A novel technique using a W-band metasurface for the purpose of transmissive fine powder layer sensing is presented. The proposed technique may allow for the detection, identification, and characterization of inhomogeneous ultrafine powder layers which are effectively hundreds of times thinner than the incident wavelengths used to sense them. Such a technique may be useful during personnel screening processes (i.e., at an airport) and in industrial manufacturing environments where early detection and quantization of harmful airborne particulates can be a matter of security or safety. Recently, characterizations of ultrathin powder layers using a novel metasurface sensing technique have been conducted at W-band1. Finally, a novel technique utilizing this metasurface will be presented, which could aid in the characterization of various dilute dielectric materials in solutions.
KEYWORDS: Radar, X band, Solid modeling, Scattering, Computer aided design, 3D modeling, Design and modelling, Data modeling, V band, Time-frequency analysis
Micro-Doppler radar signatures of helicopters and drones are gaining increasing importance. However, collecting data under controlled conditions on drones in flight can be difficult. The ability to use predictive codes to produce moving target and micro-Doppler radar data is becoming more important. In order to demonstrate the potential use of computer code predictions, this report will describe the X, V, and W-Band micro-Doppler signatures for the DJI Phantom 2 quadcopter. The predictions are generated using the Xpatch prediction code. The motion of all four propellers are simulated for realistic flight conditions. Predictions were performed at multiple viewing angles and using various PRF values. Additionally, different range resolutions were also predicted. The data is analyzed using a series of Range-Doppler spectrograms and short time Fourier transforms. The equations for the motion of the blades are examined in the context of the minimum PRF that is needed for capturing the micro-Doppler information. A discussion is included for finding the best frequency band to operate which balances the tradeoff of information content with operating frequency and PRF value. It is shown in the standard analysis that the unique shape of the blades produced patterns in the micro-Doppler signature that may be of use in target identification. Application of Time-Frequency-Analysis is also demonstrated. The predicted data is compared with micro-Doppler data measured in the laboratory using a 100 GHz compact range on a real Phantom 2 drone.
Generation of moving target (Doppler and micro-Doppler) radar data on a scale-model helicopter with an arbitrary flight path is described. Fully-polarimetric micro-Doppler radar signatures of a 1/35th scale helicopter at S-Band were measured using a 100GHz compact range and are examined under several different situations. The motion of the rotor and blades are analyzed using standard range-doppler spectrograms. In particular, the effect of radial motion toward or away from the radar is considered and the consequences this motion will have on the range/doppler spectrogram of the radar data. It is found that a helicopter exhibiting no radial velocity will experience a degeneracy of signals from the rotation of the helicopter blades. This degeneracy is lifted when a non-zero radial motion is experienced. The effect of varying the radar pulse repetition frequency will be examined.
The radar frequency backscattering behavior of rough ocean surfaces has been investigated using a physical scale modeling approach and a millimeter-wave compact radar range. The method involved fabrication of two 1/16th scale simplified rough ocean surfaces from a material that at 160 GHz behaved as the dielectric equivalent of seawater at 10 GHz. By measuring the backscattering behavior of the physical models in a 160 GHz compact range, the X-band radar frequency scattering behavior was determined. Though the physical models used were static, a scale modeling approach readily offers the ability to examine ocean backscattering phenomenology of a given surface texture over an extended range of look angles and radar frequencies, which otherwise would be challenging in a dynamic ocean environment. Computational electromagnetic modeling of the surface was also performed and compared with compact range measurements. This effort, involving more realistic ocean surfaces, builds on previous ocean modeling work presented by our team that utilized a periodic rough surface. The simulated surfaces studied here represent a natural progression toward our goal of developing reliable methodologies to characterize the backscattering behavior of land and sea clutter using physical scale modeling technologies.
KEYWORDS: Data modeling, Radar, 3D acquisition, 3D modeling, Compressed sensing, Scattering, Solid modeling, 3D metrology, Computer aided design, 3D image processing
Three-dimensional radar imaging is becoming increasingly important in modern warfare systems, leading to an increased need for deeper understanding of the 3D scattering behavior. Fully polarimetric, three-dimensional radar signature data have been collected using 1/16th scale models of tactical targets in several indoor compact radar ranges, corresponding to radar data at X-band. The high-range-resolution data has been collected through a 2D aperture in azimuth and elevation. This data has been processed into 3D coordinates using a standard 3D Fourier transform. The radar signatures have also been rendered into 3D coordinates using Interferometric ISAR techniques. The results of applying compressed sensing techniques to the analysis will be presented. Mathematical 3D correlation analysis has been used to compare the results of each method of 3D reconstruction.
