Range Ambiguity Research Articles

Dynamic and precise long-distance ranging using a free-running dual-comb laser.

Long-distance ranging is a crucial tool for both industrial and scientific applications. Laser-based distance metrology offers unprecedented precision making it the ideal approach for many deployments. In particular, dual-comb ranging is favorable due to its inherently high precision and sampling rate. To make high-performance long-range dual-comb LiDAR more accessible by reducing both cost and complexity, here we demonstrate a fiber-based dual-comb LiDAR frontend combined with a free-running diode-pumped solid-state dual-comb laser that allows for sub-µm measurement precision while offering a theoretical ambiguity range of more than 200 km. Our system simultaneously measures distance with the role of each comb interchanged, thereby enabling Vernier-based determination of the number of ambiguity ranges. As a proof-of-principle experiment, we measure the distance to a moving target over more than 10 m with sub-µm precision and high update rate, corresponding to a relative precision of 10-7. For a static target at a similar distance, we achieve an instantaneous precision of 0.29 µm with an update time of 1.50 ms. With a longer averaging time of 200 ms, we reach a precision of around 33 nm, which corresponds to a relative precision of about 3·10-9 with a time-of-flight-based approach.

High-Resolution Wide-Swath Ambiguous Synthetic Aperture Radar Modes for Ship Monitoring

This paper proposes two high-resolution, wide-swath synthetic aperture radar (SAR) acquisition modes for ship monitoring that tolerate ambiguities and do not require digital beamforming. Both modes, referred to as the low pulse repetition frequency (PRF) and the staggered (high PRF) ambiguous modes, make use of a wide elevation beam, which can be obtained by phase tapering. The first mode is a conventional stripmap mode with a PRF much lower than the nominal Doppler bandwidth, allowing for the imaging of a large swath, because the ships’ azimuth ambiguities can be recognized as they appear at known positions. The second mode exploits a continuous variation of the pulse repetition interval, with a mean PRF greater than the nominal Doppler bandwidth as the range ambiguities of the ships are smeared and are unlikely to determine false alarms. Both modes are thought to operate in open sea surveillance, monitoring Exclusive Economic Zones or international waters. Examples of implementation of both modes for TerraSAR-X show that ground swaths of 120 km or 240 km can be mapped with 2 m2 resolution, ensuring outstanding detection performance even for small ships. The importance of resolution over noise and ambiguity level was highlighted by a comparison with ScanSAR modes that image comparable swaths with better noise and ambiguity levels but coarser resolutions.

Orbital angular momentum-based dual-comb interferometer for ranging and rotation sensing.

We present a dual-comb interferometer capable of measuring both the range to a target as well as the target's transverse rotation rate. Measurement of the transverse rotation of the target is achieved by preparing the probe comb with orbital angular momentum and measuring the resultant phase shift between interferograms, which arises from the rotational Doppler shift. The distance to the target is measured simultaneously by measuring the time-of-flight delay between the target and reference interferogram centerbursts. With 40 ms of averaging, we measure rotation rates up to 313 Hz with a precision reaching 1 Hz. Distances are measured with an ambiguity range of 75 cm and with a precision of 5.9 µm for rotating targets and 400 nm for a static target. This is the first dual-comb ranging system capable of measuring transverse rotation of a target. This technique has many potential terrestrial and space-based applications for lidar and remote sensing systems.

Double Envelope Through The Aortic Valve: More than Range Ambiguity to Continuous Wave Doppler

A 70-YEAR-OLD MAN (99 kg, 185 cm) presented for aortic valve replacement owing to severe calcific aortic valve stenosis (AS). His AS was diagnosed recently after an investigation for presyncopal symptoms. Transthoracic echocardiogram demonstrated severe AS with peak and mean gradients of 84 and 48 mmHg, respectively. Asymmetric basal hypertrophy with mild left ventricular outflow tract (LVOT) obstruction also were noted. A peak LVOT gradient of 20 mmHg was measured with Valsalva maneuver, and mild mitral regurgitation was observed.

