Displaced Phase Center Antenna Research Articles

Deep learning‐based time delay estimation for motion compensation in synthetic aperture sonars

AbstractAccurate and robust time delay estimation is crucial for synthetic aperture sonar (SAS) imaging. A two‐step time delay estimation method based on displaced phase centre antenna (DPCA) micronavigation has been widely applied in motion estimation and compensation of SASs. However, the existing methods for time delay estimation are not sufficiently robust, which reduces the performance of SAS motion estimation. Deep learning is currently one of the cutting‐edge techniques and has brought about a remarkable progress in the field of underwater acoustic signal processing. In this study, a deep learning‐based time delay estimation method is introduced to SAS motion estimation and compensation. The subband processing is first applied to obtain ambiguous time delays between adjacent pings from phases of SAS echoes. Then, a lightweight neural network is utilised to construct phase unwrapping. The model of the employed neural network is trained with simulation data and applied to real SAS data. The results of time delay estimation and motion compensation demonstrate that the proposed neural network‐based method has much better performance than the two‐step and joint‐subband methods.

DPCA-Based Doppler Radar Measurements from Space: Effect of System Errors on Velocity Estimation Performance

Abstract The displaced phased center antenna (DPCA) method of clutter cancellation for ground moving target detection from airborne platforms has been in use for a number of decades. Application of the DPCA method for spaceborne Doppler weather radar velocity estimation was suggested in 2007. The initial description and analysis of the technique was followed several years ago by demonstration using a multiantenna airborne radar. Recent reviews of methods and technology for spaceborne cloud and precipitation radar have also mentioned possible use of DPCA. However, to date, analyses of the application of DPCA to spaceborne Doppler weather radar have assumed that the two channels and antennas are identical, including perfect alignment, and that the DPCA condition is well-satisfied. This study uses simulation to examine the effects of relaxing these assumptions. The simulation method and its validation are discussed, with companion analytical calculations in the appendix. Next, simulations are used to show the effects on the Doppler estimates from errors in pointing and positioning relative to the ideal DPCA. The DPCA technique is relatively robust to possible errors, indicating that a practical DPCA radar system can provide precise Doppler measurements from space. Significance Statement Analytical and simulation results show that the displaced phase center antenna approach can enable spaceborne atmospheric Doppler radar measurements with good accuracy, even in the presence of antenna mispointing and other system errors.

Null Steering in Linear Array Antennas With Electronically Displaced Phase Center Dual-Mode Antenna Elements

The potential of dynamically steering the null locations in equally spaced linear scanning array antennas using the electronically displaced phase center antenna (E-DPCA) technique is investigated for the first time in this communication. The relative coordinates, and thus, the element spacing, of the equally spaced linear array antennas are electronically varied using the E-DPCA technique to adaptively steer the null locations without any physical displacement while maintaining the same overall array length. The versatility of the proposed technique is further explored by: 1) generating consecutive nulls with a minimum resolution of 1° between them and 2) simultaneously steering the null locations and scanning the main beam.

Estimation of Doppler Velocity Degradation Due to Difference in Beam-Pointing Directions

This study investigated the effects of the difference of beam-pointing directions on the Doppler measurements from a spaceborne Ku-band precipitation radar. The Doppler measurement system was a displaced phase center antenna (DPCA) using two antennas that are displaced in the along-track direction. When the beam-pointing directions of the two antennas deviate, Doppler measurements can degrade. The well-known formula for the uncertainty in Doppler velocity measurement is extended to include the effect of the difference of the beam-pointing directions, and a simulation confirmed the formula. The results show that the uncertainty increases from 0.21 to 0.38 m/s for a case of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S/N = 20$ </tex-math></inline-formula> dB when one antenna footprint center is displaced from the other by half of the half-power beamwidth.

Recovering the Elusive Spectral Width From Spaceborne Doppler Profiling Radar Measurements: The “ExpliSyT” Approach

