Adaptive beamforming for high-frequency over-the-horizon passive radar

Target detection and tracking systems using emitters of opportunity have received significant interest recently, especially those which exploit VHF and UHF broadcasts as signal sources in so-called passive radar systems. Here, the authors discuss an experimental system in the high-frequency (HF) band, where due to long-distance ionospheric propagation of radio waves in the 3?30 MHz spectrum, the illuminator may be located well beyond the line-of-sight. In this study, live data was recorded by a high dynamic range multichannel digital receiver connected to a two-dimensional (L-shaped) antenna array, and signals from an uncooperative HF over-the-horizon (OTH) radar transmitter have been captured and analysed. As a preliminary step towards the development of a general HF-OTH passive radar system, the scope of this work is to compare the performance of conventional and adaptive spatial processing techniques in terms of their ability to cancel direct-wave interference and protect useful signal echoes to detect a small cooperative aircraft target. In particular, an alternative adaptive beamforming method specifically tailored to this application is proposed, and its practical performance is compared with classical and standard adaptive beamforming approaches. GPS data measured on-board the cooperative aircraft provided accurate ground truth of the flight path, enabling target profiles in bi-static range, Doppler frequency and direction-of-arrival (azimuth/elevation) to be calculated as a function of time. This information permitted the different processing schemes to be evaluated with a high degree of confidence. The experimental system and live data analysed are exclusively from the HF Radar program of the Defence Science and Technology Organisation (DSTO), Australia.     

Mutual interference and low probability of interception capabilities of noise radar

Recently, there has been considerable interest in noise radar over a wide spectrum of applications, such as through-wall surveillance, tracking, Doppler estimation, polarimetry, interferometry, ground penetrating or subsurface profiling, detection, synthetic aperture radar (SAR) imaging, inverse SAR imaging, foliage penetration imaging etc. Major advantages of using noise in the transmit signal are its inherent immunity from radio frequency and electromagnetic interference, improved spectrum efficiency, and hostile jamming as well as being very difficult to detect. The basic theory of digital signal processing in noise radar design is treated. The theory supports the use of noise waveforms for radar detection and imaging in such applications as covert military surveillance and reconnaissance. It is shown that by using wideband noise waveforms, one can achieve high resolution and reduced range estimation ambiguity. Mutual interference and low probability of interception capabilities of noise radar are also evaluated. The simulation results show the usefulness of the noise radar technology to improve on conventional radars.     

Improvement of high-frequency surface waves radar performances by use of multiple-input multiple-output configurations

A multiple-input multiple-output (MIMO) configuration has been studied for communication applications, offering a lot of advantages to mitigate propagation effects because of multipath and fading environments. More recently, MIMO techniques in radar have been proposed. MIMO is discussed in the context of high-frequency surface waves radar (HFSWR). After a short introduction to the MIMO radar technique (a technique which is not new; the RIAS developed by ONERA was probably the first MIMO radar), explores two different applications are explored. The first application aims at improving the resolution and accuracy of a coastal radar, proposed here in a bistatic configuration. Such a radar operates with a wide sparse frequency band and with an equivalent wide aperture, taking advantage of the MIMO configuration. Simulation over a congested area of targets demonstrates the benefit of MIMO over the conventional HFSWR radar, especially in terms of separation of targets. The second application consists in a more practical ship-borne HFWSR configuration compatible with space time adaptive processing (STAP) to improve the detection of slow targets. In this case, a limited number of receiving channels is considered whereas STAP uses the degrees of freedom offered by the transmitting array. The MIMO-STAP is compared with a conventional STAP, showing that theoretical performances should not be affected by the MIMO configuration even if the signals are no longer perfectly orthogonal, because of Doppler shift, once scattered by moving targets.     

Two-dimensional pulse-to-pulse canceller of ground clutter in airborne radar

It is well known that in the airborne radar, the location of the ground clutter spectrum in the angle- Doppler space is dependent mainly on the platform velocity and radar parameters. The authors propose a two-dimensional pulse-to-pulse canceller (TDPC) that can make full use of such prior information. The more detailed formulations of the ground clutter model and the signal model are given in a matrix?vector form. The least-squares-typical cost function associated with the filter coefficient matrix of the TDPC is established on the basis of the ground clutter model and the signal model. Like the classical displaced phase centre antenna (DPCA) processing, the proposed TDPC is also a spatial-temporal suppressor of ground clutter and can decrease the ground clutter signals, even though the DPCA condition is not satisfied. The proposed TDPC can also be used as an efficient pre-filtering tool before the conventional moving target indication (MTI) processing and the classical adaptive processing. Moreover, if only the TDPC plus the conventional MTI is used, it takes less computational time than the adaptive canceller. Experimental results show that the proposed TDPC has the satisfactory ground clutter suppression capability by using both simulated data and measured data.     

