An experimental coherent pulsed radar operating at 225 GHz is described. This system uses a pulsed, phase-locked extended interaction oscillator (EIO) transmitter and an f/4 (frequency divided by 4) subharmonic mixer pumped by a phase-locked Gunn oscillator as the receiver. A quasi-optical circular polarization duplexer combines transmitter and receiver signals into the same antenna. Results obtained with this system include the detection of targets out to ranges of 3.5 km and observation of doppler spectra of trucks and tracked vehicles, including contributions from both body and wheels/tracks.
Implementation of a multichannel radiometer, operating on thefamily of oxygen absorptions near 60 GHz, potentially useful forhorizontal temperature sounding, is discussed. Application of thisinstrument to the detection of atmospheric hazards such as wind shearand clear air turbulence is considered.
Two imaging systems have been designed and built to function as snapshot imaging spectropolarimeters; one system
made to operate in the visible part of the spectrum, the other for the long wavelength infrared, 8 to 12 microns. The
devices are based on computed tomographic imaging channeled spectropolarimetry (CTICS), a unique technology that
allows both the spectra and the polarization state for all of the wavelength bands in the spectra to be simultaneously
recorded from every spatial position in an image with a single integration period of the imaging system. The devices
contain no moving parts and require no scanning, allowing them to acquire data without the artifacts normally associated
with scanning spectropolarimeters. Details of the two imaging systems will be presented.
This paper will describe methods of measuring all of the components of the Stokes polarization vector for each pixel in a
scene using only one frame of passive optical sensor data, one radar pulse, or one radiometer integration interval. Both
active and passive sensors operating in any waveband from microwave to visible will be considered. For systems
operating in the millimeter wave and terahertz bands, the techniques developed by Dereniak and his students at the
University of Arizona will be discussed. For other wavebands, a technique developed by the author that requires the
coherent reception of two orthogonally-polarized signal components will be presented. This latter method works for both
for both broad-band and narrow-band active or passive signals, but requires focal planes and hardware in the visible and
infrared bands that may be too complicated for many applications. Results of calculations made for the millimeter and
terahertz bands will be presented.
This paper covers the design and construction of a snapshot imaging spectropolarimeter for use in the long wave
infrared, 8 to 12 micron region. This imaging device is unique in the fact that system is nonscanning, contains no
moving parts, and in a single integration period is able to record spectral data as well as the polarization state as a
function of wavelength from every spatial location in a 2D image. The system is based on the Computed Tomographic
Imaging Spectrometer, commonly referred to as CTIS, and has been modified to incorporate components of Channeled
Spectropolarimetry. The paper presents an overview of how both the CTIS and the CTICS (Computed Tomographic
Imaging Channeled Spectropolarimeter) systems work, details on the specific components used in the LWIR system, and
preliminary results from a completed LWIR CTIS system, which is the first of its kind.
The exploitation of infrared polarimetry has been shown to yield good results when applied to target discrimination in military applications and to civilian remote sensing problems. Similarly, numerous workers have shown that imaging sensors operating in the far infrared spectral bands may be useful in such counter-terror applications as concealed weapon and biological and chemical agent detection. Unfortunately, these detection and discrimination techniques have not been exploited because of the lack of suitable sensors capable of making the necessary measurements with acceptable sensitivity. In this paper we present and discuss several methods for measuring the polarization signature of a target scene using sensors with no moving parts. We also present and analyze a far infrared imaging system based on an uncooled bolometer focal plane array. The methods of measuring polarization signature with no moving parts include a coherent in-phase and quadrature approach suitable for both broad- and narrow-band sensors, a broadband sensor using channeled spectropolarimetry, a variant of this latter method that involves correlation of the spectral signatures with those of known targets, and another variant that uses an electro-optic or an acousto-optic modulator. A focal plane array of uncooled bolometers has been proposed before as a far infrared imaging system. One problem with such devices is that they are not sensitive enough to detect the low-intensity emission from a room-temperature blackbody in the far infrared bands. A potential solution to this problem is to use a high- or low-temperature blackbody to illuminate the scene to be imaged. In this paper, methods of measuring the infrared polarimetric signature and the far-infrared spatial signature of a scene will be presented and discussed.
