RF and Microwave Active Device Technologies. Semiconductor Diodes. Varactors. Schottky Diode Frequency Multipliers. Transit Time Microwave Devices. Bipolar Junction Transistors (BJTs). Heterostructure Bipolar Transistors (HBTs). Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). Metal Semiconductor Field Effect Ransistors (MESFETs). High Electron Mobility Transistors (HEMTs). RF Power Transistors from Wide Bandgap Materials. Tubes. Monolithic Microwave IC Technology
Fine resolution Synthetic Aperture Radar (SAR) requires wideband signals to be transmitted and received. Electronically steered phased-array antennas have difficulty steering wideband signals without the use of expensive and cumbersome true time delay elements. Otherwise more desirable phase shifters are by themselves inadequate to the task. Wideband radar signals can be generated from series or groups of narrow-band signals centered at different frequencies. A wideband Linear FM (LFM) chirp can be assembled from lesser-bandwidth chirp segments. The chirp segments can be transmitted as separate pulses, each with their own steering phase operation. Each chirp segment's bandwidth would essentially be narrow-band by itself. Doing so allows each pulse to be steered by phase shifters alone. This overcomes the problematic dilemma of steering wideband chirps with phase shifters alone. True time-delay elements are not required. The raw Phase History data can then be processed in a manner to reconstruct the image by combining all pulses with all chirp segments. In this manner the image will exhibit resolution consistent with the entire resolution bandwidth, which can be much larger than any individual segment's chirp bandwidth.
Speckle in SAR imagery is a by-product of constructive and destructive interference between scatterers within a resolution cell. This speckle phenomenon gives SAR imagery a "noise-like" appearance and is often exploited in near angle and/or coherent stereo pairs. However, in many cases, this speckle is unwanted and can be considered noise or interference. We use partial differential equation (PDE) methods for speckle mitigation in detected imagery and the collected complex image data. In particular, we study the effects of non-linear anisotropic diffusion filters on collected SAR image data. In the past, anisotropic diffusion (AD) techniques have been successfully used in the analysis of EO data. However, the use of these techniques on SAR image data is recent and much is yet to be done. We expect the application of AD techniques on SAR image data in combination with a fluid dynamic perspective to yield rich dividends in terms of image interpretability. Through our approach we demonstrate that it is possible to spatially maintain areas of high dynamic range (bright scatterers) and smooth areas of low dynamic range in the scene. We also exhibit the role of these non-linear filters in correlation, registration, compression, decompression, and image interpretability for SAR analysts.
In January, 2006, the New York Air National Guard requested that Sandia National Laboratories develop an X-band synthetic aperture radar to use for an experiment to detect crevasses in Antarctica. Sandia provided a MiniSAR radar that was modified to operate at X-band. Data was collected with this system in the Antarctic summer of 2006. The results from this data collection are presented in this paper.
Bistatic synthetic aperture radar (SAR) enables new defense as well as environmental applications where the characteristics of the bistatic reflectivity can be exploited. Experimental results obtained with microwave systems have been reported but not much is published using lower frequencies (<1GHz). FOI has been active in this part of the electromagnetic spectrum for many years with the development and operation of two airborne SAR sensors, i.e. CARABAS-II (20-90 MHz) and more recently LORA (200-800 MHz). During 2006 experimental work was initiated to investigate the challenges of implementing a bistatic low frequency SAR system. Various synchronization tests were made in the lab as a preparation for the first bistatic VHF SAR data registrations. An area in the vicinity of Linkoping city was illuminated using CARABAS-II as the airborne transmitter and the LORA radar electronics as a stationary roof-top mounted receiver unit. The latter was reconfigured to be able to handle the frequency interval 20-90 MHz. The approximately 4.1 km by 4.1 km large common radar scene contains urban environments, open areas and forested parts. The CARABAS-II sensor simultaneously registered monostatic SAR data to facilitate the image interpretation by comparisons although the incidence angle on receive differs considerably.
