Search

Direct path interference and clutter suppression removal algorithms are a necessity in the processing chain of a passive radar. Most echoes originating from targets of interests are many orders of magnitude weaker than the clutter. However, targets with a high radar cross-section, or close to a transmitter, or to a receiver, may contribute significant power to the recorded signal. If such a target approaches the zero-Doppler line on a range-Doppler plot, it could affect a clutter cancellation algorithm and produce significant, non-removable sidelobes. In this paper, the effects of such processing are presented for an Extensive Cancellation Algorithm. An off-grid method of estimation and removal of an endoclutter target is presented. The effectiveness of the algorithm is presented on both simulated and measured data sets.

This paper reports on two measurement campaigns aimed at investigating the importance of atmospheric variability in several applications, including passive radar. First, UHF wide-band spectrometry analyses seek to quantify diurnal statistics, power series correlation, and signal reliability. Second, long-term radar detection data further enhances the understanding of how this variability affects both clutter and moving targets. Results demonstrate the dynamic nature of the illumination environment as well as enhancement of clutter and targets. Beyond ducting enhancement, phenomena seen in the data include station dropout, unexpected illuminators, rainstorms echoes, and other unexplained diurnal features.

This paper proposes an alternative form of the Keystone Transform (KT), which is known for mitigating range walk in radar returns. While traditionally applied as an interpolation to the cross ambiguity function (CAF), the present work shows that the KT can be implemented prior to, and agnostic of, the cross correlation operation. The new time-domain KT algorithm is derived as a time-variant filter and demonstrated against simulated data. The computational cost, and architectural flexibility offered by the filter, are each examined.

Modern radio telescopes commonly use antenna arrays, and high-resolution imaging techniques exploiting radio astronomical signals collected at these antenna arrays play a critical role to achieve their missions. Beamforming techniques have been developed in radio astronomy to generate dirty images with limited image resolutions for many years. Because the manifold of a radio telescope array varies over time due to the Earth rotation, beamformers are separately designed and implemented at each time epoch, and the resulting images are averaged to form enhanced dirty images. Considering the fact that astronomical scenes are typically sparse, we present a new method through sparse reconstruction to obtain clean astronomical images. Sparse reconstruction methods that fuse the measured data observed at multiple time epochs are examined and compared. Unlike beamforming techniques which require an additional deconvolution procedure for clean image formation, the proposed technique provides clean astronomical images with accurate estimation of the source position and a high dynamic range.

Large-aperture, wide-band antenna arrays are important in both scientific (e.g., radio astronomy) and surveillance (e.g., radar) applications. The practical constraints of signal processing can limit these systems to produce a single (or few) beams, each of which is vulnerable to sidelobe interference. The disambiguation of legitimate (main lobe) signals from interference signals is of paramount importance in many applications. This paper considers time-modulated-array techniques for interference tagging as a mitigation approach and considers empirical statistics of interference detection vs. the beam-compressive ratio of a multi-beam system. The time-modulated approach described in the paper expands on our previous work, in which it was demonstrated that alternating a real-time beamformer between multiple sets of weights can multiplex several different beams (acquisition and one or more sidelobe suppression) within the beamformer output for the same computational cost as a single beamformer. Alternating the weights based on pseudo-random codes implements a CDMA-like scheme, which exploits the frequency-domain sparsity of the received signals. Further, using a single SLS beam minimizes the impact on the resultant detection SNR, but our previous work showed that this also introduces challenges when the underlying array is spatially sparse. The present work reported here will describe cost/performance tradeoffs of this technique when the number of auxiliary beams is changed; increasing this quantity will increase the probability of detection of RFI, but also will decrease the SNR (and PD) of weak target signals. This approach is compressive in the beam-space domain.

This paper reports on a simulation-based study estimating illuminator usage statistics in passive radar systems with mobile receivers. A metric termed illuminator survivability is proposed to describe the size of a geographic region, with respect to the receiver location, in which a transmitter facilitates near-optimal localization capability. We attempt to relate along-track survivability to the maximum operating region of a circling receiver using a static illuminator set. In each case, the radar's coverage is optimized using a greedy algorithm based on the multistatic range CRLB. Results suggest individual illuminator sets might provide useful localization capability over receiver flight paths of 100 km or more.

