High Pulse Repetition Frequency Radar Research Articles

Detection of hypersonic weak targets by high pulse repetition frequency radar based on multi‐hypothesis fuzzy‐matching radon transform

AbstractFor the integration detection of near space hypersonic weak targets by high pulse repetition frequency (PRF) radar, a novel method named Multi‐Hypothesis Fuzzy‐Matching Radon transform (MHFM‐RT) is proposed for the near space hypersonic target detection and tracking. For remote hypersonic target detection, to avoid range ambiguity, current radars always use a low PRF mode, which limit the number of pulse accumulations. Using the high PRF mode, the fuzzy folding will appear in the target range measurements when target trajectory crosses range fuzzy intervals. Therefore, there is a contradiction between range ambiguity and energy accumulation. The proposed method is used to match the fuzzy measurements, so as to realise the correct integration in the condition of range ambiguity. Firstly, considering the need of range ambiguity resolution, the mode of staggered PRF is used. Secondly, the first frame measurements are periodically extended for multiple‐fuzzy hypothesis. Finally, the weak target track is accumulated in MHFM‐RT domain, and the signal integration and ambiguity resolution can be realised simultaneously. The proposed method expands the Variable‐Diameter‐Arc‐Helix Radon transform (VDAH‐RT) method to fuzzy folding conditions. Compared with the existing methods, for 7‐scan measurements non‐coherent integration, the detection sensitivity of the proposed method is about 0.5–1 dB higher than that of the IMM hybrid filter algorithm, and about 1 dB higher than that of the RHT‐TBD approach, and it needs less storage space and has higher detection probability.

A Novel Method for Location Ambiguity Mitigation of High Pulse Repetition Frequency Radar Signal in Low Earth Orbit Dual-satellite Geolocation System

In order to mitigate the location ambiguity of radar signal in Low Earth Orbit (LEO) dual-satellite Time Difference Of Arrival (TDOA) location system, the ambiguous characteristics of high PRF signal are analyzed, the process of ghost location generating by ambiguous TDOA is introduced, and a method for location ambiguity mitigation by Variation Trendline Matching of TDOA (VTMT) is proposed. If the condition that the residuals between the measured TDOA series and the theoretical TDOA series corresponding to the positioning results approximately obey normal distribution is satisfied, the location where the measured TDOA line is closest to the theoretical TDOA line can be selected as the real position of radar emitter with the Euclidean distance being used to measure the similarity between the two series. Test signal of calibration station validates the effectiveness of the proposed method. Numerical simulations demonstrate that the proposed method can increase the probability of unambiguous location significantly, reduce the requirements of TDOA measurement accuracy and observation time. The proposed method has good prospect for applications to TDOA location of radar signal in long base line location systems.

Roll Calibration for CryoSat-2: A Comprehensive Approach

CryoSat-2 is the first satellite mission carrying a high pulse repetition frequency radar altimeter with interferometric capability on board. Across track interferometry allows the angle to the point of closest approach to be determined by combining echoes received by two antennas and knowledge of their orientation. Accurate information of the platform mispointing angles, in particular of the roll, is crucial to determine the angle of arrival in the across-track direction with sufficient accuracy. As a consequence, different methods were designed in the CryoSat-2 calibration plan in order to estimate interferometer performance along with the mission and to assess the roll’s contribution to the accuracy of the angle of arrival. In this paper, we present the comprehensive approach used in the CryoSat-2 Mission to calibrate the roll mispointing angle, combining analysis from external calibration of both man-made targets, i.e., transponder and natural targets. The roll calibration approach for CryoSat-2 is proven to guarantee that the interferometric measurements are exceeding the expected performance.

Multi‐frame track‐before‐detect algorithm for disambiguation

In practice, medium pulse repetition frequency (PRF) radar system is widely used as it takes both the advantages of low and high PRF radars. However, range and Doppler ambiguities appear in such a complicated system. Traditional target tracking algorithms first threshold raw measurements and obtain points from different PRF, then perform ambiguity resolutions over them to obtain the unambiguous measurements. It leads to a rather poor performance when target signal-to-noise ratio is low. While multi-frame track-before-detect (MF-TBD) is an advantageous method to track weak targets, but it fails to be applied in multiple PRF radar system directly. In this study, two algorithms based on MF-TBD are proposed. Classical ambiguity resolution (CAR) TBD first resolves ambiguities by means of a robust Chinese Remainder Theorem, where the first detection threshold is below the normal threshold, and then MF-TBD is applied for further tracking. The simulation shows that the proposed method significantly outperforms the CAR detect-before-track algorithm.

