Resolve Range Ambiguities Research Articles

Resolving Range Ambiguity Using Transmit Coding Modulation and Multi-Pulse Processing in FPC-PA Radar

Resolving Range Ambiguity Using Transmit Coding Modulation and Multi-Pulse Processing in FPC-PA Radar

High-Resolution and Wide-Swath SAR Imaging with Space–Time Coding Array

To achieve high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) images, this paper focuses on resolving the problem of separating range-ambiguous echoes with the space–time coding (STC) array. At the modeling stage, the transmit elements and pulses of the STC array are configured with time delay and phase coding modulation, which introduces extra degrees of freedom (DOFs) in the transmit domain. To separate the echoes corresponding to different range-ambiguity regions, the equivalent transmit beamforming is performed in the two-dimensional space–frequency domain. Moreover, in order to compensate for the loss of range resolution during the beamforming progress, the frequency splicing method is proposed. At the analysis stage, the distributed target simulation is provided to demonstrate the effectiveness of obtaining HRWS SAR images in the STC radar. Additionally, the performance of resolving range ambiguity is compared with the traditional radar in terms of the range-ambiguity-to-signal ratio (RASR).

High-Resolution and Wide-Swath SAR Imaging With Sub-Band Frequency Diverse Array

High-resolution and wide-swath imaging serves as an important task for synthetic aperture radar (SAR). However, the tradeoff between high azimuth resolution and wide unambiguous swath coverage has not been well optimized in traditional SAR system. Meanwhile, the ultrawideband signal is required to obtain ultrahigh range resolution imaging. It is not easy to increase the bandwidth directly due to the difficulty of system design. To this end, this article makes notable contribution on a sub-band frequency diverse array framework to realize range ambiguous echoes separation and wideband signal synthesis from narrowband signals. By introducing a small frequency increment across the elements within each subarray, the transmit steering vector within each subarray is range-angle-dependent, making it feasible to resolve range ambiguity in the spatial frequency. In addition, the transmitted waveforms of different subarrays occupy different frequency bands in range frequency domain, which play a pivotal role in achieving high-resolution imaging. With phase compensation and sub-band frequency spectrum splicing technique, the wideband signal can be obtained. Simulation results have verified the effectiveness of the proposed approach.

Monostatic Sensing With OFDM Under Phase Noise: From Mitigation to Exploitation

We consider the problem of monostatic radar sensing with orthogonal frequency-division multiplexing (OFDM) joint radar-communications (JRC) systems in the presence of phase noise (PN) caused by oscillator imperfections. We begin by providing a rigorous statistical characterization of PN in the radar receiver over multiple OFDM symbols for free-running oscillators (FROs) and phase-locked loops (PLLs). Based on the delay-dependent PN covariance matrix, we derive the hybrid maximum-likelihood (ML)/maximum a-posteriori (MAP) estimator of the deterministic delay-Doppler parameters and the random PN, resulting in a challenging high-dimensional nonlinear optimization problem. To circumvent the nonlinearity of PN, we then develop an iterated small angle approximation (ISAA) algorithm that progressively refines delay-Doppler-PN estimates via closed-form updates of PN as a function of delay-Doppler at each iteration. Moreover, unlike existing approaches where PN is considered to be purely an impairment that has to be mitigated, we propose to exploit PN for resolving range ambiguity by capitalizing on its delay-dependent statistics (i.e., the range correlation effect), through the formulation of a parametric Toeplitz-block Toeplitz covariance matrix reconstruction problem. Simulation results indicate quick convergence of ISAA to the hybrid Cramér-Rao bound (CRB), as well as its remarkable performance gains over state-of-the-art benchmarks, for both FROs and PLLs under various operating conditions, while showing that the detrimental effect of PN can be turned into an advantage for sensing.

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 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.

Folded Clutter Suppression for Pulse-Doppler Radar Based on Pulse-Agile Waveforms

Pulse-Doppler radar systems with medium/high pulse repetition frequencies (PRFs) usually produce severe range ambiguities, and strong nearby ground/sea clutter will then fold into a single range interval with weak remote target echoes, further limiting the dynamic range of radar. The existing methods for modeling or estimating clutter spectra are usually not applicable to the case of folded clutter. In recent years, pulse-agile waveforms have attracted an increasing amount of attention for resolving range ambiguities. However, cross-correlations among pulse-agile waveforms with the same frequency band, usually required for pulse-Doppler radar to coherently measure the Doppler shift, are not satisfactory for suppressing folded clutter since clutter echoes are much stronger than those of targets. In this paper, two algorithms based on the method of alternating projection (MAP) in the range-Doppler domain (RD-MAP) and the space-Doppler domain (SD-MAP) are proposed to suppress folded clutter and resolve range ambiguities for pulse-Doppler radar based on different types of pulse-agile waveforms. The effectiveness of these algorithms is verified based on several realistic simulations and a ground-based experiment. Iterative solutions and closed-form solutions are both provided for different computational complexity and accuracy requirements in certain application scenarios. Moreover, algorithm analysis, including algorithm boundary analysis, signal-to-noise ratio (SNR) loss, computational complexity and factor selection, is performed to provide an estimation of the applicable conditions and assess the performance of the proposed algorithms.