Spectrometry and polarimetry measurements are important to modern science and engineering in an extremely wide variety of fields such as atomic and chemical processes, materials identification and characterization, astronomy, remote sensing, and stress analysis. The basic principle is that when light is emitted or absorbed by, scattered or reflected from, or transmitted through a physical material, its spectral content and polarization state are often affected. Analysis of the changes imposed by these processes then has the potential to reveal useful information about the sources. Example applications are: (1) stress-induced birefringence (photoelasticity); (2) remote sensing, object discrimination, shape measurement; (3) communications (polarization shift keying, deterimental effects on fiber networks); (4) astronomy (solar magnetic fields); (5) scattering, materials identification (retinal nerve fiber layer thickness measurement); (6) ellipsometry (materials characterization, complex index of refraction, layer thicknesses); (7) atomic physics; (8) displays (color LCDs merge colorimetry and polarization).
We report on measurements of the index of refraction structure parameter Cn2 using an X-band interferometric radar operating over a 3.5 km path in Southern California. These measurements were made by observing signal fluctuations and relating them to the structure parameter. Simultaneous measurements of temperature, atmospheric pressure, and humidity were made at the end points of the range. Values of Cn2 ranging from 1.4 x 10-15 to 6.3 x 10-15 m-2/3 were observed over a propagation path that was up to 275 m above ground level. These values are generally much larger than those that would be observed at visible wavelengths under the same conditions because of the contributions of the humidity structure parameter and the cross correlation of the temperature and humidity structure parameters. We also address correlation of these measurements of Cn2 with fluctuations in the radar angle-of-arrival.
The use of uncooled bolometer focal plane arrays in long- wave infrared cameras is becoming more widespread due to reductions in price and increases in sensitivity of these devices. These improvements have made infrared cameras affordable by almost everyone and have expanded their range of usefulness to applications not previously exploited . We describe a compact, lightweight (< 500g) camera that has a 320 X 240 focal plane array with 51 micron pixel size and a 50 milli-Kelvin noise equivalent temperature difference (NETD). Research is being conducted to reduce the pixel size to 25 microns, thus quadrupling the number of pixels, and to reduce the NETD by a factor of two. Recent measurements of the spectral response of this sensor show that it has usable sensitivity out to wavelengths of 25 microns and beyond, so that it would be useful for detecting cold targets and for applications such as detecting weapons concealed beneath clothing, since the longer wavelengths have been shown to penetrate clothing more readily than the generally accepted 8 - 12 micron range of the long-wavelength infrared (LWIR) band. Examples of images obtained with this camera will be shown.
We have constructed a computed-tomography imaging spectrometer (CTIS) that uses two crossed phase-only computer generated holograms (CGH) as the dispersing elements. This imaging spectrometer collects the multiplexed spatial and spectral data simultaneously and can be used for flash spectral imaging. Previous CTIS instruments require a single CGH dispersing elements which were designed with the freedom of adjusting each element in the cell profile independently during the design process. The CHGs for this instrument are designed as identical crossed gratings to model the design parameters of a crossed 1D addressable liquid crystal spatial light modulator. Future integration of a liquid crystal spatial light modulator allows for the possibility of optical preprocessing of tomographic images. The CGH disperser pair has been designed to maintain nearly equal spectral diffraction efficiency among a 5x5 array of diffraction orders and to minimize the diffraction efficiency into higher orders. Reconstruction of the (x,y,(lambda) ) image cube from the raw, two-dimensional data is achieved by computed-tomography techniques.