Compressed sensing is a signal processing paradigm enabling the acquisition and successful reconstruction of a sparse signal from a reduced set of measurements, potentially in violation of the Nyquist sampling criterion. In this paper the results of preliminary investigations into Compressed Sensing applied to the acquisition of wide bandwidth millimeterwave compact radar range data are presented. Primary motivations for application of Compressed Sensing to compact radar range acquisition and imaging include increasing data acquisition speed as well as reducing required data storage. In this work only signal reduction in the frequency domain is examined. Compressed Sensing fully-polarimetric compact range data acquisition and imaging for both a simple canonical target (cylinder) and a complex target (Slicy) are presented as radar cross section (RCS) measurements and interferometric inverse synthetic aperture radar (IFISAR) images. Correlations of compact range data provide a measure of error between the reconstructed and complete data sets as a function of target complexity and sub-sampling rate.
Physical scale modeling of the electromagnetic backscatter behavior of static sea states using dielectric models and indoor compact radar ranges has the potential to offer a unique and advantageous method to probe ocean scattering phenomenology not feasible using conventional radar measurements on dynamic sea surfaces. As an initial step towards developing such modeling techniques, the millimeter-wave backscatter of a static, simplified rough surface made from a material that electromagnetically models the X-band dielectric properties of seawater has been measured. Computational electromagnetic modeling of the surface was performed using Xpatch and is compared with compact range measurements. By starting with simplified sea-state surfaces, the aim is to develop a reliable scale modeling approach capable of studying the backscattering behavior of realistic ocean surfaces.
KEYWORDS: 3D modeling, 3D acquisition, Data modeling, Computer aided design, Solid modeling, Radar, Scattering, 3D image processing, Rockets, Interferometry
Three-dimensional radar imaging is becoming increasingly important in modern warfare systems, leading to an increased need for deeper understanding of the 3D scattering behavior of targets as simple as a cylinder, to as complex as a main battle tank or air defense unit. Fully polarimetric, three dimensional radar signature data have been collected using 1/16th scale models of tactical targets in several indoor compact radar ranges, corresponding to data from S-band to W-band. ISAR image pairs, collected at slightly different elevations, were interferometrically processed into 3D imagery. The data collection, analysis, and 3D visualization methods are presented. Additionally, the results of mathematical 3D correlation are described. A detailed analysis of both measured and predicted 3D radar data on the UMass Lowell nominal rocket simulator target will be presented.
We present a prototype bistatic compact radar range operating at 160 GHz and capable of collecting fullypolarimetric radar cross-section and electromagnetic scattering measurements in a true far-field facility. The bistatic ISAR system incorporates two 90-inch focal length, 27-inch-diameter diamond-turned mirrors fed by 160 GHz transmit and receive horns to establish the compact range. The prototype radar range with its modest sized quiet zone serves as a precursor to a fully developed compact radar range incorporating a larger quiet zone capable of collecting X-band bistatic RCS data and 3D imagery using 1/16th scale objects. The millimeter-wave transmitter provides 20 GHz of swept bandwidth in the single linear (Horizontal/Vertical) polarization while the millimeter-wave receiver, that is sensitive to linear Horizontal and Vertical polarization, possesses a 7 dB noise figure. We present the design of the compact radar range and report on test results collected to validate the system’s performance.
With the demand for larger bandwidths and faster data speeds, wireless communication systems are expanding into the millimeter wave and terahertz region of the electromagnetic spectrum. Successful transition to higher frequencies, particularly for systems located in urban or indoor environments, will require a thorough understanding of the reflection, transmission, absorption, and scattering of a wide variety of materials. For this study, the co-polarization and crosspolarization backscattering coefficients of several dielectrics were measured in compact radar ranges operating from 160 GHz to 1.55 THz. These structures consisted of dielectric disks with various rough surfaces. The backscattering measurements of these disks were compared as a function of polarization, incident angle, roughness, and frequency.
KEYWORDS: Absorption, Databases, Transmittance, Atmospheric monitoring, Molecules, Receivers, Atmospheric propagation, Data modeling, Data acquisition, Radio frequency circuits
The terahertz frequency regime is often used as the ‘chemical fingerprint’ region of the electromagnetic spectrum since many molecules exhibit a dense selection of rotational and vibrational transitions. Water is a major component of the atmosphere and since it has a large dipole moment the propagation of terahertz radiation will be dominated by atmospheric effects. This study will present the results of high-‐resolution broadband measurements of the terahertz atmospheric absorption and detail the technique for directly measuring the pressure broadening coefficients, absolute absorption coefficients, line positions, and continuum effects. Differences between these measured parameters and those tabulated in HITRAN will be discussed. Once the water vapor absorption was characterized, the same technique was used to measure the line parameters for methanol, a trace gas of interest within Earth’s atmosphere. Methanol has a dense absorption spectrum in the terahertz frequency region and is an important molecule in fields such as environmental monitoring, security, and astrophysics. The data obtained in the present study will be of immediate use for the remote sensing community, as it is uncommon to measure this many independent parameters as well as to measure the absolute absorption of the transitions. Current models rely on tabulated databases of calculated values for the line parameters measured in this study. Differences between the measured data and those in the databases will be highlighted and discussed.