Resolving Range Ambiguity via Multiple-Input Multiple-Output Radar With Element-Pulse Coding

In this paper, we propose a novel radar framework, referred to as multiple-input multiple-output (MIMO) radar with element-pulse coding (EPC). The EPC employs Fourier basis in joint transmit elements and pulses, which has the advantages of simplicity and convenience for practical realization. It is shown that the EPC-MIMO radar is advantageous in discriminating different pulses and resolving range ambiguity. The discrimination of different pulses is achieved in the transmit spatial frequency domain, and the identifiability of range ambiguity increases with the transmit element number of the radar. Moreover, coherent processing in Doppler domain can be performed after decoding. The coding and decoding procedures are closely related though they are performed in transmitter and receiver, respectively. As the EPC-MIMO radar is capable of resolving range ambiguity, an enhanced parameter estimation approach is developed to obtain the angle and range region number estimates of target. The parameter estimation performance is assessed by the Cramer-Rao bound, which reveals useful and interesting findings. Simulation examples are provided to verify the signal properties in EPC-MIMO radar and to demonstrate its superiority in resolving range ambiguity over existing systems.

Three-dimensional target localization method based on the tensor subspace FDS-MIMO radar with a co-prime frequency offset

Target localization utilizing frequency diversity array (FDA) is one of the most popular research directions in the study of radars. Considering the problems of range ambiguity and localization accuracy, this paper investigates the issue of three-dimensional (3D) target localization based on the proposed frequency diverse-subaperturing multiple-input multiple-output (FDS-MIMO) radar. First, a fifth-order tensor signal model is established by exploiting the inherent multidimensional structure of the matched filter output. To alleviate the difficulty of range ambiguity, the idea of employing co-prime frequency offsets along the two directions in the planer array is provided. This strategy can resolve the contradiction between the range resolution and maximum unambiguous range in beam domain. The tensor-based complex- and real-valued rotational invariance technique (tensor- and unitary tensor-ESPRIT) algorithms are developed based on frequency offset structure. Given that the inherent multidimensional structure is utilized, the proposed methods can resolve range ambiguity with improved localization performance as compared with the existing ESPRIT algorithm and frequency offset manner. Theoretical analysis and numerical results demonstrate the effectiveness of the proposed approaches.

Frequency diverse array radar signal and data processing

The frequency diverse array (FDA) radar is re-explored with a special focus on how the FDA data should be correctly and efficiently processed to maximise its full potential and capacity of search and detection. Some features, such as its extraordinary Doppler tolerance and ability of resolving range ambiguity without using multiple pulse repetition frequencies, are first demonstrated. The extension of non-linear and random frequency increments is proposed in order to suppress range sidelobes and increase range resolution. Wide-area search and detection is achieved by processing the same received data set without a need of electronic scan and repeat pulse transmission and reception. This not only greatly simplifies the radar design and operation but also makes radar quieter and stealth. The limitation of the FDA radar is its sacrificed detection range. Therefore, in comparison with the conventional phased-array radar, the detection area is fan-area-like for the former and pencil-beam-like for the latter, making two radars (or radar operation modes) complementary.

Robust Adaptive Beamforming for Fast-Moving Target Detection With FDA-STAP Radar

Frequency diverse array (FDA), which employs a small frequency increment across the array elements, is able to resolve range ambiguity. However, the frequency diversity results in angle-Doppler-defocusing of target especially at a high speed in space-time adaptive processing (STAP) radar, thus, causing serious detection performance degradation. In this paper, a robust adaptive beamforming approach is proposed for the FDA-STAP radar to enhance fast-moving target detection performance. In our solution, a large feasible region is employed to include the true steering vector of target. To avoid the trivial solution, an angle-Doppler-defocusing steering vector constraint is devised and incorporated into the large feasible region. The problem is formulated as a nonconvex quadratically constrained quadratic program which is efficiently solved via semidefinite relaxation technique. Because the retrieved steering vector of target is close to the true one, the performance is significantly improved. It is demonstrated via computer simulations that the proposed algorithm is superior to the state-of-the-art methods, which includes maintaining the mainlobe of the beampattern and improving the signal-to-clutter-plus-noise ratio performance.

Ground Moving Target Detection and Imaging Using a Virtual Multichannel Scheme in HRWS Mode

Along-track multichannel synthetic aperture radar is usually used to achieve ground moving target detection and imaging. Nevertheless, there is a design dilemma between azimuth high resolution and wide swath (HRWS). To solve this problem in HRWS mode, we introduce a virtual multichannel (VMC) scheme. For each virtual channel, the low real pulse repetition frequency (PRF) improves the ability of resolving range ambiguity for wide-swath, and the high virtual PRF improves the capability of resolving Doppler ambiguity for azimuth high resolution. For multiple virtual channels, strong ground clutter is eliminated by the joint VMC processing. Furthermore, a detailed signal model of a moving target in the virtual channel is given, and the special false-peak effect in the azimuthal image is analyzed. Moreover, we propose a novel ground moving target processing method based on the VMC scheme and the clutter suppression interferometry (CSI) technique, which is called VMC-CSI. The integration of detection, location, velocity estimation, and imaging for ground moving targets can be achieved. Accounting for the unresolved main peak and false peak for a moving target, in the VMC-CSI method, we adopt a two-step scheme to estimate the radial velocity and along-track velocity, namely, rough estimation and precise estimation. Meanwhile, considering the same interferometric phases of the main peak and the false peak, we use false peaks first for the robustness of initial azimuth location estimation and remove false peaks afterward. Numerical simulations are provided for testing the effect of the false peak and the effectiveness of VMC-CSI.