How toothed whales hunt with echolocation

The late Dr. Au developed a highly influential, quantitative framework for understanding the acoustic characteristics and performance of toothed whale biosonar systems. In his honor, I will apply this framework to the question of how echolocating toothed whales use their active sensory system to find, track and subdue prey in the field. Specifically, I will present data from echo-recording Dtags on wild toothed whales to show that they can detect, discriminate and approach prey with just a few tens of clicks, but that they use several hundreds of weak, high-rate clicks in the terminal buzz phase to track movements of elusive prey for capture in a very small sensory volume. I will use captive and wild data to demonstrate that toothed whales deliberately adjust their narrow beam sonars at short target ranges to receive weak prey echo levels within a 40-dB range above their hearing thresholds, and I will go on to argue that they do so to form simpler auditory scenes with less range ambiguity. I will end the talk by providing some perspectives on how Dr. Au’s emphasis on the synergy between captive and field studies is more relevant than ever if we wish to understand how toothed whales operate their biosonars under natural and Anthropocene conditions.

Dead-time-based sequence coding method for single-photon lidar ranging

Sequence coding is beneficial for reducing range ambiguity and acquisition time in single-photon lidar (SPL) ranging. In SPL data acquisition, dead time is usually regarded as an adverse condition. However, properly utilizing echo-induced dead time in sequence coding may allow it to be useful. In this study, we propose a dead-time-based sequence coding method. We set the time intervals of the subpulses to be slightly longer than the dead time, such that the detection of a subpulse can ‘block’ incoming noise detections until the arrival of the next subpulse. Starting with a sequence of three subpulses, we determined the optimal pulse-interval criteria of ‘≈ dead time + pulse width’ when the subpulses are identical in shape and intensity. A method to extract the range with few accumulations was proposed (FARE method), incorporating the maximum cross-correlation and threshold filtering. Simulation and experiment results demonstrated that our ‘optimal interval + FARE’ method is more efficient and less ambiguous than the sequence coding method without the merit of dead time, and the conventional ‘single laser pulse’ method. Our method is promising for low range ambiguity and fast ranging, and helps reduce laser energy consumption without wasting transmitting pulses.

Clutter Rank Analysis in Airborne FDA-MIMO Radar With Range Ambiguity

Frequency diverse array (FDA), which employs a small frequency increment across its array elements to generate controllable degrees-of-freedom (DOFs) in range dimension, has attracted extensive attentions in recent years. In order to extract and exploit the DOFs in range and angle dimensions, FDA has been combined with multiple-input multiple-output (MIMO) technique, which shows sufficient performance improvement in clutter suppression in scenarios with range ambiguities. Therefore, clutter rank evaluation is an important issue for FDA-MIMO radar mounted on a moving platform. In this article, a clutter rank evaluation criterion is developed with respect to different transmit/receive array configurations and different number of range ambiguities. It is based on a subspace transformation matrix that decouples the clutter subspace and allows a direct determination of clutter rank. With the proposed clutter rank evaluation criterion, the maximum resolvable number of range ambiguities can be determined, which is bounded by the system’s DOFs. Three cases according to different relationships between uniform array element spacings are studied. Then, the aforementioned clutter rank evaluation method is extended to nonuniform array configurations. Moreover, the robustness of the method is studied in the presence of gain-phase errors. Several simulation results are provided to verify the effectiveness of the proposed method.

Cooperative Range and Angle Estimation With PA and FDA Radars

Target localization with unambiguous range and angle estimation of the signal-of-interest is a key task of a radar system, especially for wide region surveillance. In this article, we develop a novel dual-mode array radar scheme, namely phased array and frequency diverse array cooperated radar, to provide unambiguous range and angle estimation. Our approach combines the advantages of the PA in high transmit gain and that of the FDA in resolving range ambiguity. In general, two kinds of cooperative parameter estimation algorithms are derived to determine the range and angle parameters jointly. The proposed algorithms can be interpreted as different combination methods of the PA and FDA, including the uniform combination and weighted combination based on weighted least squares, where the weight is designed with user tunable angle parameters. Particularly, the design criterion for the tunable parameter is given. At the analysis stage, the Cramér–Rao bounds are studied to verify the performance of parameter estimation. Extensive simulation results demonstrate the effectiveness of parameter estimation with the dual-mode array radar.