Successful development of remote sensing and communication systems in the terahertz band requires a better understanding of the scattering behavior of various structures. Materials that could be considered homogeneous and smooth at microwave frequencies may begin to display surface and volumetric scattering behavior in the terahertz band. The co-polarization backscattering coefficient of several types of metal and dielectric structures were measured in indoor compact radar ranges operating at 100 GHz, 160 GHz, 240 GHz, and 1.55 THz. These structures consisted of roughened aluminum plates, as well as homogeneous and inhomogeneous dielectric surfaces. The roughness and inclusions of the measured samples were tailored in order to systematically investigate various scattering effects. Polarimetric backscattering measurements of these materials were collected at elevation angles from 5 to 75 degrees. Analysis of the backscatter data supports a better understanding of surface and volumetric scattering behavior of materials at terahertz frequencies.
The terahertz frequency regime is often used as the ‘chemical fingerprint’ region of the electromagnetic spectrum due to
the large number of rotational and vibrational transitions of many molecules of interest. This region of the spectrum has
particular utility for applications such as pollution monitoring and the detection of energetic chemicals using remote
sensing over long path lengths through the atmosphere. Although there has been much attention to atmospheric effects
over narrow frequency windows, accurate measurements across a wide spectrum are lacking. The water vapor continuum
absorption is an excess absorption that is unaccounted for in resonant line spectrum simulations. Currently a semiempirical
model is employed to account for this absorption, however more measurements are necessary to properly
describe the continuum absorption in this region. Fourier Transform Spectroscopy measurements from previous work are
enhanced with high-resolution broadband measurements in the atmospheric transmission window at 1.5THz. The
transmission of broadband terahertz radiation through pure water vapor as well as air with varying relative humidity
levels was recorded for multiple path lengths. The pure water vapor measurements provide accurate determination of the
line broadening parameters and experimental measurements of the transition strengths of the lines in the frequency
region. Also these measurements coupled with the atmospheric air measurements allow the water vapor continuum
absorption to be independently identified at 1.5THz. Simulations from an atmospheric absorption model using
parameters from the HITRAN database are compared with the current and previous experimental results.
The radar cross section of spherical retroreflectors operating at terahertz frequencies is investigated. Several spherical
retroreflectors with diameters ranging from 2 mm to 8 mm were fabricated and their radar cross section was measured at
100 GHz, 160 GHz, and 350 GHz. A frequency selective surface was applied to the retroreflectors to demonstrate proof
of concept of narrow-band terahertz retroreflection.
Remote sensing over long path lengths has become of greater interest in the terahertz frequency region. Applications
such as pollution monitoring and detection of energetic chemicals are of particular interest. Although there has been
much attention to atmospheric effects over narrow frequency windows, accurate measurements across a wide spectrum
is lacking. The water vapor continuum absorption spectrum was investigated using Fourier Transform Spectroscopy.
The continuum effect gives rise to an excess absorption that is unaccounted for in just a resonant line spectrum
simulation. The transmission of broadband terahertz radiation from 0.300THz - 1.5THz through air with varying relative
humidity levels was recorded for multiple path lengths. From these data, the absorption coefficient as a function of
frequency was determined and compared with model calculations. The intensity and location of the strong absorption
lines were in good agreement with spectral databases such as the 2008 HITRAN database and the JPL database.
However, a noticeable continuum effect was observed particularly in the atmospheric transmission windows. A small
discrepancy still remained even after accounting for continuum absorption using the best available data from the
literature. This discrepancy, when projected over a one kilometer path length, typical of distances used in remote
sensing, can cause a 30dB difference between calculated and observed attenuation. From the experimental and resonant
line simulation spectra the air-broadening continuum parameter was calculated and compared with values available in
the literature.
We have designed, built, and tested an uncooled THz imager based on optical readout photomechanical imaging technology, in which a MEMS-based sensor chip transduces the THz scene into a visible signal that is captured by a CCD imager. The performance of the 130x90 resolution, 100 μm pitch, 30 fps uncooled THz imager was measured using the λ = 119 μm (2.52 THz) emission line of a CO2-pumped methanol gas laser. Excellent linearity of the responsivity was observed over a wide range of laser power. The noise equivalent power (NEP), limited by shot noise from the optical readout, was 76 pW/Hz1/2. Switching to a high-capacity CCD imager to reduce shot noise and tailoring the photomechanical pixel structure for THz absorption will yield an NEP of less than 1 pW/Hz1/2. In addition, the uncooled THz imager successfully profiled the output beam of a λ=134 um (2.24 THz) quantum cascade laser (QCL) in real time, with performance far superior to a commercial pyroelectric array camera.