This paper presents results from model-based work investigating the influence of atmospheric refractivity on the performance of UHF passive radars, using parabolic-equation (PE)-based propagation techniques. It is shown that, since PE-based models are monochromatic, modeling the illuminator bandwidth can smooth their usual multipath power variations; this smoothing can affect the maximum predicted range of the radar. Further, it is shown that changing atmospheric conditions (refractivities) can impart meaningful variations in illumination and path loss at a variety of ranges and altitudes, ultimately affecting the radar's coverage. Finally, it is shown that, although specific refractivities do affect the realized radar performance, they do not appear to strongly influence the selection of optimal illuminators in channel-constrained passive radars.

Time modulation is applied to an array pattern by alternating a beamformer between two sets of beamforming coefficients. The coefficients are selected so signals arriving in the sidelobes are BPSK modulated by the beam switching, while signals arriving in the main lobe are not. This technique permits sidelobe blanking using a single beamformer, which is simpler than other approaches. Methods of array synthesis are presented along with a simulation example using a 32-element linear array.

This paper discusses the development of a testbed for real-time experimental studies of time-modulated arrays. The proposed architectural design will be described, and the current state of progress will be presented through simulation and over-the-air tests conducted in a lab. The TMA antennas and radios are designed to operate at about 2.3 GHz for a target application of passive radar using XM Radio, in which the TMA capabilities will be targeted at multibeaming and interference suppression.

This paper presents techniques for predicting the resolution of the radar imaging point spread function (PSF). These techniques are useful for predicting the performance of multistatic passive synthetic aperture radars (P-SARs), and passive inverse SARs (P-ISARs), for which the PSF depends on a multitude of factors. Greedy algorithms are used in combination with the fast techniques to optimize the PSF resolution for both incoherent and coherent multistatic imaging scenarios, subject to constraints on combining methods, sparsity, and target isotropy.

This paper compares results of a simulation study and experimental measurement campaign designed to evaluate the performance of a DTV-based passive radar. The simulations considered a tower-mounted receiver in Springfield, Ohio, and predicted the accuracy of localizing airborne targets using the illuminations of regional transmitters. The predicted coverage area was found to vary with the selection of the propagation loss model used by the simulator. Supporting experiments at the site did not favor any particular model, but showed 10 dB variations in both the direct path and clutter over several hours. These results illustrate the need to accurately model tropospheric radio propagation in passive radar performance predictions.

著者人数: 87名, Accepted: 2011-02-08, 資料番号: SA1004099000

The Search for ExtraTerrestrial Intelligence (SETI) finally has its own full-time telescope. The Allen telescope array (ATA) in Northern California was dedicated on October 11, 2007. This array, which will eventually be composed of 350 small radio antennas, each 6.1 m in diameter, is being built as a partnership between the SETI Institute and the University of California Radio Astronomy Laboratory. Last October, Paul G. Allen (who provided the funds for the technology development and the first phase of array construction) pushed a silver button and all 42 antennas of the current ATA-42 slewed to point in the direction of the distant galaxy M81. Specialized electronic backend detectors attached to the ATA began making a radio map of that galaxy and simultaneously began SETI observations of HIP48573, a G5V star near M81 on the sky and a distance of 264 light years from Earth. The Allen telescope array will greatly improve the speed of conducting SETI searches over the next few decades, and it will allow a suite of different search strategies to be undertaken. This paper summarizes some of the earliest SETI observations from the array, and describes the search strategies currently being planned.