This paper describes work that considered two Joint Time-Frequency Transforms (JTFTs) for use in a SAR-based (single sensor/platform Synthetic Aperture Radar) 3D imaging approach. The role of the JTFT is to distinguish moving point scatterers that may become collocated during the observation interval. A Frequency Domain Velocity Filter Bank (FDVFB) was compared against the well-known Short Time Fourier Transform (STFT) in terms of their maximal Time-Frequency energy concentrations. The FDVFB and STFT energy concentrations were compared for a variety of radar scenarios. In all cases the STFT achieved slightly higher energy concentrations while simultaneously requiring half the computations needed by the FDVFB.
The detection and tracking of humans and vehicles on the battlefield using radar systems operating at microwave frequencies was first achieved almost 40 years ago. The subsequent generation of radars designed to detect personnel and vehicles on the battlefield has seen improvements due to increased signal processing capability. To date, most of the self-contained human detection radars have incorporated a co-located (monostatic) transmitter and receiver operated by humans. Approximately, three decades ago the bistatic radar was introduced and used for security at high value target sites. These bistatic "fence" radars employ a transmitter located at one end of a bistatic baseline and a receiver at the other end of the baseline. The receiver is tuned to the transmitter. Operation is simple; an intruder crosses the bistatic baseline and is detected after simple signal processing is performed on the bistatic signature produced by the intruder. The experiments demonstrate that passive bistatic radar can be used to detect humans and vehicles. This paper describes "quick-look" experiments that have been conducted in the Atlanta, Georgia area to detect humans and vehicles using a passive radar configuration requiring no coordination between the receiver and transmitter. The illumination source (transmitter) is a High Definition Television (HDTV) broadcast transmitter located approximately 13.5 miles from the test area. The transmitter is broadcasting a 6 MHz wide digital signal with a pilot carrier on a frequency of 548.310 MHz. The continuous wave (CW) pilot carrier HDTV signal component is processed to extract the signature of the walking human or the signature of a vehicle. The experimental receiving system utilizes a commercial off-the-shelf (COTS) communications receiver. A set of multi-element back to back Yagi antennas are used to provide a reference signal and the signal from the area where the human subject is located. The walking human generates micro-Doppler that can be detected using micro-Doppler signal processing techniques. Vehicular targets can be detected without applying micro-Doppler processing due to a vehicle's larger radar cross section (RCS) and higher Doppler shift (higher velocity). The technical challenges that are addressed in the following sections include receiver stability, common signal cancellation, multipath environments, and geometries. The technique has also been tested inside of a building and it has been found that walking humans can be detected through walls and down long halls.
One of the challenges of using synthetic aperture radar (SAR) to detect and classify an object behind a wall consists of determining the amount of signal attenuation introduced by the signal's propagation through the wall. This attenuation is difficult to determine because the electromagnetic properties of the wall, along with its thickness are normally not known a priori. We describe a procedure for determining the necessary parameters given that the SAR has high enough resolution such that the front and the rear surfaces of a uniform wall or cinder block wall can be determined from the SAR image. In addition, we provide a procedure for estimating the signal level behind the wall, or equivalently the attenuation due to the wall, from measured returns from its front and rear surfaces. We demonstrate the effectiveness of this procedure using data generated by XPATCH simulations.
The quality and reliability of through-the-wall radar imagery is governed, among other things, by the knowledge of the wall characteristics. Ambiguity in wall characteristics has a two-fold effect. It smears and blurs the image, and also shifts the imaged target positions. Higher order standardized moments have been shown to be suitable measures of the degree of smearing and blurriness of through-the-wall images. These moments can be used to tune the wall variables to achieve autofocusing. It is noted that the solution to the autofocusing problem is not unique, and there exist several assumed wall characteristics, in addition to the exact, that lead to similar focused images. In this paper, we analyze the dependency of the estimated autofocusing wall parameters on the imaged scene, specifically target density and location, in the presence of single uniform wall. We consider single and multiple target cases with different scene complexity and population. Supporting simulation results are also provided.
