Passive millimetre-wave imaging is a potential solution to surveillance problems in the presence of obscurants. Research has shown that passive millimetre-wave imaging can form high contrast, natural imagery even through foul weather. The application of passive acquisition methods on a UAV is important for strategical and tactical reasons. In mountainous regions or in highly urbanized areas an almost Nadir looking imaging scheme for the data acquisition is beneficial. In order to achieve sufficient ground resolution from a few kilometers altitude, the use of synthetic methods is proposed. The method of aperture synthesis for high resolution passive imaging is discussed, and a simulated system design example using the UAV platform characteristics is illustrated. Since the aperture size even for W band is in the order of several meters, the imaging performance is influenced by many parameters, which are otherwise negligible. Within this context the feasibility of such a system on a UAV platform is discussed.

The need to develop passive millimetre wave imaging systems which have better spatial resolution and sensitivity, to offer powerful discrimination capabilities, is great. On the other hand, many deployment scenarios dictate the system footprint to be smaller than that of existing imagers. This dilemma may be resolved by replacing the bulky quasi-optics of existing imaging systems with two dimensional antenna arrays and electronic processors. This paper discusses initial design work for a demonstrator of an all electronic passive millimetre wave imager. This approach to the all electronic imaging system, endeavours to create ultimately an 'imager skin', which has essentially a two dimensional architecture, the minimum requirement for the collection of millimetre wave radiation for imaging. The physics of the imaging process and possible routes to electronic imaging are discussed. Possible arrays are investigated and a point spread function of the demonstrator imager is modelled. The results of using this imager system for a security scanning application are presented and conclusions about relevant technologies are given.

This paper describes a new opto-mechanical scanner that is able to meet the established requirements for mm-wave imaging in remote sensing and security applications. The ideal system would employ a 2-D array of electronically scanned receivers, but at present their cost is prohibitively high. Fortunately low cost, high performance, opto-mechanically scanned imagers are able to meet the current requirements. They can establish the market and pave the way for lower-cost receiver developments, necessary for electronic scanning. An opto-mechanical scanner should be able to scan a 2-D image of the scene in real-time, with a linear raster scan pattern. It should have high optical efficiency so that an imager can achieve the required thermal sensitivity with the minimum number of receivers. It should be compact to fit inside a small space envelope. The size of the imager should be dominated by the size of the collection aperture and not by any relay optics. In mm-wave imaging this size is controlled by the required spatial resolution and the space available. It is also desirable that the scanner employs the minimum number of frequency-selective optical components. This ensures that it can easily operate at any wavelength, and be active or passive. The new scanning arrangement meets these requirements and is being developed into a high performance, low-cost, compact prototype system that hopefully will meet the present and future needs for mm-wave and terahertz imaging.

A real-time passive millimeter-wave imaging system with a wide-field of view and 3K temperature sensitivity is described. The system was flown on a UH-1H helicopter in a flight test conducted by the U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate (NVESD). We collected approximately eight hours of data over the course of the two-week flight test. Flight data was collected in horizontal and vertical polarizations at look down angles from 0 to 40 degrees. Speeds varied from 0 to 90 knots and altitudes varied from 0' to 1000'. Targets imaged include roads, freeways, railroads, houses, industrial buildings, power plants, people, streams, rivers, bridges, cars, trucks, trains, boats, planes, runways, treelines, shorelines, and the horizon. The imaging system withstood vibration and temperature variations, but experienced some RF interference. The flight test demonstrated the system's capabilities as an airborne navigation and surveillance aid. It also performed in a personnel recovery scenario.