Nonmelanoma skin cancers are the most common form of cancer. Continuous wave terahertz imaging has the
potential to differentiate between nonmelanoma skin cancers and normal skin. Terahertz imaging is non-ionizing
and offers a high sensitivity to water content. Contrast between cancerous and normal tissue in transmission mode
has already been demonstrated using a continuous wave terahertz system. The aim of this experiment was to
implement a system that is capable of reflection modality imaging of nonmelanoma skin cancers. Fresh excisions of
skin cancer specimens were obtained from Mohs surgeries for this study. A CO2 optically pumped far-infrared
molecular gas laser was used for illuminating the tissue at 584 GHz. The reflected signal was detected using a liquid
Helium cooled Silicon bolometer. The terahertz images were compared with sample histology. The terahertz
reflection images exhibit some artifacts that can hamper the specificity. The beam waist at the sample plane was
measured to be 0.57 mm, and the system's signal-to-noise ratio was measured to be 65 dB.
Operational temperature increase of CW THz QCLs to 77 K has enabled us to employ solid nitrogen (SN2) as the
cryogen. A roughing pump was used to solidify liquid nitrogen and when the residual vapor pressure in the nitrogen
reservoir reached the pumping system's minimum pressure the temperature equilibrated and remained constant until
all the nitrogen sublimated. The hold time compared to liquid helium has thereby increased approximately 70-fold,
and at a greatly reduced cost. The milliwatt CW QCL was at a temperature of approximately 60 K, dissipating 5 W
of electrical power. To measure the long-term frequency, current, and temperature stability, we heterodyned the
free-running 2.31 THz QCL with a CO2 pumped far-infrared gas laser line in methanol (2.314 THz) in a corner-cube
Schottky diode and recorded the IF frequency, current and temperature. Under these conditions the performance
characteristics of the QCL, which will be reported, exceeded that of a device mounted in a mechanical cryocooler.
The ability to accurately classify targets is critical to the performance of automated/assisted target recognition (ATR)
algorithms. Supervised machine learning methods have been shown to be able to classify data in a variety of disciplines
with a high level of accuracy. The performance of machine learning techniques in classifying ground targets in two-dimensional
radar imagery were compared. Three machine learning models were compared to determine which model
best classifies targets with the highest accuracy: decision tree, Bayes', and support vector machine. X-band signature
data acquired in scale-model compact ranges were used. ISAR images were compared using several techniques
including two-dimensional cross-correlation and pixel by pixel comparison of the image against a reference image. The
highly controlled nature of the collected imagery was ideally suited for the inter-comparison of the machine learning
models. The resulting data from the image comparisons were used as the feature space for testing the accuracy of the
three types of classifiers. Classifier accuracy was determined using N-fold cross-validation.
Spectroscopic studies are useful in a range of areas, from remote sensing and radio astronomy to the medical
community. Since spectroscopy can provide information used to identify chemicals, new techniques have been
developed for high-resolution measurements of chemical absorption frequencies. These high-resolution measurements
not only enhance accuracy of the molecule's energy level transitions, but also allow for environmental
information to be gathered through collisional broadening of the spectral lines. High-resolution measurements,
made possible by far-infrared lasers coupled with Schottky diodes, were supplemented with data taken by variable
pathlength FTIR measurements in the terahertz region. Chemicals investigated include methanol, nitromethane,
water and its isotopes.
This paper discusses the effect of periodic roughness and surface defects on the electromagnetic scattering of
terahertz waves from cylindrical objects. The cylinders, possessing periodic roughness imparted during their fabrication
process, had average roughness values ranging from approximately 0.1 μm - 0.50 μm. Metallic cylinders were
fabricated from lathe-turned aluminum rods and dielectric cylinders were fabricated using a rapid prototype technique
(stereolithography). The scattering behavior of the rough cylinders was measured in 160 GHz and 350 GHz compact
radar ranges. In addition, the effect of seams and grooves on the scattering behavior of cylinders will also be presented.
Research into absorption spectra is useful for detecting chemicals in the field. Each molecule absorbs a set of
specific frequencies, which are dependent on the molecule's structure. While theoretical models are available for
predicting the absorption frequencies of a particular molecule, experimental measurements are a more reliable
method of determining a molecule's actual absorption behavior. The goal of this research is to explore chemical
markers (absorption frequencies) that can be used to identify highly energetic molecules of interest to the
remote sensing community. Particular attention was paid to the frequency ranges located within the terahertz
transmission windows of the atmosphere. In addition, theoretical derivations, with the purpose of calculating
the detection limits of such chemicals, will also be presented.