Accurate Doppler Radar Noncontact Vital Sign Detection Using the RELAX Algorithm

We consider using a Doppler radar for accurate noncontact vital sign detection. The Doppler radar first captures and downconverts the wireless signal that is phase modulated by the physiological movements, and then identifies the human heartbeat and respiration rates by processing the baseband signal. When nonlinear Doppler phase modulation is employed to monitor vital signs without contact, one of the challenges that we encounter is the presence of undesired harmonic terms and intermodulations other than the sinusoids of interest. A spectral estimation algorithm is needed to accurately estimate the sinusoidal frequencies before identifying the heartbeat and respiration rates. The conventional periodogram cannot reliably separate the rich sinusoidal components since it suffers from smearing and leakage problems, particularly for the case of limited data samples. A parametric and cyclic optimization approach, referred to as the RELAX algorithm, is instead suggested to mitigate these difficulties. Both simulated and experimental results are provided to validate the superiority of using the RELAX algorithm for accurate noncontact vital sign detection.      

Using a van-mounted FM-CW radar to detect corner-reflectorroad-boundary markers

The motivation for this experimental work is to explore a car-borne radar to assist the river of a vehicle in determining where the vehicle is located within the road during adverse (blizzard and/or foggy) weather conditions. This paper discusses the advantage of frequency-modulated continuous wave (FM-CW) radar to achieve such a system. Results are reported of detecting corner reflectors placed on the shoulders of the road with a van-mounted FM-CW radar. The corner reflectors were used as markers of road boundaries. These reflectors were ranged with two radar channels operating simultaneously in real time. The radar signals were processed using a digital signal processing (DSP) circuit board. It is shown that the radar could map the corner reflectors on single and/or two road shoulders in real-time operation at a speed up to 50 km/h. This kind of car-borne radar aids snow-plowing work in a heavy snow regions     

Co-channel interference suppression by time and range adaptive processing in bistatic high-frequency surface wave synthesis impulse and aperture radar

Intense co-channel interference (CCI) severely depresses the target detection in high-frequency surface wave radar (HFSWR). In this study, the CCI cancellation algorithm by time and range adaptive processing is proposed for a novel HFSWR ? bistatic HF surface wave synthesis impulse and aperture radar. With the real data, the interference is firstly modelled and then its features are investigated. The analyses show that the same interference prevails over a few but different bins through different channels, whereas the echoes are relatively weak and exist in all bins; in range domain, however, the interference takes over all the bins including positive and negative bins and will spread over the same and considerable Doppler area through different channels, whereas the echoes appear only in partially positive bins. On the basis of the features, the interference covariance matrix can be obtained by selecting the samples whose average power is much higher than that of the others in time domain and in range domain; the samples from either or both of beyond the detectable bins and negative bins can be selected for training. The interference can be cancelled by projecting the polluted data into the orthogonal subspace, constructing the projecting matrix with the eigenvectors associated with large eigenvalues of the covariance matrix. Finally, the segment handling and samples requirement are also discussed for reducing the computation burden. The experimental results are provided to demonstrate the performance of the method.     

Bubble sizing with high spatial resolution

The authors propose a technique that allows them to size bubbles with the same accuracy as with the double-frequency method and to locate them with the same range resolution as with the pulsed Doppler velocimeter. They demonstrate that the signal scattered by the bubble insonified by a high-frequency pulsed ultrasonic field and a low-frequency pumping field is a low-frequency signal sampled at the repetition frequency rate and in which the amplitude is maximum when the bubble resonates. However using a conventional Doppler flowmeter, the maximum amplitude is not detectable when the repetition frequency is a multiple of the pump frequency. The modifications of the signal processing needed to overcome this drawback are discussed and implemented in the conventional Doppler flowmeter. Using this modified setup the lateral and the longitudinal range resolution are the same as in conventional Doppler flowmeters. The resonance frequency thus obtained is also compared to the resonance frequency measured by the double Doppler frequency method. Some practical improvements are proposed to make the system easy to use. Using this latest version, the resonant requencies for ten different bubble sizes are measured and compared. The case in which the nonlinearity effect (due to a bubble at resonance) generates out-of-phase upper and lower sidebands is discussed, and it is demonstrated that this effect is so feeble that it is negligible.     

