaying method fits GRIS better.

This paper describes the general requirements and an approach to scene simulation in millimetre wave radiometric imaging that is based on multi faceted semitransparent layered media in the earth’s three-dimensional geometry. The driving attributes in this field are essentially the transparency of clothing for security scanning and the transparency of fog, cloud, rain and dust for all weather flight. Out-door illumination and the physics of the interaction of millimetre waves with the atmosphere and obscurants are discussed, together with the interaction of millimetre waves with multi layer material surfaces, giving rise to transmission, reflection and emission. The physics of these interactions are discussed in the context of computer graphics. These considerations enable a powerful polarimetric modelling capability to be developed that can be used to simulate all scenarios, including artificial or burst illumination, from in-doors to imaging from satellites.