We present several techniques for maximizing the contact time between low Earth orbiting satellites (LEOs) and a ground station (GS). The GS comprises an adaptive array of electronically steered space-fed lenses (SFLs). Each SFL is manufactured as a low-cost printed circuit with the result that it exhibits scanning loss. By differently orienting the boresights of the SFLs in the adaptive array, the SFL's scanning losses can be made to optimally complement the path loss of the LEO, thereby reducing the cost of the GS while maximizing the download capacity of the satellite link. The optimization, implemented with a genetic algorithm (GA), can be viewed as a kind of pattern synthesis. Such arrays will benefit Earth exploration satellite service (EESS) and telemetry applications, promising a decreased cost and increased reliability as compared with GSs consisting of a large dish antenna. We show that a network of these GSs comprising a total of fourteen small antennas can achieve an average daily data rate that is comparable with that of a single large dish antenna for the Earth Observing One (EO-1) satellite, without increasing the output power of the satellite. We also analyze the case in which the satellite transmits with a variable bit rate (VBR). Furthermore, we show that by selectively populating the focal surface of the SFL with feeds, simultaneous communications with multiple satellites can be achieved with a single GS.

We present the Allen Telescope Array Twenty-centimeter Survey (ATATS), a multi-epoch (12 visits), 690 deg(2) radio image and catalog at 1.4 GHz. The survey is designed to detect rare, very bright transients as well as to verify the capabilities of the ATA to form large mosaics. The combined image using data from all 12 ATATS epochs has rms noise sigma = 3.94 mJy beam(-1) and dynamic range 180, with a circular beam of 150 '' FWHM. It contains 4408 sources to a limiting sensitivity of 5 sigma = 20 mJy beam(-1). We compare the catalog generated from this 12 epoch combined image to the NRAO VLA Sky Survey (NVSS), a legacy survey at the same frequency, and find that we can measure source positions to better than similar to 20 ''. For sources above the ATATS completeness limit, the median flux density is 97% of the median value for matched NVSS sources, indicative of an accurate overall flux calibration. We examine the effects of source confusion due to the effects of differing resolution between ATATS and NVSS on our ability to compare flux densities. We detect no transients at flux densities greater than 40 mJy in comparison with NVSS and place a 2 sigma upper limit of 0.004 deg(-2) on the transient rate for such sources. These results suggest that the greater than or similar to 1 Jy transients reported by Matsumara et al. may not be true transients, but rather variable sources at their flux density threshold.

A 3-D pattern-space representation is presented for volumetric arrays. In this representation, the radiation pattern of an array is formed by the evaluation of the 3-D pattern-space on a spherical surface. The scan angle of the array determines the position of this surface within the pattern-space. This pattern-space representation is used in conjunction with a genetic algorithm to minimize the sidelobe levels exhibited by a thinned volumetric array during scanning.

We report radio SETI observations on a large number of known exoplanets and other nearby star systems using the Allen Telescope Array (ATA). Observations were made over about 19000 hours from May 2009 to Dec 2015. This search focused on narrow-band radio signals from a set totaling 9293 stars, including 2015 exoplanet stars and Kepler objects of interest and an additional 65 whose planets may be close to their Habitable Zone. The ATA observations were made using multiple synthesized beams and an anticoincidence filter to help identify terrestrial radio interference. Stars were observed over frequencies from 1- 9 GHz in multiple bands that avoid strong terrestrial communication frequencies. Data were processed in near-real time for narrow-band (0.7- 100 Hz) continuous and pulsed signals, with transmitter/receiver relative accelerations from -0.3 to 0.3 m/s^2. A total of 1.9 x 10^8 unique signals requiring immediate follow-up were detected in observations covering more than 8 x 10^6 star-MHz. We detected no persistent signals from extraterrestrial technology exceeding our frequency-dependent sensitivity threshold of 180 - 310 x 10^-26 W / m^2., Comment: 225 pages including very long table, 9 figures, 7 tables, resubmitted to Astrophysical Journal

This paper describes the design and fabrication of an 8-element wideband Vivaldi antenna array with an integrated feed network. This array was created for use in an X/Ku-band measurement system for characterizing dynamic rocket plasma, extending previous measurements of this phenomenon. The array is instrumented to help distinguish plume-induced mechanical changes from electrical ones.

The formulation of multibeaming in time modulated arrays (TMAs) is generalized for nonsinusoidal basis functions. This can overcome ambiguities between angle and frequency present in TMA harmonic beams and detrimental to radars and spectrometers. This concept is used to analyze the Doppler VHF Omni Range (DVOR), and it is shown that the DVOR is a TMA using a single-tone frequency-modulated basis function.