Micro-Doppler refers to Doppler scattering returns produced by non rigid-body motion. Micro-Doppler gives rise to many detailed radar image features in addition to those associated with bulk target motion. Targets of different classes (for example, humans, animals, and vehicles) produce micro-Doppler images that are often distinguishable even by nonexpert observers. Micro-Doppler features have great potential for use in automatic target classification algorithms. Although the potential benefit of using micro-Doppler in classification algorithms is high, relatively little experimental (non-synthetic) micro-Doppler data exists. Much of the existing experimental data comes from highly cooperative targets (human or vehicle targets directly approaching the radar). This research involved field data collection and analysis of micro-Doppler radar signatures from non-cooperative targets. The data was collected using a low cost Xband multiple frequency continuous wave (MFCW) radar with three transmit frequencies. The collected MFCW radar signatures contain data from humans, vehicles, and animals. The presented data includes micro-Doppler signatures previously unavailable in the literature such as crawling humans and various animal species. The animal micro-Doppler signatures include deer, dog, and goat datasets. This research focuses on the analysis of micro-Doppler from noncooperative targets approaching the radar at various angles, maneuvers, and postures.
In support of the U.S. Army's need for intelligence on the configuration, content, and human presence inside enclosed areas (buildings), the Army Research Laboratory is currently engaged in an effort to evaluate RF sensors for the "Sensing Through The Wall" initiative (STTW).Detection and location of the presence of enemy combatants in urban settings poses significant technical and operational challenges. This paper shows the potential of hand held RF sensors, with the possible assistance of additional sources like Unattended Aerial Vehicles (UAV), Unattended Ground Sensors (UGS), etc, to fulfill this role. In this study we examine both monostatic and multistatic combination of sensors, especially in configurations that allow the capture of images from different angles, and we demonstrate their capability to provide comprehensive information on a variety of buildings. Finally, we explore the limitations of this type of sensor arrangement vis-a-vis the required precision in the knowledge of the position and timing of the RF sensors. Simulation results are provided to show the potential of this type of sensor arrangement in such a difficult environment.
Technology that can be used to unobtrusively detect and monitor the presence of human subjects from a distance and through barriers can be a powerful tool for meeting new security challenges, including asymmetric battlefield threats abroad and defense infrastructure needs back home. Our team is developing mobile remote sensing technology for battle-space awareness and warfighter protection, based on microwave and millimeter-wave Doppler radar motion sensing devices that detect human presence. This technology will help overcome a shortfall of current see-through-thewall (STTW) systems, which is, the poor detection of stationary personnel. By detecting the minute Doppler shifts induced by a subject's cardiopulmonary related chest motion, the technology will allow users to detect personnel that are completely stationary more effectively. This personnel detection technique can also have an extremely low probability of intercept since the signals used can be those from everyday communications. The software and hardware developments and challenges for personnel detection and count at a distance will be discussed, including a 2.4 GHz quadrature radar single-chip silicon CMOS implementation, a low-power double side-band Ka-band transmission radar, and phase demodulation and heart rate extraction algorithms. In addition, the application of MIMO techniques for determining the number of subjects will be discussed.
It is well-known that Non-Linear FM (NLFM) chirp modulation can advantageously shape the transmitted signal's Power Spectral Density such that the autocorrelation function (i.e. matched filter output) exhibits substantially reduced sidelobes from its Linear FM (LFM) counterpart. Consequently, no additional filtering is required and maximum Signal-to-Noise Ratio (SNR) performance is preserved. This yields a 1-2 dB advantage in SNR over the output of a LFM waveform with equivalent sidelobe filtering. However precision NLFM chirps are more difficult to design, produce, and process. This paper presents design and implementation techniques for Nonlinear FM waveforms. A simple iterative design procedure is presented that yields a NLFM phase/frequency function with the desired inherent sidelobe response. We propose to then generate the NLFM waveform by using a cascaded integrator/accumulator structure. Several specific architectures are examined to meet target performance criteria, including bandwidth constraints and sidelobe reduction goals. We first examine a fixed parameter set to generate a fixed polynomial phase function. Polynomial coefficients are selected to be constant during the pulse. Alternatively, a NLFM waveform can be generated via integrating a stepped parameter set, whereby parameters are constant over specific intervals, with the pulse width encompassing multiple intervals. The parameter changes in steps during the course of the pulse as a function of time. Alternatively yet, the parameter steps can be made a function of the pulse's instantaneous frequency.
In this paper, vehicle-mounted ultra-wide band (UWB) radar is studied for detection of roadside improvised explosive devices (IEDs). Simulations and measurements have been performed to study the radar system set-up, target scattering, and data processing. The challenges of UWB radar for target visibility within the ground clutter, locating with limited angular diversity, and discrimination of buried bomb shells have been encountered and explored. Possible solutions to overcome these challenges are investigated.