A low loss, hollow, flexible waveguide was designed and fabricated for the transmission of terahertz
radiation. Polycarbonate and glass tubing was chosen as the base material of the waveguide. The inner surface of the
dielectric tube was coated with silver using a wet chemistry method. The waveguides were fabricated in several
diameters namely, 2.4 mm, 3.2 mm, 3.8 mm, and 4.6 mm. The attenuation coefficients for the waveguides were
measured at a wavelength of 215 μm using an optically pumped terahertz laser. The free space Gaussian mode from
the terahertz laser source was coupled into the lowest loss TE11 mode inside the waveguide. The mode profiles of
the output beam from the waveguides were shown. A loss of 3.5 dB/m was measured for the silver coated dielectric
tube of diameter 4.6 mm.
Continuous wave terahertz imaging has the potential for diagnosing and delineating skin cancers. While contrast has
been observed between cancerous and normal tissue at terahertz frequencies, the source mechanism behind this contrast
is not clearly understood.1Transmission measurements of 240μm thick sections of nonmelanoma skin cancer were taken
at two frequencies of 1.39 THz and 1.63 THz that lie within and outside the tryptophan absorption band, respectively.
Two CO2 pumped Far-Infrared molecular gas lasers were used for illuminating the tissue while the transmitted signals
were detected using a liquid Helium cooled Silicon bolometer. At both THz frequencies 2-dimensional THz transmission
images of nonmelanoma skin cancers were acquired with better than 0.5mm spatial resolution. The resulting images
were compared to the sample histology and showed a correlation between cancerous tissue and decreased transmission.
The results of the imaging experiments will be presented and discussed.
A coherent transceiver using a THz quantum cascade laser as the transmitter and an optically pumped molecular laser as
the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire
high-resolution images of fully illuminated targets, including scale models. Phase stability of the received signal,
sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by
frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the
target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric
attenuation at 2.408 THz (2.0 dB/m at 40% RH), the coherence length of the QCL transmitter will allow coherent
imaging over distances up to several hundred meters. In contrast to non-coherent heterodyne detection, coherent
imaging allows signal integration over time intervals considerably longer than the reciprocal of the source, or signal
bandwidth, with consequent improvement in the signal-to-noise ratio. Image data obtained with the system will be
presented.
Coherent terahertz radar systems, using CO2 laser-pumped molecular lasers have been used during the past decade for
radar scale modeling applications, as well as proof-of-principle demonstrations of remote detection of concealed
weapons. The presentation will consider the potential for replacement of molecular laser sources by quantum cascade
lasers. While the temporal and spatial characteristics of current THz QCLs limit their applicability, rapid progress is
being made in resolving these issues. Specifications for satisfying the requirements of coherent short-range THz radars
will be reviewed and the feasibility of incorporating existing QCL devices into such systems will be described.
We report on the first THz balanced mixer/upconverter using a Schottky diode MMIC chip. Using an optically pumped
laser at 1562 GHz as an LO source with a coupled power of about 1 mW, and 1 mW input at an IF frequency of 10 GHz,
we obtained a sideband output power of 23 uW (sum of two sidebands). As a mixer, at an LO of 1621 GHz, we obtain a
conversion loss of 12.4 dB DSB and a noise temperature of 5600 K DSB. Response is believed to be similar over a band
1250-1650 GHz. New diodes have been designed for easier application as mixers up through 3 THz, and a new wafer
run is in process.
Continuous wave terahertz imaging has the potential to offer a safe, non-invasive medical imaging modality for detecting
different types of human cancers. The aim of this study was to identify intrinsic biomarkers for non-melanoma skin
cancer and their absorption frequencies. Knowledge of these frequencies is a prerequisite for the optimal development of
a continuous wave terahertz imaging system for detecting different types of skin cancers. The absorption characteristics
of skin constituents were studied between 20 and 100 cm-1 (0.6 THz - 3 THz). Terahertz radiation is highly absorbed by
water. Thus, the high water content of human tissue necessitates a reflection based imaging modality. To demonstrate a
reflection based, high resolution, terahertz imaging system, a prototype imaging system was constructed at 1.56 THz.
The system resolution was determined to be 0.5 mm and the system signal to noise ratio was found to be 70 dB. Data
from the terahertz spectroscopy experiments and reflection based terahertz images at 1.56 THz are presented.
A Terahertz imaging system intended to demonstrate identification of objects concealed under clothing was designed, assembled, and tested. The system design was based on a 2.5 m standoff distance, with a capability of visualizing a 0.5 m by 0.5 m scene at an image rate of 2 frames per second. The system optical design consisted of a 1.56 THz laser beam, which was raster swept by a dual torsion mirror scanner. The beam was focused onto the scan subject by a
stationary 50 cm-diameter focusing mirror. A heterodyne detection technique was used to down convert the backscattered signal. The system demonstrated a 1.5 cm spot resolution. Human subjects were scanned at a frame rate of 2 frames per second. Hidden metal objects were detected under a jacket worn by the human subject. A movie including data and video images was produced in 1.5 minutes scanning a human through 180° of azimuth angle at 0.7° increment.