ISAR imaging of manoeuvring targets with the range instantaneous chirp rate technique

The conventional range instantaneous Doppler (RID) algorithm is a well accepted inverse synthetic aperture radar (ISAR) imaging method for manoeuvring targets. In the RID imaging, the cross-range resolution depends on the instantaneous Doppler of scatterers at the imaging instant. For a high manoeuvring target, the instantaneous Doppler of scatterers may be small at some imaging instants and the satisfactory RID images may not be obtained. On the other hand, a large instantaneous chirp rate is often present for the same scatterer at the same instant for RID imaging. In order to obtain some additional information of a manoeuvring target, a novel ISAR imaging approach, referred to as the range instantaneous chirp (RIC), is proposed based on instantaneous chirp rate of scatterer to provide cross-range resolution. Using the proposed imaging algorithm, with the same received data of RID, a RIC image is generated at the same instant with a different `view`. Therefore the RIC image may provide some additional information that is not shown in the RID image. With both the RIC and RID images, a better target recognition and identification can be achieved for high-manoeuvring targets. The proposed RIC algorithm is verified by raw radar data.     

Comparison of pulse subtraction Doppler and pulse inversion Doppler

In this paper, a new Doppler technique based on pulse subtraction imaging (PSD) is described and compared with pulse inversion Doppler (PID). Combining a nonlinear contrast agent imaging technique with a Doppler process provides a tool for detecting motion of both contrast agents and tissues. This has potential in targeted imaging in which attached microbubbles need to be separated from moving ones and surrounding tissues. The results from both simulation and experiment show that PSD is able to differentiate bubble motion from tissue motion. For Doppler processing conducted at the fundamental frequency, the contrast-to-tissue ratio (CTR) in PSD was 3.3 (±0.4) times higher on average than PID at a mechanical index (MI) of 0.1. At the harmonic frequency, PID was shown to have a 3.1 (±0.4) times higher CTR than PSD. Overall, taken in their optimum processing conditions, PID has a CTR up to 1.9 (±0.4) times higher than PSD. The CTRs for both techniques have also been shown to increase with increasing MI. However, for the same axial Doppler resolution. PSD also allows less energy to be transmitted into the medium, which makes it less disruptive. The relative performances of PSD and PID in terms of the bandwidth of the imaging system are also discussed.     

Pulse inversion Doppler: a new method for detecting nonlinear echoes from microbubble contrast agents

A novel technique for the selective detection of ultrasound contrast agents, called pulse inversion Doppler, has been developed. In this technique, a conventional Doppler or color Doppler pulse sequence is modified by inverting every second transmit pulse. Either conventional or harmonic Doppler processing is then performed on the received echoes. In the resulting Doppler spectra, Doppler shifts from linear and nonlinear scattering are separated into two distinct regions that can be analyzed separately or combined to estimate the ratio of nonlinear to linear scattering from a region of tissue. The maximum Doppler shift that can be detected is 1/2 the normal Nyquist limit. This has the advantage over conventional harmonic Doppler that it can function over the entire bandwidth of the echo signal, thus achieving superior spatial resolution in the Doppler image. In vitro measurements comparing flowing agent and cellulose particles suggest that pulse inversion Doppler can provide 3 to 10 dB more agent to tissue contrast than harmonic imaging with similar pulses. Similar measurements suggest that broadband pulse inversion Doppler can provide up to 16 dB more contrast than broadband conventional Doppler. Nonlinear propagation effects limit the maximum contrast obtainable with both harmonic and pulse inversion Doppler techniques.     

Doppler-Based Airborne Ultrasound for Detecting Surface Discontinuities on a Moving Target

This paper presents a novel concept based on the acoustic Doppler effect for detecting defects on the surface of a rapidly moving object. The proof-of-concept tests show the feasibility of this approach. By impinging and detecting airborne ultrasound on a rotating target with surface notches, the Doppler effect was clearly observed in the spectral domain at the time when the transducer passed over the flawed zone. For continuous monitoring, the gated spectral magnitude is used to discriminate the signals returned from a flawed region against those from sound regions. In addition, two real-time signal processing techniques are proposed.     

Raising the Accuracy of a Panoramic Monopulse Radar

A method is considered for raising the accuracy and noise immunity of a panoramic angular Doppler pulse radar by means of coherent signal processing between periods for frequency modulation unknown a priori. The method gives an algorithm simple to implement and applicable in digital primary processing.     

Bistatic clutter analysis and range ambiguity resolving for space time adaptive processing

The clutter characteristics of bistatic airborne radar are more complex than those of monostatic airborne radar. The clutter spectra not only vary severely with range, but also vary with bistatic configuration. The problem of range dependence is more serious in monostatic airborne radar. In this paper, the geometry of arbitrary bistatic airborne radar configuration is firstly analysed, and a formula for Doppler frequency calculation with the variables of azimuth angle and bistatic range is deduced, which is an efficient tool for bistatic clutter analysis. Because of the severe clutter range dependence, the processing of compensation is indispensable in space time adaptive processing (STAP). However, when range ambiguity occurs, the compensation is difficult to be applied to each clutter range cell. To solve this problem, a range ambiguity resolving approach is further proposed by utilising azimuth elements in phased array. Because this approach will result in spatial degrees of freedom (DOF) loss, the overlapped subarray processing is introduced in order obtain enough spatial DOF for STAP. By doing so, the compensation for mitigating range dependence can be applied effectively to bistatic clutter.     