Synthetic aperture radar (SAR) imagery is formed using radar data collected from a moving platform (aircraft, vehicle, human, etc.). The radar transmits and receives backscatter signals in the down-range direction at a fixed pulse repetition interval (PRI) while the platform moves along the cross-range direction (called along-track) to generate a synthetic aperture. In the ideal situation, the platform moves at a constant speed and as a result, the radar will collect the phase-history data that are uniformly sampled along the aperture. However, in many situations the radar platform cannot be kept at a constant speed, e.g. a helicopter maneuvering over an imaging area for surveillance. The problem is even worse in the case of urban warfare with human-borne radar. A soldier moves at his own speed and creates erratic aperture sections with phase-history data that are either sparse or dense. The collected SAR data in such situation will result in SAR imagery with severe artifacts that might prevent us from detecting targets of interest. In this paper, we will present the SAR imagery of non-uniform aperture data formed using the backprojection image formation algorithm. Although the backprojection image former is well suited to an arbitrary radar aperture, the SAR image artifacts are obvious from the nonuniform aperture. Using the nonuniform aperture phase-history data, we interpolate the data using a uniform grid along the aperture. We will show the resulting imagery with reduced artifacts. We use both simulated data and the Army Research Lab BoomSAR data to illustrate the artifacts generated by nonuniform sampling and the improvement using this interpolation technique.
Technology for the detection of enemies from behind barriers and for securing of ports and perimeters with minimal threat to warfighters is essential in modern threat scenarios. We are developing a network of small scattered Doppler radar sensors which lie in wait and report on change or motion within a targeted perimeter. Most sensors are simple radar receiver "nodes" capable of short range communications and long operation life with minimal power requirements, while a few are more advanced radar transceiver "beacons" capable of active interrogation and long range communications. Radar nodes and beacons could be scatter-deployed from a distance, creating a need for post-deployment localization in order to provide useful reconnaissance. A beacon is designed to have absolute position knowledge by strategic deployment of GPS, produces an interrogation signal, and analyzes locally received echoes for signs of motion activity in the targeted area. Scattered nodes in the targeted vicinity form an ad-hoc network which also receives and compares the beacon signal and its target echoes, and reports sensed activity to the beacon. This paper introduces such a system and discusses radar node localization based on signal strength using kernel methods and distributed learning algorithms which take energy constraints into account.
In this paper a Ku-band (15GHz), dual polarization, combined short-pulse scatterometer-radiometer is developed for short distance remote sensing of the water surface, bare soil and snow cover, as well as for simultaneous and coincident measurements of the microwave reflective and emissive characteristics of the observed medium under laboratorycontrolled conditions. The system allows us carry out polarimetric (vv, vh, hh, hv), simultaneous and coincident microwave active-passive measurements of the observed surface (soil, vegetation, snow and water surface) parameters at angles of incidence from 0-60°. The originality of the developed system is in the spatial-temporal combination of microwave active and passive channels of observation and its application for short distance sensing (the minimum operational range for the scatterometer is ~6m) from low altitude platforms under far field conditions for both radar and radiometric observations.
In this paper a C-band (~5.6GHz), double channel, polarimetric, combined short-pulse scatterometer-radiometer system is described. The system was developed for short distance remote sensing application (from 6m up to 100m), from stationary fixed platforms or vessels. The minimum operational range for the scatterometer is 6m. This capacity allows study correlative features between microwave reflective and emission characteristics of the observed surfaces and medium under control-test laboratory conditions. Although the system was developed for polarimetric (vv, vh, hh, hv), simultaneous and spatially coincident microwave active-passive measurements of water surface, bare and vegetated soils and land snow cover parameters, it may be successfully used for atmospheric boundary layer remote survey too.
A description of the design parameters for a scaled RF environment is presented. This scaled RF environment was developed for purposes of simulating and investigating multipath phenomena in urban environments. A number of experiments were conducted with this scaled urban environment including a series of tests with eight spatially distributed receivers and one transmitter. Details with regard to the instrumentation system along with the measurement philosophy are provided. The primary focus of this paper is a detailed treatment of data analysis and exploitation techniques for the multipath data generated by this scaled RF environment. A portion of the material on multipath data analysis and exploitation is focused on developing techniques for identifying a optimum placement of receiver pairs for purposes of maximizing information content on a embedded target. In other words, data from the eight distributed receiver locations are analyzed and techniques are presented that allow for the selection of receiver pairs that provide the most information on targets that are embedded within the multipath environment. The last section of the paper discusses visualization and pseudo-imaging techniques for targets embedded in multipath environments.