An Automatic Range Ambiguity Solution in High-Repetition-Rate Airborne Laser Scanner Using Priori Terrain Prediction

In this letter, a fully automatic method to resolve range ambiguities for high-repetition-rate airborne laser scanner data is introduced. The method combines position and orientation system data with a rough digital elevation model to make adaptive predictions for the range intervals of laser pulses and is compatible with a wide range of LiDAR systems. Results show its capability in avoiding the intrinsic constraints of multiple pulse repetition rate techniques in state-of-the-art commercial ALS systems while retaining efficient computational performance and robustness under complex terrain circumstances.

Range ambiguity resolution technique applying pulse-position modulation in time-of-flight scanning lidar applications

Time-of-flight (ToF) range measurements rely on the unambiguous association of each received echo signal to its causative emitted pulse signal. This definite association is difficult when measuring long ranges at a high repetition rate, resulting in ambiguous range measurements. While methods and algorithms exist to overcome this fundamental problem of ToF measurement techniques like radar these may not be directly applied to lidar without adaptations. Especially, in airborne laser scanning, up to now it was a requirement to strictly avoid range ambiguities during data acquisition. We present a new method for resolving range ambiguities fully automatically in scanning lidar, enabling measurements exceeding the maximum unambiguous measurement range by far. As a theoretical foundation of our approach, we introduce a specific model of the lidar transmission path (i.e., emitter–target–receiver) accounting for the time-variability of consecutive measurements. Based on this model, we discuss the influence of intentional variation of the intervals between pulse emissions on the intervals of successively received echoes and delineate an algorithm for automated, definitive association of pulse emissions and their resulting echoes. Simulation results indicate a probability of incorrect associations of <10 −5 , which we positively proved by applying this technique to real-world scan data.

Resolving range ambiguities in high-repetition rate airborne light detection and ranging applications

Correctly determining a measurement range in light detection and ranging instruments, based on time-of-flight measurements on laser pulses, requires the association of each received echo pulse with its causative emitted laser pulse. Without further precautions, this definite association is only possible under specific conditions constraining the usability of range finders and laser scanners with very high measurement rates. We give an introduction to known techniques for avoiding and resolving range ambiguities. The specific disadvantages of these methods led to the development of a technique based on pulse-position modulation (PPM) of the laser pulse train using a pseudorandom noise signal and the subsequent analysis of the impact of the PPM on groups of consecutive range measurements. The error probability of our approach has been evaluated by simulations based on real scan data. The use of a discrete uniform distribution of amplitudes as a modulation signal shows high detection robustness even for difficult terrain like forest areas. The encouraging results of the simulations led to the practical implementation of PPM to our laser scanners and the development of the associated software RiMTA, which resolves range ambiguities without the necessity of a priori information on the target situation and without any user interaction required.

Resolving range ambiguity in a photon counting depth imager operating at kilometer distances

Time-correlated single-photon counting techniques have recently been used in ranging and depth imaging systems that are based on time-of-flight measurements. These systems transmit low average power pulsed laser signals and measure the scattered return photons. The use of periodic laser pulses means that absolute ranges can only be measured unambiguously at low repetition rates (typically <100 kHz for > 1 km) to ensure that only one pulse is in transit at any instant. We demonstrate the application of a pseudo-random pattern matching technique to a scanning rangefinder system using GHz base clock rates, permitting the acquisition of unambiguous, three-dimensional images at average pulse rates equivalent to >10 MHz. Depth images with centimeter distance uncertainty at ranges between 50 m and 4.4 km are presented.

The White Sands Range and Range-Rate System

This paper discusses the concept, design, and design verification of the White Sands Range and Range-Rate System. Development of the system has been completed only through the design phase. The system is designed to meet requirements for high-accuracy midcourse tracking under severe target dynamics at the White Sands Missile Range. It is a multistatic Doppler and range tracker which operates at X band and incorporates transmitter, transponder, receiver, and baseline subsystems. The transmitter includes specially designed digital circuitry to synthesize test signals for target simulation during checkout of the system. The transponder signal is processed by a receiver which has been established theoretically to be the optimum realizable processor of continuous tracking data. The receiver incorporates specially designed carrier acquisition circuitry and digital VCO, and directly provides digital Doppler and tone phase data to facilitate real-time processing. The system utilizes data from other tracking systems at the Range for spatial acquisition, for aiding carrier acquisition in the receiver, and for resolving range ambiguities.