This article presents a novel method to retrieve spectral widths from measurements of spaceborne Doppler profiling radars operating in low-Earth orbit. The proposed “ExpliSyT” approach, is based on a formulation of the Doppler broadening in terms of the distributions of reflectivity and velocity in the footprint. This formulation makes it possible to estimate the broadening and remove it from the spectral-width measurement. Two implementations of the ExpliSyT correction are described: 1) a numerical implementation that uses Wiener’s deconvolution, and 2) an analytical implementation based on Taylor expansions of the radar observations. The ExpliSyT corrections enable retrievals of higher-order Doppler information such as posterior power spectra. Results are shown for simulations of the radar of EarthCARE (Earth Cloud Aerosol Radiation Explorer developed by the European and Japanese space agencies) and a Displaced Phase Center Antenna (DPCA) configuration, for which the ExpliSyT corrections perform significantly better. A novel <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">canonical Doppler resampling diagram</i> is introduced to explain the correlation between non-uniform beamfilling (NUBF) and spectral broadening. This tool shows that 1) the NUBF of reflectivity modulates the spacecraft-induced spectral broadening, and 2) the NUBF of velocity can either broaden or <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">shrink</i> the Doppler spectrum. This spectral shrinkage, although important, is not typically thought of when dealing with spaceborne Doppler radars. The ability to estimate spectral widths accurately from space paves the way for future microphysical and dynamical characterizations of hydrometeors in shear or turbulence.

The Impact of Channel Errors in Passive Coherent Location Radar using FM Base Stations

This paper presents the impact of channel errors for an FM based passive bistatic radar system mounted on mobile platforms for ground moving target indication (GMTI). Reciprocal filter, which is effective compared to conventional matched filter (MF), is performed for the pulse compression stage to remove the time-variant structure of the signal. The displaced phase centre antenna (DPCA) method is applied for the clutter cancellation and target detection. This technique is effective if the hardware is well calibrated. Thereby, the influences of calibration errors between the receiving channels are studied for different FM waveforms. The simulation results of the amplitude and phase errors are analysed separately.

Sidelobe Reductions in Linear Array Antennas Using Electronically Displaced Phase Center Antenna Technique

A novel approach to sidelobe reduction in uniformly-excited, equally-spaced linear arrays using the electronically displaced phase center antenna (E-DPCA) technique is presented for the first time. Antenna elements with the E-DPCA capability are employed in nine- and 21-element linear array antennas for the proof of concept. The element spacing of these physically periodic arrays is electronically tapered by displacing the phase center location, and thus the relative coordinates, of the base elements to reduce the sidelobe and minor lobe levels. Compared to the pre-existing synthesis techniques, the proposed technique provides not only comparable sidelobe reductions but also an additional capability of adaptively changing the array configuration to generate desired patterns without any physical means, while maintaining the same overall array length.

Mind the Gap - Part 3: Doppler Velocity Measurements From Space

Convective motions and hydrometeor microphysical properties are highly sought-after parameters for evaluating atmospheric numerical models. With most of Earth’s surface covered by water, space-borne Doppler radars are ideal for acquiring such measurements at a global scale. While these systems have proven to be useful tools for retrieving cloud microphysical and dynamical properties from the ground, their adequacy and specific requirements for spaceborne operation still need to be evaluated. Comprehensive forward simulations enable us to assess the advantages and drawbacks of six different Doppler radar architectures currently planned or under consideration by space agencies for the study of cloud dynamics. Radar performance is examined against the state-of-the-art numerical model simulations of well-characterized shallow and deep, continental, and oceanic convective cases. Mean Doppler velocity (MDV) measurements collected at multiple frequencies (13, 35, and 94 GHz) provide complementary information in deep convective cloud systems. The high penetration capability of the 13 GHz radar enables to obtain a complete, albeit horizontally under-sampled, view of deep convective storms. The smaller instantaneous field of view (IFOV) of the 35 GHz radar captures more precise information about the location and size of convective updrafts above 5–8 km height of most systems which were determined in the portion of storms where the mass flux peak is typically located. Finally, the lower mean Doppler velocity uncertainty of displaced phase center antenna (DPCA) radars makes them an ideal system for studying microphysics in shallow convection and frontal systems, as well as ice and mixed-phase clouds. It is demonstrated that a 94 GHz DCPA system can achieve retrieval errors as low as 0.05–0.15 mm for raindrop volume-weighted mean diameter and 25% for rime fraction (for a −10 dBZ echo).

Research on sea clutter model of emulating aircraft motion based on shore-based multichannel radar

ABSTRACT Space-time adaptive processing (STAP) is an important clutter suppression technique in airborne multichannel radar. It can be a challenging problem to apply STAP to the maritime airborne radar without knowledge of sea clutter space-time coupling characteristics. The radar sea clutter data with space-time coupling characteristics can be obtained by airborne multichannel radar, but the flight testing is very expensive. A cost-effective technique is presented for emulating airborne motion based on shore-based multichannel radar. The technique is implemented by the same array shared with transmit and receive antenna, where the transmit array uses an inverse displaced phase centre antenna to emulate platform motion and the receive array is used for receiving data. Based on the above technique, a sea clutter model of aircraft motion emulation is proposed. Then, we verify the model by quantitatively comparing the number of eigenvalue and the spread width of the space-time power spectrum between the simulated and measured data. The result shows that the eigenvalue number and the spread width of 3 dB clutter ridge of measured data are more than 79.5% of that belonging to simulated data. It means that the simulated radar sea clutter data generated by this model can better fit the measured data under sea states classes 1–4, which can provide support for the analysis of sea clutter space-time characteristics.