We report results from a demonstration of a midwave-infrared non-scanning, high speed imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. High speed spectral imaging was demonstrated by collecting spectral and spatial snapshots of blackbody targets and combustion products. The instrument is based on computed tomography concepts and operates in a mid-wave infrared band of 3.0 to 5.0 micrometers . Raw images were recorded at a frame rate of 60 fps using a 512 x 512 InSb focal plane array. Reconstructed object cube estimates were sampled at 46x46x21 (x, y,(lambda) ) elements, or 0.1 micrometers spectral sampling. Reconstructions of several objects are presented.
Channeled spectropolarimetry is a technique for measuring the spectral dependence of the polarization state of light. Passive polarization optics are used to encode the spectral dependence of the four Stokes components sk into a single irradiance spectrum. We treat the technique as a linear operator and compute its singular value decomposition numerically. The resulting singular functions divide into three distinct groups representing s0, s1 and mixtures of s2 and s3. The corresponding singular values indicate that measurements of the latter two groups will have signal-to-noise ratios reduced form that of s0 by factors of 0.6 and 0.4 respectively. The structure of the singular vectors is in agreement with a separate estimate of the system's resolution.
We present and analyze a technique for snapshot imaging spectropolarimetry. The technique involves the combination of channeled spectropolarimetry with computed tomography imaging spectrometry (CTIS). Channeled spectropolarimetry uses modulation to encode the spatial dependence of all four Stokes parameters in a single spectrum. CTIS is a snapshot imaging spectrometry method in which a computer-generated holographic disperser is employed to acquire dispersed images of the target scene, and both spatial and spectral information is reconstructed using the mathematics of computed tomography. The combination of these techniques provides the basis for a snapshot imaging complete Stokes spectropolarimeter which can be implemented with no moving parts. We present results of a simulation that we did using four input Stokes vectors that varied with wavelength. The reconstruction took into account dispersion from the retarders and that low frequency components will be missing in CTIS.
We report results of experimentation with a new, high- resolution MWIR non-scanning, snapshot imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. The instrument is based on computed tomography concepts and operates in a mid-wave infrared band of 3.0 to 5.0 micrometer. High speed spectral imaging was demonstrated by collecting spectro-spatial snapshots of an artificial target in the lab. Raw images were recorded using a 512 X 512 InSb focal plane array in snapshot mode.
Improvement in the capabilities of infrared, millimeter- wave, acoustic, and x-ray, sensors has provided means to detect weapons concealed beneath clothing and to provide wide-area surveillance capability in darkness and poor light for military special operations and law enforcement application. In this paper we provide an update on this technology, which we have discussed in previous papers on this subject. We present new data showing simultaneously obtained infrared and millimeter-wave images which are especially relevant because a fusion of these two sensors has been proposed as the best solution to the problem of concealed weapon detection. We conclude that the use of these various sensors has the potential for solving this problem and that progress is being made toward this goal.
Recent advances in millimeter-wave (MMW) radar technologies provide new applications for law enforcement use over-and- above the venerable speed timing radar. These applications include the potential to detect weapons under clothing and to conduct surveillance through walls. Concealed Weapon Detection and covert surveillance are of high interest to both the Department of Defense in support of Small Unit Operations and the Justice Department for civilian law enforcement applications. MMW sensors are under development which should provide the needed capabilities including radiometric sensors at 95 GHz, active 95 GHz real aperture radars, active focal plane array (FPA) radars, and holographic radars. Radiometric sensors include 2D FPA systems, 1D FPA, scanned systems, and single element scanned sensors. Active FPA radars include illuminated radiometric systems and coherent radar systems. Real aperture MMW radar systems include raster scanned and conical scanned sensors. Holographic systems ruse mechanical scanners to collect coherent data over a significant solid angular sector.