In this paper we present SPIRA (Scanning Polarimetric Imaging Radiometer), a fully polarimetric and transportable imager at 91 GHz that is being developed at IAP. It will be used for measuring signatures of the Earth surface in all four Stokes parameters and can be transported and put into an imaging-ready status by two persons within hours. The mechanical imager scans in azimuth and elevation whereby an image of 120×120 pixels (field of view 30°× 30°) is acquired within less than 600 s. The instrument is controlled by a PC under Linux OS, achieving a nearly real-time performance by using a specially designed kernel module. The brightness temperature distribution of a scene is sampled with a 90° offset antenna at an angular resolution of 0.5°. The polarized signal is separated by an orthomode transducer into two orthogonal components and then down converted to an intermediate frequency band between 2 and 4 GHz by two sub-harmonic mixers that are pumped by a common local oscillator. The four Stokes parameters are extracted with an analog broadband correlator, consisting of a hybrid network and quadratic detectors. Raw images in the four Stokes parameters are immediately displayed and stored for postprocessing. Calibration is performed by adding noise from a switchable noise diode in each channel and by an external load at ambient temperature. Polarimetric calibration of the whole system is done by using coherent and thermal polarization generators.

We describe a holographic or "phaseless" technique for measurement of complex antenna patterns that is particularly suited to planar antennas at very high frequencies (mm-wave thru long-wave infrared) where phase coherent detection is impractical. It has been implemented in a compact automated antenna range, and validated on a previously reported planar slot-ring antenna at 95 GHz. In the IR, the interferometric technique must be refined in order to reduce to acceptable levels the uncertainties due to positioning and alignment errors. This measurement of the complex antenna pattern is the most problematic part of a full accounting for the optical coupling efficiency "budget" in a microantenna-coupled, imaging sensor array.

Typical radio frequency style millimeter-wave detectors for passive millimeter-wave imaging are limited by the thermal noise floor inherent in microwave and millimeter-wave circuits. This thermal noise, or Johnson noise, can be derived from Rayleigh-Jeans approximation of Planck's blackbody radiation law and is a fundamental limit to the noise levels in such systems. For this reason low-noise amplifiers are typically used at the front end of such high sensitivity receivers to improve their noise equivalent difference temperature. However, such approaches are undesirable in high-noise environments where high signal levels could damage such amplifiers or in large pixel count arrays where the cost of amplification of each pixel becomes prohibitive. Herein, we present an alternate approach to achieving low difference temperatures, which involves upconversion to optical frequencies with subsequent filtering and square-law detection. Through this process the normal thermal noise floor limitations are not relevant as, at such high frequencies, the Rayleigh-Jean’s approximation is no longer valid. In fact, the thermal noise present in the optical regime is diminished to negligible levels at room temperatures and quantum noise becomes the fundamental limit to system performance. In this paper, we present the theoretical and practical limits to reduction of noise equivalent power using optical amplifiers and relate the noise contributions of such amplifiers to the quantum noise limit.

In this paper we presented the modeling, design of a millimeter wave (MMW) photonic modulator operating at high Giga-Hertz frequency, i.e. 94GHz. Our approach combines high quality factors (Qs) photonic microdisks with nonlinear properties of electro-optic (EO) material at the present of applied millimeter wave to achieve optical modulation. The high Q values and strong field densities associated with WGMs in microdisk resonators result in the enhancement of the nonlinear interaction of optical fields with a material that exhibits coherent attributes, even with weak millimeter waves. The coupled mode theory (CMT) is applied to incorporate the nonlinearity of EO material in the optical analysis. In the simulations, two EO materials, LiNBO3 and GaAs, are considered and designed to realize the MMW photonic modulation. In addition, carefully design and analysis is taken by using rigorous electromagnetic (EM) algorithm to accurately determine EM field in the MMW resonator, which will provide maximum modulation.

We present an optical approach to 1-D broadband microwave imaging. The imager uses a Fourier optical beamformer to generate a squinted broadband image which is then spectrally resolved by burning a spatial distribution (an image) of spectral signals into a spectral-hole burning material. This spatial-spectral image corresponds to the spectral content of the image at each resolveable spatial point. These narrowband images may be sequentially read out with a chirped laser, scaled to compensate for beam squint, and summed to form a broadband microwave image.