We report a novel radio autocorrelation (AC) search for extraterrestrial intelligence (SETI). For selected frequencies across the terrestrial microwave window (1-10 GHz) observations were conducted at the Allen Telescope Array to identify artificial non-sinusoidal periodic signals with radio bandwidths greater than 4 Hz, which are capable of carrying substantial messages with symbol-rates from 4-1000000 Hz. Out of 243 observations, about half (101) were directed toward sources with known continuum flux > ~1 Jy over the sampled bandwidth (quasars, pulsars, supernova remnants, and masers), based on the hypothesis that they might harbor heretofore undiscovered natural or artificial, repetitive, phase or frequency modulation. The rest of the targets were mostly toward exoplanet stars with no previously discovered continuum flux. No signals attributable to extraterrestrial technology were found in this study. We conclude that the maximum probability that future observations like the ones described here will reveal repetitively modulated emissions is less than 1% for continuum sources and exoplanets, alike. The paper concludes by describing a new approach to expanding this survey to many more targets and much greater sensitivity using archived data from interferometers all over the world., 33 pages, 4 figures, 3 tables

This paper presents results of a simulation study quantifying the expected importance of cochannel interference (CCI) in a passive radar using Advanced Television Systems Committee (ATSC) illuminators. CCI stations broadcasting on the same frequency as the illuminator are modeled as additional direct path interference. It is found that spatial nulls alone can be insufficient to suppress CCI in small airborne radars, which might observe up to 10 significant CCIs for each illuminator. A proposed hybrid algorithm uses remodulation to suppress the strongest CCI and nulls only CCI that cannot be decoded. This method improves predicted coverage as compared to nulls alone.

This paper reports on analytical and simulation-based investigations of the Keystone transform (KT), which is used to remove range walk in radar systems. The KT modifies the cross ambiguity function to fix a radar target at its initial range, allowing longer integration times and higher SNRs. Limits on the useful length of integration time are explored, demonstrated, and found to depend on residual range migration caused by the acceleration of the target. A second order double-Keystone transform is then presented as an alternative that corrects for both velocity-based and acceleration-based range walk and enables chirp detection of an accelerating target.

Accurately predicting propagation loss is critical to determining if an area can support passive bistatic radars (PBRs), which must rely on illuminators of opportunity and might operate near the threshold of detection for weak targets. Models available to calculate propagation loss vary in their complexity and might include factors such as terrain, refraction, clutter, and multipath. This paper reports on work comparing two such models, the Advanced Refractive Effects Prediction System (AREPS) and the Irregular Terrain Model (ITM). An interface between AREPS, ITM, and MATLAB was developed to enable model comparisons for similar configurations. Results were incorporated into a PBR simulator to examine how the model differences affect localization performance predictions.

The affinity of lightning to a charged rocket plume depends on the plume's electrical properties, many of which are poorly characterized in theoretical models. The vulnerability of rockets to lightning-induced vehicle failure motivates study of this phenomenon. This paper describes experimental efforts to determine the plasma frequency and electron number density of plumes from static firings of amateur high power rocket motors, by analysis of the microwave transmission characteristics. Two new techniques presented include time delay measurements and using reflectors to sample multiple paths in the plume.

This paper presents a design for a passive tag to be used as a transponder for unmanned aircraft systems. The tag is designed to scatter digital television signals with an imposed modulation, negating the need for an onboard transmitter. The ground station processes the backscattered signal using passive coherent location techniques. Results suggest the feasibility of communication between a ground station and aerial vehicle with relatively high data rate, long range, and minimal power consumption without further congesting existing bands such as 1030/1090 or 978 MHz.

Time modulation is explored as a tool for inexpensive sidelobe blanking in a digital beamformer. Rapid switching of the steering coefficients encodes acquisition and sidelobe suppression beams within a single output during beamforming so that they are separable by a synchronized decoder. The computational cost is the same as traditional beamforming, making this method advantageous versus other sidelobe blanking methods. Experimental results are shown using data from the Allen Telescope Array.