Phase Mismatch Calibration for Dual-Channel Sliding Spotlight SAR-GMTI

This article investigates channel phase mismatch calibration during the application of displaced-phase-center antenna (DPCA) in dual-channel sliding spotlight synthetic aperture radar (SAR) for ground moving target indication (GMTI). In sliding spotlight SAR, the utilization of beam progressive sweeping in azimuth causes antenna phase centers to be misaligned from the sensor path, resulting in the phase mismatch between channels. Then, spatial channel co-registration required in the DPCA cannot be achieved directly by an azimuth time shift. In this study, a calibration method based on scanning geometry of the dual-channel sliding spotlight SAR is developed to address this issue. Moreover, the effect of the phase mismatch calibration on the estimation of azimuth time difference between the two channels is derived and analyzed in depth. The clutter suppression results processed from experimental data acquired by a C-band dual-channel SAR system (Gaofen-3) operated in sliding spotlight mode are shown for the first time to demonstrate the effective phase mismatch calibration.

Motion compensation using joint-sub-band phase unwrapping for synthetic aperture sonar.

Displaced phase center antenna (DPCA) micro-navigation has been widely applied in the motion compensation of synthetic aperture sonars (SASs). Estimating the time delay is the most important step for DPCA-based motion compensation. However, at present, the existing methods of estimating the time delay in motion compensation are not sufficiently accurate, which limits the improvement of imaging quality of SASs. This paper proposes a time delay estimation method using joint-sub-band phase unwrapping, which achieves much higher estimation accuracy than the reference method. The experimental results demonstrate that the proposed method dramatically improves the SAS imaging quality, compared to the reference method.

Dually Supervised Track-Before-Detect Processing of Multichannel Video SAR Data

Track-Before-Detect (TBD) algorithm has been used to track weak moving target shadow in video synthetic aperture radar (SAR), where strong maneuverability may deteriorate tracking performance. Fortunately, the Doppler characteristic of target can additionally provide velocity guidance for tracking. This article presents ground moving target indication (GMTI) results of the multi-channel video SAR raw data, and a tracking approach is proposed based on the Doppler supervision in an improved dynamic programming-based TBD (DP-TBD) framework that uses a dual-domain merit function. Both the sequential high-resolution SAR images and low-resolution Range-Doppler (RD) spectra are used, and the clutter in RD spectrum is suppressed by using an adaptive displaced phase center antenna (ADPCA) technique. Fine states can be searched in dual-domain through resampling from random expansion in state initialization and transition. The inverse shadow amplitude and Doppler energy of the same potential target are simultaneously integrated to determine whether the target exists. The proposed Dual-DP-TBD can deal with the tracking of a time-varying number of targets in successive measurements. Compared to other TBD algorithms, this approach has fewer missing alarms and false alarms thanks to precise velocity estimation constrained from diverse features both in SAR image and RD spectrum.

SAR(Synthetic Aperture Radar) 위성 개발현황 및 향후 HRWS(High Resolution Wide Swath) SAR 위성 개발전략

This paper is made to suggest a future strategy for the Korean High Resolution Wide Swath Synthetic Aperture Radar (HRWS SAR) satellite development by surveying the current trends for the SAR satellite technologies. From the survey, the latest SAR technology trends are revealed of using Digital Beam-Forming (DBF), SCan-On-Receive (SCORE), Displaced Phase Center Antenna (DPCA), interferometry, and polarimetry for exploiting the SAR imagery. Based on the latest SAR technology trends and the foreign HRWS SAR development cases, the strategy for the future HRWS Korean SAR satellite development is suggested to develop the DPCA and SCORE technologies by using the KOrea Multi-Purpose SATellite-6 (KOMPSAT-6) which is going to launch in a few years, and consequently to develop the HRWS SAR satellites which can monitor the whole Earth at weekly intervals.