We present the first mm-wave characterization of Semimetal Semiconductor Schottky (S3) diodes for direct detector applications from 94 GHz to 30 THz. The S3 devices use molecular-beam epitaxy growth of binary compounds that are closely lattice-matched and crystallographically perfect across the heterointerface to reduce 1/f and burst noise while maintaining ultra-high-frequency performance. The S3 diodes are fabricated from an InAlGaAs/InP based material system with both the Schottky layer and contact layer having n and n+ doping levels. The semimetal Schottky contact is ErAs which is grown in-situ during the MBE growth. By varying the InAlAs percentage content in the epitaxial layer structure, the diode dc I-V characteristics and its zero bias responsivity are optimized. Diode s-parameter data from dc-100 GHz is used to determine the diode responsivity as a function of frequency and diode capacitance and resistance. These measurements then allow the device intrinsic and extrinsic equivalent-circuit elements to be optimized for direct detection from 94 GHz to ~30 THz.

The figure of merit for RF detectors, noise-equivalent power (NEP), is determined by the noise divided by the sensitivity. Thus, the challenge is to design a diode structure that has low junction resistance while maintaining a large nonlinearity. This work presents sensitivity and noise measurements for Sb-heterostructure backward diodes with varying barrier thicknesses and cross-sectional areas. Nominal diode areas are 2x2mm² and 3x4mm² with 15Å and 20Å barriers. The best NEP measured to date is 1.19 pW/rtHz at 36.5 GHz.

In this paper, we report on our investigations of novel imaging techniques such as holography, the generation of limited diffraction beams with large depths of focus and the use of binary optics for millimeter wave systems. Holography, widely used at visible wavelengths is simulated and tested in a simple optical sep-up at 100 GHz using an off-axis lensless configuration. Such a technique can be used to measure absorption characteristics of materials, and can also help classify radiating horns and lens antennas. Gaussian Beam Mode Analysis is used as an efficient computational technique to investigate the propagation of non-diffracting beams, and in particular, Bessel beams, at millimeter wavelengths. Because of the limited throughput of millimeter-wave systems, due to the long wavelength and the need for compact optics for practical applications, modal analysis is a very computationally efficient means for computing propagation characteristics. Typically, the axicon, or conical lens, is the most common optical component used for the generation of such zeroth order Bessel beams, but we show that holographic simulation can be used to design binary holograms for the generation of higher order non-diffracting beams. Furthermore, we describe a practical design for such a simple alternative to the axicon through the manufacture of a binary analogue of this component, which successfully produces diffraction invariant beams.

The ability of millimetre-waves (mm-wave) to penetrate obscurants, be they clothing, fog etc., enables unique imaging applications in areas such as security screening of personnel and landing aids for aircraft. When used in an outdoor application, the natural thermal contrast provided by cold sky reflections off of objects allow for direct imaging of a scene. Imaging at mm-wave frequencies in an indoor situation requires that a thermal contrast be generated in order to illuminate and detect objects of interest. In the case of a portal screening application the illumination needs to be provided over the imaged area in a uniform, omni-directional manner and at a sufficient level of contrast to achieve the desired signal to noise ratio at the sensor. The primary options are to generate this contrast by using active noise sources or to develop a passive thermally induced source of mm-wave energy. This paper describes the approaches taken to developing and implementing an indoor imaging configuration for a mm-wave camera that is to be used in people screening applications. The camera uses a patented mechanical scanning method to directly generate a raster frame image of portal dimensions. Imaging has been conducted at a range of frequencies with the main focus being on 94GHz operation. Experiences with both active and passive illumination schemes are described with conclusions on the merits or otherwise of each. The results of imaging trials demonstrate the potential for using mm-wave imaging in an indoor situation and example images illustrate the capability of the camera and the illumination methods when used for personnel screening.