In response to the growing interest in developing terahertz imaging systems for concealed weapons detection, the Submillimeter-Wave Technology Laboratory (STL) at the University of Massachusetts Lowell has produced full-body terahertz imagery using coherent active radar measurement techniques. The proof-of-principle results were readily obtained utilizing the compact radar range resources at STL. Two contrasting techniques were used to collect the imagery. Both methods made use of in-house transceivers, consisting of two ultra-stable far-infrared lasers, terahertz heterodyne detection systems, and terahertz anechoic chambers. The first technique involved full beam subject illumination with precision azimuth and elevation control to produce high resolution images via two axis Fourier transforms. Imagery collected in this manner is presented at 1.56THz and 350GHz. The second method utilized a focused spot, moved across the target subject in a high speed two dimensional raster pattern created by a large two-axis positioning mirror. The existing 1.56THz compact radar range was modified to project a focused illumination spot on the target subject several meters away, and receive the back-reflected intensity. The process was repeated across two dimensions, and the resultant image was assembled and displayed utilizing minimal on-the-fly processing. Imagery at 1.56THz of human subjects with concealed weapons are presented and discussed for this scan type.
As short range, ground based, surveillance systems operating at terahertz frequencies continue to evolve,
increasing attention is being directed towards the behavior of dielectric materials at terahertz frequencies as well as the
behavior of optical components used to control terahertz radiation. This work provides an overview of several terahertz
optical components such as frequency selective filters, laser output couplers, artificial dielectrics, and electromagnetic
absorbers. In addition, a database was established that contains terahertz properties of common materials that have been
largely unexplored in this region of the spectrum. The database consists of transmittance and reflectance spectra of a
variety of materials measured using Fourier transform infrared spectroscopy techniques from 175 GHz - 2 THz. In
addition, ultra-stable, CO2 optically pumped, far-infrared gas lasers were used to collect fixed-frequency transmittance
data at 326 GHz, 584 GHz, and 1.04 THz. A Gunn oscillator was used for measurements at 94 GHz.
Construction of the new 350GHz compact range has been completed and it is able to collect fully polarimetric scaled X-band radar data with 6-inch full-scale range resolution. In order to investigate the reproduction of X-band data using scale models, fully polarimetric high-resolution radar signature data has been collected on several targets which include a high-fidelity in-house built 1/16th scale T72 Main Battle Tank (MBT) and a commercially available 1/35th scale model T72 modified to match its features. A correlation study of ISAR images has been performed between the X-band data sets collected on these models, a full-scale T72, a 1/35th scale model heavy equipment transporter, and several different 1/16th scaled targets of similar size. The ISAR images formed from the data were compared using several techniques which include a two-dimensional cross-correlation of the images against one another, and the comparison of the images pixel-by-pixel to measure the percentage differences. It will be shown that the T72 data sets compare well across the three different radar platforms. It has also been found that there are persistent sharp features in the two-dimensional cross-correlation maps that are located where the real target is matched even when other parameters have changed by a significant amount. These features continue to occur when the target has been imbedded in a complex two-target scene with the heavy equipment transporter.
The HH and VV-polarized backscattering behavior of homogeneous ground clutter has been investigated by measuring the radar cross section per unit area of rough surface terrain. The X, Ka, and W-band behavior was investigated by analyzing ISAR imagery of 1/16th scale terrain collected in compact radar ranges operating at 160 GHz, 520 GHz, and 1.56 THz. An array of scale model ground planes was fabricated with the appropriate roughness to model relatively smooth to rough soil terrain. In addition to studying terrain backscatter as a function of surface roughness, the dependence on soil moisture content was also characterized by tailoring the dielectric constant of the scale models. The radar cross section per unit illuminated area (?0) was calculated as a function of elevation angle between 15° and 75°. The results of this work have been used in the fabrication of scale model ground planes for collection of radar imagery from scaled threat targets situated in realistic environments. Backscattering data are presented and compared to clutter data found in the literature.
Radar detection and identification of ground targets in diverse environments is a subject of continuing interest. It has long been known that different radar bands have advantages for different environmental conditions. For example, it has been shown that detection of targets under foliage is more easily accomplished using longer wavelength radars since there is less attenuation at these frequencies. However, higher frequency radars offer greater resolution that is crucial in target identification. Because each radar band has its own unique strengths and weakness, one current approach is the use of dual-band radar platforms. With two radar bands working simultaneously, the strengths of each radar band can be used to compliment the other. ERADS has constructed two full polarimetric compact radar ranges to acquire X-Band and UHF ISAR imagery data using 1/35th scale models. The new compact ranges allow data to be taken that can simulate a multi-frequency radar platform with frequencies low enough to detect obscured targets and high enough to provide useful resolution to aid in target identification once they have been detected. Since both compact ranges use the same scale factor, this allows measurement of the same target at the two spectral regions simply by moving the target model from one compact range to the other. Data can thus be taken whose differences in scattering are due only to the difference in radar frequency, eliminating variations due to differences in target models as well as the surrounding ground clutter. Detailed descriptions of the new compact ranges will be presented along with results from sample data sets.