An Improved Scattered Wave Deceptive Jamming Method Based on a Moving Jammer Beam Footprint Against a Three-Channel Short-Time SAR GMTI

To effectively generate a verisimilar false moving target against a three-channel synthetic aperture radar ground moving target indication (SAR GMTI), a scattered wave deceptive jamming method performed by the time-delay modulation and the phase modulation (STP) is first proposed by using traditional deceptive jamming strategies. In addition, a novel three-channel short-time SAR GMTI method (TCSTSG) is heuristically inspired by the short-time Fourier transform (STFT) method and proposed with anti-jamming capacity. The procedure of the TCSTSG method is to divide the whole-time data into shorter segments, and each segment will be processed by displaced phase center antenna (DPCA) technique and along-track interferometry (ATI) technique. With the TCSTSG method, the moving target will have different positions in each segment. However, the false target generated by the traditional deceptive jamming method based on multiple jammers (DJ-MJ) and the STP method has the same position in each segment and a verisimilar false moving target cannot be generated. To meet this challenge, an improved scattered wave deceptive jamming method based on the moving jammer beam footprint (SMJBF) is proposed. Compared with the traditional scattered wave jamming methods, the improvement is that the jammer can control the azimuth position and velocity of the footprint. The position and velocity are calculated by comparing the detection results of the moving target and those of the false target with the TCSTSG method. Then the false target will be similar to the moving target. Finally, simulations are provided to verify the effectiveness of the proposed methods.

A Novel Two-Step Scheme Based on Joint GO-DPCA and Local STAP in Image Domain for Multichannel SAR-GMTI

Multichannel synthetic aperture radar ground moving target indication has been widely used and get great attention nowadays. Traditional adaptive methods like space-time adaptive processing (STAP) need to calculate and search the maximum of the test statistic pixel by pixel over a large range of radial velocity, which will cause great computational burden. Nonadaptive methods like displaced phase center antenna (DPCA) will cause signal loss of moving targets, thereby degrading the performance of moving targets detection and radial velocity estimation. To improve the problem mentioned above, this article proposes a novel two-step scheme based on greatest of (GO)-DPCA and Local STAP. In step I, GO-DPCA utilizes the complex images of the channels with different baselines for joint moving target detection. Then, in step II, only the local data of moving targets detected in step I needs to be extracted from the original image domain for clutter suppression and radial velocity estimation simultaneously by STAP. Since the processing gain of moving targets depends on the radial velocity and baseline length, the detection performance of moving targets with different radial velocity can be improved by proposed GO-DPCA compared to conventional DPCA. Considering the moving targets usually distribute sparsely in the scene, step II can avoid most computational burden caused by processing pixel by pixel. Finally, the experimental results of both simulated and acquired data are given to validate the effectiveness of the proposed method.

A Multiple-Jammer Deceptive Jamming Method Based on Particle Swarm Optimization Against Three-Channel SAR GMTI

Traditional deceptive jamming methods effectively generate false targets against three-channel synthetic aperture radar ground moving target indication (SAR GMTI). However, effectiveness of traditional methods is limited to azimuth distance between two jammers. When the distance requirement is not satisfied, the jamming amplitude coefficients will be too high to be used by jammers. To overcome this disadvantage, a novel multiple-jammer deceptive jamming method is proposed. The intercepted radar signal is retransmitted and performed by amplitude and time-delay modulation. Then, jamming signal is first modeled as a polynomial function where the polynomial order is equal to the number of jammers. The interferometric phase difference between a moving target and a false target is utilized as an objective function, the azimuth relocated position difference between a moving target and a false target is utilized as the constraint to reduce computational burden, and the optimal jamming amplitude coefficients are estimated by solving the optimization problem based on the particle swarm optimization (PSO). Hereby, the optimal jamming amplitude coefficients and gain calculated with the proposed method will be lower than those calculated with traditional methods when the distance requirement is not satisfied. By utilizing the optimal coefficients and jamming amplitude gain, a false target generated with the proposed method will be detected by displaced phase center antenna (DPCA) technique and be relocated in the desired position by along-track interferometry (ATI) technique. Experimental results verify the effectiveness of the proposed method.

A Three-Stage Inter-Channel Calibration Approach for Passive Radar on Moving Platforms Exploiting the Minimum Variance Power Spectrum.