We describe a low-cost passive millimeter wave (MMW) scanning camera for detecting concealed weapons and contraband. It is based on a focal plane array of 64 radiometric channels that employ MMICs operating at 94 GHz. Equipped with a 125 mm primary optic, the camera achieves a 26 26 degrees field of view by means of a rotating optic that performs 10 conical scans of the scene per second. The resulting 10 Hz rate images are of size 28 by 28, yielding a spatial resolution of 5 cm at a range of 1.6 meters from the camera. The radiometric sensitivity, at the maximum frame rate, is given by a median of under 3 Kelvin. With a size of 8 in. 8 in. 22 in. and a weight of 26 lbs., the camera is very compact and portable. This development may constitute the first affordable, commercially available passive MMW scanning camera. When operated at the slower frame rate of 1 Hz, the resulting time integration improves the image to less than 1 Kelvin, making the camera well suited for the detection of a wide variety of threats at security checkpoints. At finer camera sensitivity levels, the possibility arises of the exposure of anatomic details of the scanned subjects. In view of this, we have developed specialized display software that allows the presentation of the MMW scanning results in a manner that overcomes privacy concerns.

A novel polarimetric millimeter-wave imaging technique has been developed at the Pacific Northwest National Laboratory (PNNL) for concealed weapon detection applications. Wideband millimeter-wave imaging systems developed at PNNL utilize low-power, coherent, millimeter-wave illumination in the 10-100 GHz range to form high-resolution images of personnel. Electromagnetic waves in these frequency ranges easily penetrate most clothing materials and are reflected from the body and any concealed items. Three-dimensional images are formed using computer image reconstruction algorithms developed to mathematically focus the received wavefronts scattered from the target. Circular polarimetric imaging can be employed to obtain additional information from the target. Circularly polarized waves incident on relatively smooth reflecting targets are typically reversed in their rotational handedness, e.g. left-hand circular polarization (LHCP) is reflected to become right-hand circular polarization (RHCP). An incident wave that is reflected twice (or any even number) of times prior to returning to the transceiver, has its handedness preserved. Sharp features such as wires and edges tend to return linear polarization, which can be considered to be a sum of both LHCP and RHCP. These characteristics can be exploited for personnel screening by allowing differentiation of smooth features, such as the body, and sharper features present in many concealed items. Additionally, imaging artifacts due to multipath can be identified and eliminated. Laboratory imaging results have been obtained in the 10-20 GHz frequency range and are presented in this paper.

We present passive indoor imagery of human subjects in the 100 - 1000 GHz band. In order to obtain adequate sensitivity, a cryogenically cooled (4 K), broadband antenna-coupled, superconducting microbolometer with optical noise equivalent power NEP < 2 pW/rtHz was used as the sensor. Mechanical scanning of the collecting aperture, a 30 cm diameter spherical mirror, was used to slowly accumulate the images. While not yet practical for deployable real-time cameras, this system provides valuable phenomenological comparisons with similar imagery obtained with actively illuminated systems.

Image registration is a crucial part of the success of the super-resolution algorithms. In real applications, atmospheric turbulence is an important factor that brings further degradation to the low-resolution image sequence (video frames), besides other degradations such as global motion due to movement of the optical device, blurring due to the point spread function of the lens, and blurring due to the finite size of the detector array. In this paper, the degradation of the atmospheric turbulence to the low-resolution images is modeled as per-pixel motion in the high-resolution plane and is assumed to be spatially local and temporally quasi-periodic. The registration is a two-stage process: first, the global motion between frames is estimated using the phase-correlation method to remove "jitter" and stabilize the sequence; then, an optical flow method with quasi-periodic constraint is used to estimate the per-pixel motion. A threshold is used to separate the relatively larger object movement from per-pixel atmospheric turbulence. After registration, the shift map of each frame is obtained, along with a prototype of the high-resolution image. A maximum a posteriori (MAP) based super-resolution algorithm is therefore applied to reconstruct the high-resolution image. Experiments using synthetic images are conducted to verify the validity of the proposed method. Finally, conclusions are drawn.