The next generation of hot electron bolometric (HEB) mixer receivers for terahertz frequencies is under development. In order to improve sensitivity and integration time, terahertz focal plane arrays with HEB elements are required. We have designed, fabricated, and tested a three-element focal plane array with HEB devices. We implemented a quasi-optical power coupling scheme using three elliptical silicon lenses. Recently developed wideband (0.5 GHz to 12 GHz) MMIC low noise amplifiers were directly integrated with HEB devices in a single block. The array was tested using an FIR laser as the LO source and a side band generator as the signal source. This is the first heterodyne array for a frequency above 1 THz, and the suitability of HEB elements in a terahertz FPA has thus been demonstrated. This development is also geared toward investigating new architectures for much larger arrays utilizing HEB elements. Additional issues to be resolved include an improved antenna design for efficient LO injection, compact and low power IF amplifiers, and cryogenic optimization.
The demand for high-resolution ISAR data on tactical targets at all radar bands has been growing steadily. Here we describe a new 350GHz compact range currently being constructed to acquire fully polarimetric X-band data using 1/35th scale models. ERADS currently operates compact ranges from X to W-band using 1/16th scale models. The addition of this new compact range using 1/35th scale models will permit the measurement of larger targets and the measurement of multiple targets arrangeed in a scene. It will also allow us to take advantage of teh large number of commercially available models at 1/35th scale. The 350GHz transceiver uses two high-stability optically pumped far-infrared lasers, microwave/laser 350GHz mixer side-band generation for frequency sweep, and a pair of waveguide mounted diode receivers for coherent integration. The 35GHz bandwidth at a center frequency of 350GHz will allow the X-band transceiver system to collect data with up to 6-inch down range resolution, with a round trip half power beam diameter corresponding to 60 feet. Tactical targets may be measured in free space or on various ground planes, which simulate different types of terrain. Compact range measurements of simple calibration objects have been performed and compared to theoretical results using computer code predictions. A correlation study of X-band data using field measurements, 1/35th scale models and 1/16th scale models is planned upon completion of compact range construction. Available results of the diagnostic testes and the correlation study will be presented.
Using the high-frequency terahertz compact range developed recently for measurement of polarimetric return of scale modesl of tactical targets, we have developed several techniques to produce 3D data sets. Fully polarimetric 3D ISAR data has been collected on several 1/16th scale model tactical targets in free space at individual look angles. The 3D scattering coordinates are calculated by viewing the target through a 2D angular aperture in both azimuth and elevation while simultaneously performing a linear frequency chirp to measure the down-range coordinate. Due to the high frequency of W-band radar, this technique produces high-resolution cross-range images from relatively small (approximately 1 degree) angular integrations. Several techniques for calculation of the 3D coordinates have been developed. In addition to the technique described above, a new method utilizing the phase change of the scattering centers due to differentially small changes in angle will be described. Data collected using this technique can be processed to produce 3D scattering information similar to that obtained by monopulse systems. Results from this analysis will be shown.
Based on the excellent performance of NbN HEB mixer receivers at THz frequencies which we have established in the laboratory, we are building a Terahertz REceiver with NbN HEB Device (TREND) to be installed on the 1.7 meter diameter AST/RO submillimeter wave telescope at the Amundsen/Scott South Pole Station. TREND is scheduled for deployment during the austral summer season of 2002/2003. The frequency range of 1.25 THz to 1.5 THz was chosen in order to match the good windows for atmospheric transmission and interstellar spectral lines of special interest. The South Pole Station is the best available site for THz observations due to the very cold and dry atmosphere over this site. In this paper, we report on the design of this receiver. In particular, we report on HEB mixer device performance, the quasi-optical coupling design using an elliptical silicon lens and a twin-slot antenna, the laser local oscillator (LO), as well as the mixer block design and the plans for coupling the TREND receiver to the sky beam and to the laser LO at the AST/RO telescope site.
Fully polarimetric high-resolution W-band target signature data has been collected on 7 high fidelity 1/16th scale model main battle tanks. Data has been collected at several different elevation angles and target poses. Additionally, targets have been measured both on 1/16th scale simulated ground terrain and in free-space. ISAR images were formed from this data for use in several different target identification algorithms. These algorithms include using the data in both linear and circular polarization. The results of the inter-comparisons of the data using different algorithms are presented. Where possible the data has been compared with existing W-band Full-scale field measurements. The data is taken using a 1.55THz compact range designed to model W-band. The 1.55THz transceiver uses two high- stability optically pumped far-infrared lasers, microwave/laser Schottky diode side-band generation for frequency sweep, and a pair of Schottky diode receivers for coherent integration.