We provide performance information on the reduction of sidelobe levels in sparse linear phased arrays operating in the near-field. Iterative optimization techniques are used to choose inter-element spacings which reduce grating lobe magnitudes at select distances, and array configurations are generated with constraints on the minimum spacing between elements. The performance of these array configurations is then analyzed over a scan angle of 180 degrees. We show that it is possible to generate arrays which outperform the side-lobe level of far-field-optimized array configurations in the near-field by as much as 3 dB.

Time-modulation is applied to an array pattern by alternating a beamformer between two sets of beamforming coefficients. These are selected so signals arriving in the sidelobes are BPSK modulated by the beam-switching while signals in the main lobe are not. This technique enables sidelobe blanking using a single beamformer, which is simpler than other approaches.

This paper describes passive radar observations using the ATSC digital television signal as an illuminator. The system has two unique attributes. First, it recovers the reference signal by remodulation of the surveillance channel, a process which permits a compact single-antenna receiver. Second, it is shown that keystone formatting of the cross-ambiguity data mitigates range walk of moving targets and allows meaningful increases to the integration time and sensitivity.

A method of communications is presented based on the backscatter modulation of terrestrial digital television signals by low-complexity tags. The modulation is sensed by receivers implementing passive coherent detection algorithms similar to those used in passive radar. This method enables shared use of the UHF television band for low-data-rate applications. Analyses and experiments suggest the feasibility of this technique but also highlight the unique challenges for designing such a system.

We present performance bounds obtained from the optimization of the sidelobe levels of aperiodic linear arrays. The antennas comprising these arrays are large compared to the distance between neighboring antennas, a case not addressed in previously published work. This optimization is performed in pattern-space and is applicable over a wide range of scan angles. We show that grating lobes can be suppressed even when the elemental antennas are several wavelengths in size, provided that the ratio of the antenna size to the average spacing between the antenna center-points does not exceed 80%.

Beamforming methods of maximal ratio combining (MRC) and equal gain combining (EGC) are compared in the presence of calibration errors. It is found that sufficient uncertainties in the antenna sensitivities make MRC less effective than EGC. Furthermore, the threshold noise temperature for including a new antenna in a time-limited survey is explored, and is found to be lower than previously-described results.

RF Tomography is a technique to reconstruct an image of the area of interest using distributed transmitters and receivers assumed to be cooperating. This paper explores the possibility of using RF tomography when the sources' locations and waveforms are not known. With this added complexity, the system must 1) sense the environment to detect radar waveforms of opportunity 2) estimate the source's locations 3) use a tomographic reconstruction based upon match-filtered data, where the phase information of targets is practically lost. This paper addresses these steps through a new formalization of the tomographic model and associated signal processing. Simulations and experimental results are shown.

This paper describes the simulation approach and model for a software tool used to analyze passive radar scenarios. A database of known transmitters includes more than 250,000 sources from FM radio, digital television, cellular, and aeronautical radio-navigation aids. Terrain is considered using digital elevation maps and the Irregular Terrain Model. The tool calculates the expected detection and localization performance for PBR receivers, and can be used to guide receiver design.

This paper reports on early observation results from the S-band Array for Bistatic Electromagnetic Ranging Technology Demonstrator Array. This system is a demonstration platform for a passive ground-based primary radar station for monitoring aircraft. This station is unique in that it utilizes existing satellite-based radio signals as transmitters rather than terrestrial signals. Using algorithms that enable long integration times, this system might provide accurate position and velocity measurements at ranges of tens of nautical miles from the ground station for most targets, and up to 100 nautical miles for larger targets. In these measurements, XM-Radio was used as the illuminator and the SABER-TDA station as the receiver. Different types of aircraft were observed in different scattering geometries. Comparison with truth data is in progress.

This paper presents a remote, human-controlled, immersive Unmanned Robotic System (URS) designed to assist in search and rescue operations. The system is being designed and constructed by the Embry-Riddle Aeronautical University Department of Electrical, Computer, Software, and Systems Engineering 2011/2012 Senior Design Team. Described in this paper are two different implementations of the URS - one is controlled using a handheld tablet and the other is controlled by way of a Human Robotic Interaction (HRI) head-mounted display. This paper outlines the architecture, implementation, and design considerations for each system.