Analysis and extraction of stepped frequency radar signature for micro-motion structure

Radar signature produced by micro-motion structures contains movement and structure information which is useful for radar target recognition. For stepped frequency (SF) radar, the Doppler modulation is coupled with the stepped carrier frequency. It shifts and smears the high-resolution range profile and makes it difficult to analyse and extract the micro-motion information from the range profile directly. The authors propose to comprehend the SF radar as a special pulse Doppler radar, and the range profile as a Doppler profile for the convenience of motion analysis. The signature of moving targets for SF radar is analysed from this point of view and the equivalent instantaneous Doppler frequency (EIDF) is introduced. Then, a typical micro-motion, rotation, is taken as an example. The sinusoidal vibration of the peaks in the range profile sequence is explained in detail, especially the relationship between the sinusoid parameters and the rotation parameters. An approach to extract the rotation parameters from the range profile or EIDF spectrum sequence is proposed based on the Hough transform. Simulated and experimental results validate the theoretical analysis and the feature extraction method.     

Distributed splitting-tree-based medium access control protocol for multiaccess wireless networks with multipacket reception

The conventional medium access control (MAC) protocols assume that only one packet can be received at a given time. However, with the advent of spread spectrum, antenna arrays or sophisticated signal processing techniques, it is possible to achieve multipacket reception (MPR) in wireless communication networks. A distributed splitting-tree-based MAC protocol that can exploit the MPR capability in the networks is proposed. For the MPR MAC protocol, a closed-form expression of the system throughput is derived based on a Markov chain model. The experimental results show that the MPR protocol can considerably increase the spectrum efficiency compared with the splitting-tree algorithm with its conventional collision resolution method.     

Doppler-Based Airborne Ultrasound for Detecting Surface Discontinuities on a Moving Target

Abstract

This paper presents a novel concept based on the acoustic Doppler effect for detecting defects on the surface of a rapidly moving object. The proof-of-concept tests show the feasibility of this approach. By impinging and detecting airborne ultrasound on a rotating target with surface notches, the Doppler effect was clearly observed in the spectral domain at the time when the transducer passed over the flawed zone. For continuous monitoring, the gated spectral magnitude is used to discriminate the signals returned from a flawed region against those from sound regions. In addition, two real-time signal processing techniques are proposed.     

Applicability of Doppler weather radar based rainfall data for runoff estimation in Indian watersheds – A case study of Chennai basin

Traditionally, India has been vulnerable to various hazards such as floods, droughts and cyclones. About 8% of the total Indian landmass is prone to cyclones. A number of Doppler weather radars are installed in India and their products are utilized for weather predictions and detection of cyclones approaching the Indian coast. Radar-based hydrological studies in various countries have proven that computation of runoff using radar rainfall data could outperform rain gauge network measurements. There are no reported studies on their utilization for hydrological modelling and/or flood-related studies in Indian river basins. A comparison study between Doppler weather radar (DWR) derived rainfall data and the conventional rain gauge data was carried out with hourly inputs at one of the watersheds of Chennai basin, Tamil Nadu, India using HEC-HMS model. The model calibration and validation were performed by comparing the simulated outflow with the observed daily outflow data. The calibrated model was used to predict runoff from two post-monsoon cyclonic storm events with hourly inputs. It was noticed that the discrepancy in the runoff volume was small, but the difference in the peak flow was substantial. Additionally, there was a variation at the time to peak flow using daily and hourly inputs. The results show that the use of radar data may be optional for runoff volume estimation for the watersheds with sufficient rain gauge density, but highly desirable for peak flow and time to peak estimation. Therefore, the DWR derived rainfall data is a promising input for runoff estimation, especially in urban flood modelling.     

高频雷达海杂波混沌特性的实验研究

利用高频地波雷达实测数据 ,从实验角度应用动力学方法对高频海杂波的动力特性作出分析 ,首次得到了高频海杂波呈现混沌特性的结论。这一结论对高频雷达目标探测和海态遥感以及海洋气象的研究都具有重要意义。     

两种超分辨ISAR成象算法

为提高逆合成孔雷达（ＩＳＡＲ）图象的分辨率,本文介绍了连续Ｈｏｐｆｉｅｌｄ神经网络和ＥＳＰＲＩＴ两种超分辨技术。通过对实验ＩＳＡＲ回波数据的处理表明,与常规的ＦＦＴ算法以及普通超分辨成象算法相比较,这两种新算法明显地提高了图象分辨率。