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.
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.
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.
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.
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.
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.
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.
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.
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