Range-Dependent Ambiguous Clutter Suppression for Airborne SSF-STAP Radar

Range-dependent clutter suppression is a challenging problem in nonsidelooking, bistatic, and conformal antenna array airborne radar, especially in the presence of range ambiguity. Superimposed stepped frequency (SSF) radar employs a small frequency increment across the stepped signals in each pulse, which introduces a dimension related to slant range. In this article, an airborne radar framework is established, which takes SSF signal as the transmit waveform. Thus, a two-stage adaptive clutter suppression method is proposed, which utilizes the degrees of freedom in range, space, and time domains provided by airborne SSF radar. In the first stage, the secondary range dependence compensation approach is adopted to distinguish the clutter of each range region in the carrier frequency domain. Then, a target-free covariance matrix is estimated by the compensated data, and the covariance matrix is used for the range-ambiguous clutter separation in the carrier frequency domain. Thus, the space-time snapshot of each range region can be extracted. In the second stage, a clutter segmentation processing method is devised for residual clutter suppression. For the near-range unambiguous clutter, the traditional clutter compensation approach is applied to further align the spectrum distribution of clutter, while the conventional space-time adaptive processing is utilized to suppress far-range clutter directly. Since the proposed method takes full advantage of the range dimension and the two stages of adaptive processing are data-dependent, excellent clutter suppression performance can be obtained. The proposed method can also extract the real range of the target since each range region is processed separately. Simulation results demonstrate the effectiveness of the proposed method.

Adaptable Pulse Compression in ϕ-OTDR With Direct Digital Synthesis of Probe Waveforms and Rigorously Defined Nonlinear Chirping

Recent research in Phase-Sensitive Optical Time Doman Reflectometry (ϕ-OTDR) has been focused, among others, on performing spatially resolved measurements with various methods including the use of frequency modulated probes. However, conventional schemes either rely on phase-coded sequences, involve inflexible generation of the probe frequency modulation or mostly employ simple linear frequency modulated (LFM) pulses which suffer from elevated sidelobes introducing degradation in range resolution. In this contribution, we propose and experimentally demonstrate a novel ϕ-OTDR scheme which employs a readily adaptable Direct Digital Synthesis (DDS) of pulses with custom frequency modulation formats and demonstrate advanced optical pulse compression with a nonlinear frequency modulated (NLFM) waveform containing a complex, rigorously defined modulation law optimized for bandwidth-limited synthesis and sidelobe suppression. The proposed method offers high fidelity chirped waveforms, and when employed in resolving a 50-cm event at ∼1.13 km using a 1.2-µs probe pulse, matched filtering with the DDS-generated NLFM waveform results in a significant reduction in range ambiguity owing to autocorrelation sidelobe suppression of ∼20 dB with no averages and windowing functions, for an improvement of ∼16 dB compared to conventional linear chirping. Experimental results also show that the contribution of autocorrelation sidelobes to the power in the compressed backscattering responses around localized events is suppressed by up to ∼18 dB when advanced pulse compression with an optical NLFM pulse is employed.

The Staring Mode Properties and Performance of Geo-SAR Satellite with Reflector Antenna

Geosynchronous synthetic aperture radar (Geo-SAR) with a short revisit time can obtain wide-area images. This paper advances a new two-dimensional pitch and roll squint controlling (2D-PRSC) method that can make satellites continuously stare at any scene in the whole orbital period. The maximum attitude steering angle is less than ±7.6 degrees, and the attitude controlling time can be greatly shortened compared with the yaw steering method. Furthermore, a Geo-SAR staring mode model is illustrated and compared with that of low earth orbital SAR (Leo-SAR). Finally, Geo-SAR’s ambiguity property is discussed. The simulation results illuminate that the cross-term ambiguity to signal ratio (CASR) also needs to be considered in addition to the azimuth and range ambiguity to signal ratio (AASR, RASR), and the whole orbital ergodic analysis should be carried out. To ensure that RASR, AASR, and CASR meet the requirement of −20 dB, it is necessary to select an appropriate PRF in the range of a few hundred Hertz.

Signal Simulation of Dual-Polarization Weather Radar and Its Application in Range Ambiguity Mitigation

In this paper, a dual-polarization weather radar echo signal simulation method is proposed for the evaluation of the performance enhancement of dual-polarization weather radar systems, the optimization of signal processing algorithms and the improvement of scanning strategies. The actual weather radar base data are used in the simulation as the reference weather scene, which avoids using a complex algorithm for weather modeling. Moreover, based on radar weather equations, the radar system parameters are added into the radar echo signal modeling to establish the relationship between the simulated echo signal and radar system. As a result, the final simulated echo signal not only shows both the time and frequency domain characteristics of the weather target, but also includes the effects of the important performance of the dual-polarization weather radar system. In addition, to evaluate the performance of range ambiguity mitigation using phase coding and batch working modes, two different simulation methods for the radar signal are established on the method above; one is for batch working mode with long-PRT (pulse repetition time) and short-PRT transmission and receiving, and the other is for phase-coded mode with phase-coded transmission and phase-uncoded receiving. Under the same weather scene, the observation of these two different methods of range ambiguity mitigation are simulated and compared. Results show that the performance of the phase coding mode for mitigating range ambiguity is better than that of the batch mode. Obviously, the simulation method can be used to directly show the observation of different algorithms for mitigation range ambiguity under the same weather process, and quickly compare and evaluate the algorithm’s performance, which is not possible for real radars.