This paper analyzes a simple low-cost scan system for concealed weapon detection (CWD) on a cooperative subject. The passive imaging system is based on a cylindrical sensing geometry, realized by mechanical vertical scan of a horizontal circle, filled with many diffraction-limited antennas surrounding the subject, over the whole body height. This system is dimensioned to scan an ideally coaxial cylindrical subject of known radius with a fixed spatial resolution. Several system parameters influence the capability of anomaly detection: horizontal spatial resolution (constrained by diffraction limitations on the sensing circle), vertical spatial resolution and radiometric sensitivity (both related to vertical scan settings). Spatial resolution calculations are carried out in function of the working frequency, and achievable resolutions according to diffraction limitations are discussed. A qualitative and quantitative study is done to determine how high radiometric sensitivity (achievable with well-established commercial components) could overcome the poor spatial resolution related to low working frequencies, in view of dielectric anomaly detection; the optimal dwell time (giving a good radiometric/spatial resolution trade-off) is evaluated. Sub-pixel resolution capabilities are briefly considered, together with a least square matching criterium. Performance of an alternative configuration, consisting of a rotating vertical array, is derived from the circular system. Finally, the data fusion from both configurations is suggested.

A poor inherent resolution capability of the passive millimeter-wave (PMMW) imaging becomes a problem in many applications. The need for efficient post-processing to achieve resolution improvement is being increasingly recognized. To obtain high- and super-resolution PMMW imaging, many restoration methods have been developed and evaluated. In this paper, two recent advanced wavelet based methods are discussed; Fourier-wavelet regularized deconvolution (ForWaRD) and multiscale entropy method. The ForWaRD is a linear deconvolution algorithm that performs noise regularization via scalar shrinkage in both the Fourier and wavelet domains. The ForWaRD has been reported to be efficient and applicable to all ill-conditioned deconvolution problems. The multiscale entropy method, which generalized the wavelet-regularized iterative methods, is advance of the maximum entropy method (MEM), which is more effective and leads to efficient restoration. These two methods have not been applied and analyzed in the PMMW images which were highly blurred and low signal to noise circumstance. We have studied the restoration performance of wavelet-based methods in the PMMW imaging comparing with particular reference to the Lorentzian method. The evaluation has been performed with actual radiometer imaging with the 94 GHz mechanically scanned radiometer as well as simulation. In the actual radiometer imaging, a simple blind restoration method was exploited with blur identification. To compare the restored image fidelity, objective and subjective criteria were used, and the super-resolution capability was also checked. Comparison of the linear and non-linear methods revealed the preferable bandwidth extension of the non-linear methods. In the non-linear methods, the multiscale entropy and Lorentzian, they showed their strength and weakness.

The problem of compensation for various distortions is solved with the aid of diminishing of PSF O distortions. The laws of distortions are presented as scalar product I(y)=(O, Ix), Ix is the undistorted image, which is localized as PSF O, I(y) is a value of the image Iy in the point x=y. The pointed algorithm of compensation for PSF O distortions is as follows. The scalar products I(z)=(R, Iy) are calculated in analogous way for all the points x=z, Ix ~ Iz, Iz is a image with the compensated distortions. The small dimensions variation problem for resolving function R is a well definite one. It is necessary to calculate all the different resolving functions Rx, if the distorted image Iy has the domains with the different distortions because of the different PSF Ox. That means, we may successfully use a pointed ultra-resolution method in all the modern multi-sensors (digital cameras) or multi-rays (microwave imaging) receiving systems. For the compensation of distortions we do not solve Fredholm equation of the first kind, our small dimension solutions for R cannot, in principle, contain the high frequency spatial oscillations. We emphasize that these high frequency oscillations in operator analogue of R are the main obstacle of the compensational methods based on solution of Fredholm equation of the first kind. Examples of applying of the pointed ultra-resolution method in microwave imaging are considered.