The VV-polarized W-band backscattering behavior of homogeneous ground clutter has been investigated by measuring the radar cross section per unit area of 1/16th scale rough surface terrain in a 1.56 THz compact radar range. An array of scale model ground planes was fabricated with the appropriate roughness to model smooth to rough soil terrain. In addition to studying the backscattering behavior as a function of surface roughness, the dependence on soil moisture content was also characterized by tailoring the dielectric constant of the scale models. Radar imagery of the rough surfaces were acquired in a 1.56THz compact radar range by collecting single frequency backscatter data over a solid angle in both azimuth and elevation. The data were Fourier transformed in both the azimuth and elevation directions to produce two-dimensional imagery. The backscattering coefficient per unit illuminated area ((sigma) 0) was calculated as a function of elevation angle between 5 degree(s) and 85 degree(s). The results of this work have been used in the fabrication of scale model ground planes for collection of W-band radar imagery from scaled threat targets in realistic environments. Backscattering data, including clutter statistics, are compared to W-band clutter data found in the literature.
KEYWORDS: Polarimetry, Scattering, Polarization, Ka band, 3D modeling, Data modeling, Automatic target recognition, Radar, Target recognition, 3D acquisition
In this study the polarization scattering matrices (PSM) of individual scatterers from a complex tactical ground target were measured as a function of look angle. Due to the potential value of PSMs in studies of automatic target recognition, a fully polarimetric, 3D spot scanning radar modeling system was developed at 1.56 THz to study the W- band scattering feature behavior from 1/16th scale models of targets. Scattering centers are isolated and coherently measured to determine the PSMs. Scatterers of varying complexity from a tactical target were measured and analyzed, including well-defined fundamental odd and even bounce scatterers that maintain the exact normalized PSM with varied look angle, scatterers with varying cross- and co-pol terms, and combination scatterers. Maps defining the behavior of the position and PSM activity over varying look angle are likely to be unique to each target and could possibly represent exploitable features for ATR.
With the continuing interest in ATR, there is a need for high-resolution fully polarimetric data on tactical targets at all radar bands. Here we describe a newly developed system for acquiring W-band data with 1/16 scale models. The NGIC sponsored ERADS project capability for obtaining fully polarimetric ISAR imagery now extends from X to W band.
HEB technology continues to extend the sate-of-the-art for THz low-noise receivers. This talk discusses recent measured noise temperatures for NbN HEB receivers form which we infer intrinsic noise temperatures which approach the quantum noise limit within a factor of 3-5. We discuss the feasibility of achieving noise temperatures even close to the quantum limit noting that this limit has been reached both at lower frequencies and at higher frequencies. Another approach for considerably enhancing the speed with which THz receivers collect data is to employ a focal plane array system. We will discuss our design approach and general constraints for such a system.
A new very high-frequency compact radar range has been developed to measure scale models of tactical targets. This compact range has demonstrated very good signal-to-noise and is useful in measuring low observable targets. In addition to normal ISAR imaging of targets (range vs. horizontal cross- range), the system can also produce two-dimensional images in azimuth and elevation (vertical cross-range vs. horizontal cross-range). The 1.56 THz transceiver uses two high-stability optically pumped far-infrared lasers, microwave/laser side- band generation for frequency sweep, and a pair of Schottky diode receivers for coherent integration. Measurements made on 1/16th scale models of tactical targets, simulating W-band frequencies, allows the formation of images of very high cross-range resolution (3.5 cm full scale) while still integrating over a reasonably small angular extent (2.5 degrees). The results from several targets that have been recently measured will be presented.
We have developed prototype HEB receivers using thin film superconducting NbN devices deposited on silicon substrates. The devices are quasi-optically coupled through a silicon lens and a self-complementary log-specific toothed antenna. We measured DSB receiver noise temperatures of 500 K (13 X hf/2k) at 1.56 THz and 1,100 K (20 X hf/2k) at 2.24 THz. Noise temperatures are expected to fall further as devices and quasi-optical coupling methods are being optimized. The measured 3 dB IF conversion gain bandwidth for one device was 3 GHz, and it is estimated that the bandwidth over which the receiver noise temperature is within 3 dB of its minimum value is 6.5 GHz which is sufficient for a number of practical applications. We will discuss our latest results and give a detailed description of our prototype setup and experiments. We will also discuss our plans for developing focal plane arrays with tens of Hot Electron Bolometric mixer elements on a single silicon substrate which will make real time imaging systems in the THz region feasible.
Molecular spectroscopy was the earliest application in the terahertz spectral region and remains one of the most important. With the development of modern technology, spectroscopy has expanded beyond the laboratory and is the basis for a number of important remote sensing systems, especially in atmospheric science and studies of the interstellar medium. Concurrently, these spectroscopic applications have been one of the prime motivators for the development of terahertz technology. This paper will review these issues in the context of the requirements placed on future technology developments by spectroscopic applications.
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