A Novel Time-Domain Frequency Diverse Array HRWS Imaging Scheme for Spotlight SAR

The frequency diverse array (FDA) technique is a novel scheme for high resolution wide swath synthetic aperture radar (SAR) imaging, which employs a frequency increment across the array elements. This introduces a range-angle-dependence to the transmission steering vector, which is exploited for range ambiguity resolution in strip-map SAR. Generally in spotlight mode, scatterers dispersively distributed in azimuth have different Doppler histories, in which the range ambiguity resolution for strip-map SAR fails. To extend the flexibility of FDA, in this paper a novel FDA imaging scheme for spotlight mode SAR is proposed. Exploiting the property of the same illuminated period of all scatterers in spotlight mode, the proposed scheme is carried out entirely in the azimuth time domain, which allows for higher processing efficiency and real-time implementations. Still, excessive Doppler history differences among scatterers deteriorate the scheme performance for azimuth-edge scatterers. Aiming at this situation, a residual angle phase compensation in time domain is considered for the cases of a large azimuth beam width, improving the applicability of the proposed scheme. Compared with existing methods, the proposed spotlight FDA-SAR offers the possibility of achieving simultaneously high azimuth resolution and wide swath performance with high efficiency. Simulations and analyses are performed to demonstrate the effectiveness of the proposed spotlight FDA-SAR scheme.

Range Ambiguous Clutter Suppression Approach With Elevation Time Diverse Array

When range ambiguity and range dependence coexist in forward-looking ground moving target indication radar, the traditional space-time adaptive processing (STAP) approaches may fail to work. To cope with this issue, a range-ambiguous clutter suppression approach based on elevation time diverse array (TDA) radar system and azimuth phase coding technique is presented in this article. Due to the advantages of frequency scanning, the elevation TDA radar is able to cover all directions by transmitting a single time-shifted waveform. Therefore, a frequency bandpass filter is devised for each single range bin to achieve the mainlobe echo extraction procedure. In the sequel, the azimuth phase decoding and Doppler filtering are performed to deal with the residual sidelobe ambiguous energy. With two-dimensional filtering, one can extract the unambiguous signals for each range region independently. Then, the traditional clutter compensation and STAP methods are employed to suppress the ground clutter. Finally, the effectiveness of the proposed framework in resolving range ambiguity is verified by the numerical simulation experiments.

Method of Range Ambiguity Suppression Combining Sparse Reconstruction and Matched Filtering

Method of Range Ambiguity Suppression Combining Sparse Reconstruction and Matched Filtering

Method of Range Ambiguity Suppression Combining Sparse Reconstruction and Matched Filter

Synthetic Aperture Radar (SAR) images are often affected by range ambiguity due to antenna sidelobe characteristics and pulse operating mechanism. The work of range ambiguity suppression focuses on both SAR system design and signal processing. On the one hand, ideas have been proposed to modify the transmitting and receiving waveforms to block the ambiguous energy, such as up and down chirp modulation, multiple elevation beams, and azimuth phase coding techniques. On the other hand, signal processing methods in echo and image domains have been used to reduce the range ambiguity energy, such as up and down chirp processing and sparse regularization methods. This paper proposes a range ambiguity suppression method that combines sparse reconstruction and matched filter. There are four main steps: performing the sparse reconstruction to obtain the ambiguity area, estimating the ambiguity signal using the ambiguity area image and reconstruction matrix, separating ambiguity signal from the whole signal to obtain the signal of primary imaging area, using matched filter to the primary signal to obtain the main lobe area image. In this method, sparse reconstruction improves the accuracy of ambiguity signal estimation, and matched filter ensures the efficiency of imaging processing procedure. Simulation results show that the proposed method can effectively suppress range ambiguity. When the ambiguity energy is relatively strong, the Focal Underdetermined System Solver (FOCUSS) algorithm can get better results than the classical Orthogonal Matching Pursuit (OMP) algorithm. Moreover, the weak targets and the details of the main lobe image are well preserved.