The unknown radio propagation delay of the ionosphere is well-known as a potentially-limiting effect of microwave navigation systems such as GPS. We report on an analysis on the effects of the ionosphere on passive bistatic radar using non-cooperative satellite-based transmitters. Ionospheric data for a one-year period during 2010 are analyzed in the context of the bistatic geometry. Both geostationary (e.g., XM-Radio) and low Earth-orbiting spacecraft (e.g., Iridium) are considered. It is found that the worst-case effects of the ionosphere on a radar of this type might be meaningful for systems attempting to achieve the best accuracy at the longest range, but that the effect is small enough to be neglected for close-range measurements.

We demonstrate a signal processing concept for imaging the sky at millisecond rates with radio interferometers. The "Pocket Correlator" (PoCo) correlates the signals from multiple elements of a radio interferometer fast enough to image brief, dispersed pulses. By the nature of interferometry, a millisecond correlator functions like a large, single-dish telescope, but with improved survey speed, spatial localization, calibration, and interference rejection. To test the concept, we installed PoCo at the Allen Telescope Array (ATA) to search for dispersed pulses from the Crab pulsar, B0329+54, and M31 using total-power, visibility-based, and image-plane techniques. In 1.7 hours of observing, PoCo detected 191 giant pulses from the Crab pulsar brighter than a typical 5 sigma sensitivity limit of 60 Jy over pulse widths of 3 milliseconds. Roughly 40% of pulses from pulsar B0329+54 were detected by using novel visibility-based techniques. Observations of M31 constrain the rate of pulses brighter than 190 Jy in a three degree region surrounding the galaxy to, 13 pages, accepted to ApJ

This research reports on the construction of a demonstration platform for a passive ground-based primary radar station for monitoring aircraft. The key to the system is the use of existing satellite transmitters as illuminators for the radar system. This proposed system can provide accurate position and velocity measurements of aircraft or small unmanned aircraft at ranges of tens to hundreds of nautical miles away from the ground station. This station is a light-weight, low-cost, portable, and field-deployable option to supplement deficiencies in the National Airspace System (NAS) and homeland security surveillance networks. Potential applications include providing coverage in remote mountainous regions, low-altitude en-route primary radar coverage throughout the continental United States, and low-altitude interdiction efforts in coastal areas. As a field-deployable system, this could also be used to quickly restore primary radar coverage in the event that a disaster or terrorist event disables existing primary radars. Additionally, the portable and non-emitting nature of the radar permits use in a wide range of applications where emitting radars are unacceptable. Previous research in the area of passive radar has used terrestrial sources as illuminators since satellite sources have been traditionally viewed as too weak. What makes this research unique is that it takes specialized signal processing techniques typically used to detect objects in deep space for radio astronomy applications and applies them to the domain of terrestrial bistatic radar. The system has been constructed and characterization of system performance is in process.

Using a generous grant from the IEEE Life Member Foundation, the IEEE Daytona Section has acquired, built, and operated a Small Radio Telescope as a tool to support Engineering Education in the Middle and High Schools, as well as Colleges and Universities. This paper describes the project and its educational goals and purposes.

The communication system presented in this paper, uses a directional antenna tracking system to minimize required power consumption on a remote system. The system utilizes the telemetry data from a remote system to determine the required azimuth and elevation angles necessary to track and maintain communications with the vehicle. The system is intended for fully mobile applications where the orientation of the ground tracking station is arbitrary and unknown. This paper describes a self-contained system integrating onboard sensors to automatically calibrate the antenna tracking station.

[1] The Allen Telescope Array (ATA) at the Hat Creek Radio Observatory (HCRO) is a wide-field panchromatic radio telescope currently consisting of 42 offset-Gregorian antennas each with a 6 m aperture, with plans to expand the array to 350 antennas. Through unique back-end hardware, the ATA performs real-time wideband beamforming with independent subarray capabilities and customizable beam shaping. The beamformers enable science observations requiring the full gain of the array, time domain (nonintegrated) output, and interference excision or orthogonal beamsets. In this paper we report on the design of this beamformer, including architecture and experimental results. Furthermore, we address some practical considerations in large-N wideband beamformers implemented on field programmable gate array platforms, including device utilization, methods of calibration and control, and interchip synchronization.