Sensors are needed for concealed weapon detection which perform better with regard to weapon classification, identification, probability of detection and false alarm rate than the magnetic sensors commonly used in airports. We have concluded that no single sensor will meet the requirements for a reliable concealed weapon detector and thus that sensor fusion is required to optimize detection probability and false alarm rate by combining sensor outputs in a synergistic fashion. This paper describes microwave, millimeter wave, far infrared, infrared, x-ray, acoustic, and magnetic sensors which have some promise in the field of concealed weapon detection. The strengths and weaknesses of these devices are discussed, and examples of the outputs of most of them are given. Various approaches to fusion of these sensors are also described, from simple cuing of one sensor by another to improvement of image quality by using multiple systems. It is further concluded that none of the sensors described herein will ever replace entirely the airport metal detector, but that many of them meet needs imposed by applications requiring a higher detection probability and lower false alarm rate.
Recent advances in passive and active imaging and non- imaging sensor technology offer the potential to detect weapons that are concealed beneath a person's clothing. Sensors that are discussed in this paper are characterized as either non-imaging or imaging. Non-imaging sensors include wide band radar and portal devices such as metal detectors. In general the strength of non-imaging sensors rest with the fact that they are generally inexpensive and can rapidly perform bulk separation between regions where persons are likely to be carrying concealed weapons and those regions that are likely to contain persons who are unarmed. The bulk process is typically accomplished at the expense of false alarm rate. Millimeter-wave (MMW), microwave, x-ray, acoustic, magnetic, and infrared (IR) imaging sensor technologies provide with greater certainty the means to isolate persons within a crowd that are carrying concealed weapons and to identify the weapon type. The increased certainty associated with imaging sensors is accomplished at the expense of cost and bulk surveillance of the crowd. CWD technologies have a variety of military and civilian applications. This technology focus area addresses specific military needs under the Defense Advanced Research Projects Agency's (DARPA) operations other than war/law enforcement (OOTW/LE). Additionally, this technology has numerous civilian law enforcement applications that are being investigated under the National Institute of Justice's (NIJ) Concealed Weapons Detection program. This paper discusses the wide variety of sensors that might be employed in support of a typical scenario, the strengths and weaknesses of each of the sensors relative to the given scenario, and how CWD breadboards will be tested to determine the optimal CWD application. It rapidly becomes apparent that no single sensor will completely satisfy the CWD mission necessitating the fusion of two or more of these sensors.
Recent advances in millimeter-wave (MMV), microwave, and infrared (IR) technologies provide the means to detect concealed weapons remotely through clothing and is some cases through walls. Since the developemnt of forward-looking infrared instruments, work has been ongoing in attempting to use these devices for concealed weapon detection based on temperatrue differences between metallic weapons and in the infrared has led to the development of techniques based on lower frequencies. Focal plane arrays operating MMW frequencies are becoming available which eliminate the need for a costly and slow mechanical scanner for generating images. These radiometric sensors also detect temperature differences between weapons and the human body background. Holographic imaging systems operating at both microwave and MMW frequencies have been developed which generate images of near photographic quality through clothing and through thin, nonmetallic walls. Finally, a real- aperture radar is useful for observing people and detecting weapons through walls and in the field under reduced visibility conditions. This paper will review all of these technologies and give examples of images generated by each type of sensor. An assessment of the future of this technology with regard to law enforcement applications will also be given.
Approximately half of all aircraft related deaths are caused by weather. Perhaps the most dangerous and unpredictable weather phenomenon is wind shear, which usually takes the form of a strong downdraft which is especially hazardous for large jet transports on landing approach. This paper describes a three channel radiometer operating on the low- frequency wing of the 60 GHz oxygen absorption line which should be able to detect a wind shear event at a distance of up to 10 km in addition to measuring its distance and temperature.
This paper describes a compact, high speed 95 GHz scanned antenna designed for collecting radar data from an airborne platform. This antenna consists of a corrugated horn feeding an f∶1 parabolic reflector through a hole in the scanning mirror, as shown in Figure 1. Radiation exiting the horn is thus collimated by the parabola and is incident on the flat scanning mirror, from whence it is steered to the desired angle. The collimated output is incident on the scanner in such a way that the angle is doubled, so that greater angular coverage can be obtained for a given mirror angular velocity. The scanner operates in only one axis; coverage of the other dimension is given by the forward motion of the aircraft.