Single frequency (Doppler) radars cannot be used in target range estimation due to the associated large range ambiguities. An additional frequency can be used to increase the maximum unambiguous range to values adequate for range estimation of moving targets within buildings and enclosed structures. The dual-frequency technique uses phase comparison of the transmitted and received CW signals to provide an estimate of the target range. It offers the benefit of reduced complexity, fast computation, and real time target tracking. However, the dual-frequency approach for range estimation can be compromised due to the presence of drift in frequency, I/Q mismatch, and noise. In this paper, we analyze the effect of I/Q mismatch and noise on the bias and variance of the target range estimate. We consider targets with both linear and simple harmonic motions. Computer simulations are provided for illustrating the performance as a function of signal-to-noise ratio.
Radar systems have long been recognized as an effective tool for detecting moving targets--a problem commonly referred to as moving target indication (MTI). Recent advances, including Space Time Adaptive Processing (STAP), allow for even more precise determination of a target's location relative to the radar. Still, most of these methods approach MTI from the point of view of parameter estimation, and this sort of an approach can become problematic when the target speed is low and its associated Doppler frequency is near zero. In such cases the target signature is masked by the stationary, background clutter. Another potential drawback to STAP techniques arises from the fact that they require a relatively large number of receive channels, adding additional complexity to the radar system hardware. In this paper we present a moving-target-indication (MTI) technique that is based on a change detection paradigm. That is, rather than estimating the Doppler frequency associated with a target's motion, we propose to detect subtle differences between simultaneously collected, complex SAR images. We use simulated data to illustrate the feasibility of the approach under several different operating scenarios.
This paper describes the development of an algorithm for detecting multiple-scattering events in the 3D Geometric Theory of Diffraction (GTD)-based Jackson-Moses scattering model. This approach combines microlocal analysis techniques with geometric-invariant theory to estimate multiple-scattering events. After multiple-scattering returns were estimated, the algorithm employed the Generalized Radon Transform to determine the existence of multiple scattering within the measured data. The algorithm was tested on an X-band simulation of isotropic point scatterers undergoing unknown rotational motion.
The Euler decomposition, when applied to the polarization scattering matrix, attempts to extract phenomenological information about the scattering target. Because the Euler parameters constitute a more physically relevant set of parameters than the traditional HH-VV ISAR representations, they have potential to improve ATR performance. The Euler parameter's usefulness in target recognition, however, is effected by several layers of signature variability. Unfortunately, many of the variability layers are often omitted in a typical ATR study. A complete ATR algorithm was therefore developed that allows for all layers of variability and requires no previous knowledge of the target's position, orientation, or average reflectivity. The complete ATR algorithm was then used to assess the effectiveness of Euler ISAR imagery in target recognition when all layers of variability are considered. The general approach and sub-methods used to construct the complete ATR system will be presented, including the methods to determine the targets orientation, registration, and to compare it to a library of pre-rendered target images. Finally, the performance of the Euler parameters in target recognition using the complete ATR algorithm will be presented.
The Rapid Terrain Visualization interferometric synthetic aperture radar was designed and built at Sandia National Laboratories as part of an Advanced Concept Technology Demonstration (ACTD) to "demonstrate the technologies and infrastructure to meet the Army requirement for rapid generation of digital topographic data to support emerging crisis or contingencies." This sensor was built by Sandia National Laboratories for the Joint Programs Sustainment and Development (JPSD) Project Office to provide highly accurate digital elevation models (DEMs) for military and civilian customers, both inside and outside of the United States. The sensor achieved better than HRTe Level IV position accuracy in near real-time. The system was flown on a deHavilland DHC-7 Army aircraft. This paper presents a collection of images and data products from the Rapid Terrain Visualization interferometric synthetic aperture radar. The imagery includes orthorectified images and DEMs from the RTV interferometric SAR radar.