An Advanced Scheme for Range Ambiguity Suppression of Spaceborne SAR Based on Blind Source Separation

Due to the minimum antenna area constrain of synthetic aperture radar (SAR), high-resolution and wide-swath (HRWS) imaging is difficult to be achieved using classical modes, such as scan, spotlight, and so on. Range ambiguity is one of the main technical challenges limiting HRWS imaging in spaceborne SAR. Insipred by the cocktail party effect, an advanced scheme based on blind source separation is put forward to suppress the range ambiguity of spaceborne SAR in this paper. In the scheme, multiple subantennas are used to collect multiple echo data sets, which are given different patterns. In this way, the echo signal of the desired region and that of the ambiguous region are weighted using different coefficients. During postprocessing, a detailed flow with the blind source separation algorithm is proposed to separate the desired echoes from those data sets. To verify the scheme, point target and scene simulations based on GF3 are carried out. The results indicate that the range ambiguity can be reduced by more than 15 dB using the proposed scheme, the processing flow is effective, and the scheme has a good performance without increasing the complexity of the system design under the current SAR system conditions. The scheme has the potential to be adopted in future spaceborne SAR missions.

Pattern Synthesis Algorithm for Range Ambiguity Suppression in the LT-1 Mission via Sequential Convex Optimizations

The innovative spaceborne Earth observation mission LuTan-1 (LT-1) deploys advanced full polarimetric L-band synthetic aperture radar (SAR) to obtain high-precision, multidimensional ground feature information. As the quad-pol SAR system, LT-1 suffers from strong ambiguities that degrade the quality of the observation products. Antenna pattern synthesis algorithm can suppress the range ambiguities without the increase of the azimuth ambiguities and the system complexity and therefore has great potential to improve the overall ambiguity performance of the quad-pol SAR system. However, the previous research on this kind of method is generally limited to specific situations and lacks of the analysis of actual measurement results. Based on the application requirements of LT-1, this article proposes a novel pattern synthesis algorithm that suppresses the range ambiguities via sequential convex optimizations. In simulation and comparison, the proposed algorithm effectively suppresses the strong co-polarized range ambiguities and shows the flexibility, efficiency, and stability that are significantly better than the previous algorithms. What is more, the analysis of practical performance loss is performed based on the measurement result of the LT-1 antenna, and the practicality and validity of the algorithm are strongly verified.

A Novel Range-Azimuth Joint Modulation Scheme for Range Ambiguity Suppression

Range ambiguity is a technical challenge for current spaceborne synthetic aperture radar (SAR) systems. To this end, a novel range-azimuth joint modulation transmission scheme is proposed, and the corresponding imaging processing and performance analysis are detailed. Compared with the azimuth phase coding (APC) technique, this scheme fully exploits the sampling margins of the range and azimuth dimensions, resulting in the range ambiguities experiencing a double suppression effect. Starting from the range-azimuth joint modulation scheme, to obtain the best ambiguity suppression performance, the design of a nonlinear frequency modulation (NLFM) waveform with a continuous piecewise linear instantaneous frequency is formulated and tackled via a MATLAB optimization toolbox. The detailed simulation results based on LuTan-1 (LT-1) parameters illustrate that the proposed methodologies outperform the APC method and provide considerable ambiguity suppression.

Method of High Signal-to-Noise Ratio and Wide Swath SAR Imaging Based on Continuous Pulse Coding

In synthetic aperture radar (SAR), increasing the pulsewidth of signal is an effective way to achieve high signal-to-noise ratio (HSNR) imaging. However, when the pulse repetition frequency (PRF) is fixed, increasing the pulsewidth will reduce the maximum unambiguous swath width of SAR. In order to solve this contradiction, a method based on continuous pulse coding (CPC) which increases the average transmit power by multiple pulses with varying high PRF is proposed in this article. Due to the small interval between pulses, the echo will have range ambiguity and occlusion problems. To obtain the complete echo, we first use the form of CPC signal and the swath width of radar to construct a linear equation set to model the ambiguous echo of each receiving window. Next, the established linear equations are split according to the distribution law of receiving window. Finally, the echo energy accumulation of multiple pulses is accomplished by solving the split sublinear equations. Therefore, the echo with both swath width corresponding to narrow pulse and HSNR corresponding to wide pulse is obtained to realize HSNR and wide swath SAR imaging. The experimental results have confirmed the effectiveness of the method proposed in this article.