Research in passive radar has moved its focus towards passive radar on moving platforms in recent years with the purpose of moving target indication and ground imaging via synthetic aperture radar. This is also fostered by the progress in hardware miniaturization, which alleviates the installation of the required hardware on moving platforms. Terrestrial transmitters, commonly known as illuminators of opportunity in the passive radar community, usually emit the signals in the Very High Frequency (VHF) or Ultra High Frequency (UHF) band. Due to the long wavelengths of the VHF/UHF band, there are constraints on the size of the used antenna elements, and therefore, the number of antenna elements to be employed is limited, especially as the platform carrying the passive radar system is intended to be small, potentially even an unmanned aerial vehicle. In order to detect moving targets hidden by Doppler shifted clutter returns, one common approach is to suppress the clutter returns by applying clutter suppression techniques that rely on spatial and temporal degrees of freedom, such as Displaced Phase Center Antenna (DPCA) or Space-Time Adaptive Processing. It has been shown that the DPCA approach is a meaningful technique to suppress the clutter if two antenna elements are employed. However, if the employed receiving channels are not carefully calibrated, the clutter suppression is shown to be not effective. Here, we suggest a three-stage calibration technique in order to perform the calibration of two receiving channels, which involves the exploitation of the direct signal, a data-adaptive amplitude calibration, and finally, a data-adaptive calibration of phase mismatches between both receiving channels by the estimation of the Minimum Variance Power Spectrum of the clutter. The validity of the proposed approach is shown with simulated data and demonstrated on real data from a fast ground moving platform, showing improved clutter cancellation capabilities.

Bistatic Forward-Looking SAR MP-DPCA Method for Space–Time Extension Clutter Suppression

Echoes of bistatic forward-looking synthetic aperture radar (BFSAR) disperse to multiple range cells and exist spatial frequency extension as well as Doppler spectrum extension (i.e., namely space–time extension). Furthermore, the characteristics of BFSAR clutter are strongly nonstationary and spatial variants. Because of the abovementioned issues, clutter and moving targets are fully overlapped in the initial 3-D space–time–range raw data domain, and the clutter cannot be suppressed effectively. To solve this problem, a BFSAR multipulse displaced phase center antenna (MP-DPCA) method is proposed in this article. First, keystone transform without Doppler ambiguity is applied to remove the coupling between the space–time and range domains. Hence, the overall complex 3-D processing in the space–time–range domain is reduced to independent 2-D processing in each space–time domain. Subsequently, a spatial-dechirp processing is applied in the space–time domain to eliminate the spatial frequency extension. Meanwhile, Doppler parameters of clutter point scatterers are equalized by nonlinear chirp scaling processing in the frequency and time domains. Accordingly, the Doppler spectrum extension of point scatterers can be eliminated by a uniform azimuth dechirp processing. After aforesaid three steps, the clutter and moving targets are separated in the space–time domain. Finally, based on the linear phase difference of clutter between the channels, a multipulse canceller can be designed to suppress the clutter. Compared with the existing space–time adaptive procession (STAP) and DPCA methods, this method not only overcomes the nonstationary problem in BFSAR but also conquers the strict application conditions of DPCA. Simulation results are given to verify the effectiveness of the proposed method.

Reduction of False Alarm Rate in SAR-MTI Based on Weighted Kurtosis

Moving target indication (MTI) is considered one of the most important applications of synthetic aperture radar (SAR) in military operations and traffic monitoring. Although many studies have focused on detectors based on the statistical clutter models, a mismatch between the measured data and the applied statistical model often leads to unreliable MTI results, particularly in terms of false alarm rates. To reduce the number of false alarms, we propose an efficient MTI scheme consisting of conventional MTI techniques (the displaced phase center antenna (DPCA) and the interferogram's magnitude and phase (IMP) methods), which operate in dual-receive antenna (DRA) mode in an SAR system. These techniques are coupled with a new discrimination stage based on a new detection metric-weighted kurtosis. Using simulated and real measured data from TerraSAR-X, the proposed MTI scheme demonstrates robust performance in the reduction of false alarm rates with only a slight increase in the computation time compared with conventional schemes.

Road-Aided Along-Track Baseline Estimation in a Multichannel SAR-GMTI System

In this letter, a novel method is proposed to estimate the along-track baseline for a multichannel synthetic aperture radar (SAR) system, intended for ground moving target indication (GMTI) applications. First, an adaptive spectrum filtering technique is proposed to compensate the terrain interferometric phase caused by the cross-track baseline and then the ground clutter is rejected by applying the joint-pixel displaced phase center antenna (JPDPCA). After performing the moving target detection, the target radial velocity is estimated according to the azimuth position offset by exploiting the road-aided information. Finally, the along-track baseline is derived based on the subspace projection (SP). The effectiveness of the proposed method is validated by data from the experimental airborne system.