2D Frequency Domain Fully Focused SAR Processing for High PRF Radar Altimeters

Fully-focusing of radar altimeters is a recent concept that has been introduced to allow further improvement of along-track resolution in high pulse repetition frequency (PRF) radar altimeters. The straight potentiality of this new perspective reflects into a more accurate estimation of geophysical parameters in some applications such as sea-ice observation. However, as documented in a recent paper, such capability leaves unsolved the problem of the high computational effort required. In this paper, we face the problem of adapting for altimeters the Omega-Kappa SAR focusing algorithm that is performed in the two-dimensional wavenumber domain, accounting for the difference existing between SAR and altimeter from geometry (looking and swath width) and instrument (echoes are deramped onboard on receiving) point of view. Simulations and an application using in-orbit data show the effectiveness of the proposed approach and the highly reduced computational effort.

Multi‐target tracking in clutter using a high pulse repetition frequency radar

In this study, two algorithms of single-target tracking in clutter using a high pulse repetition frequency radar are extended: the Gaussian mixture measurement likelihood-integrated track splitting (GMM-ITS) algorithm and the enhanced multiple models (MM) to multi-target tracking algorithm, that is, the GMM-joint ITS algorithm and the enhanced MM-joint probabilistic data association algorithm, respectively. Both algorithms are extended on the basis of the optimal Bayes approach that creates track clusters for determining the nearby tracks that share measurements by enumerating and evaluating all the feasible joint measurement allocations. In all cases, the track trajectory probability density function is a Gaussian mixture, and both algorithms enable false track discrimination using the probability of target existence.

Fixed lag smoothing target tracking in clutter for a high pulse repetition frequency radar

Abstract A new method to smooth the target hybrid state with Gaussian mixture measurement likelihood-integrated track splitting (GMM-ITS) in the presence of clutter for a high pulse repetition frequency (HPRF) radar is proposed. This method smooths the target state at fixed lag N and considers all feasible multi-scan target existence sequences in the temporal window of scans in order to smooth the target hybrid state. The smoothing window can be of any length N. The proposed method to smooth the target hybrid state at fixed lag is also applied to the enhanced multiple model (EMM) tracking algorithm. Simulation results indicate that the performance of fixed lag smoothing GMM-ITS significantly improves false track discrimination and root mean square errors (RMSEs).

Target tracking in clutter using a high pulse repetition frequency radar

Target tracking in clutter using a high pulse repetition frequency radar

Two helicopter classification methods with a high pulse repetition frequency radar

In this study, two helicopter classification methods with a high pulse repetition frequency radar are proposed. By analysing the returned signal from a main rotor, the authors find that the essential difference between rotors appears when none of blades is orthogonal to the incident waves. Based on this difference, the authors propose two methods to classify the helicopter. The first one is based on the matched filters (MFs), including the time MF and the time-frequency MF. The second one derives from time-frequency masks. Experimental results demonstrate that both methods have the ability to classify helicopters with the same micro-Doppler parameters at a low signal-to-noise ratio. And they are robust against errors of the estimated parameters and the gesture of the helicopter.

Range ambiguity resolving of HPRF radar based on hybrid filter

In this paper, a novel method of range ambiguity resolving based on hybrid filter is proposed for high-pulse repetition frequency (HPRF) radars. By considering the discrete-valued pulse interval number (PIN) which is described as the radial range of the target divided by the maximum unambiguous range for a certain HPRF and its incremental variable as target states, the problem of range ambiguity resolving is converted to the hybrid state estimation problem. In the initial time steps, since the range measurement is ambiguous, the hybrid state initialization is achieved by utilizing multiple HPRFs based on Euclidean distance. Then, to avoid the choice of the thresholds, the hybrid model based on six-dimension state vector is made equivalent to several hybrid models based on five-dimension state vector according to the values that PIN incremental variable may take. The hybrid estimation is achieved by comparing 2-norm of the innovations obtained from several hybrid filters. Simulation results demonstrate that the proposed method can converge more rapidly than the existing multiple hypothesis method when only one HPRF has target measurement and overcome the mistake that multiple hypothesis method makes when PIN varies.

Mitigating Range Ambiguity in High PRF Radar using Inter-Pulse Binary Coding

The paper proposes a way to increase the energy within a coherent processing interval (CPI) using more pulses instead of longer pulses. Long coded pulses result in masking targets at close range and poor Doppler tolerance. Increasing the number of pulses implies high pulse repetition frequency (PRF), which suffers from range ambiguity and target folding. These drawbacks of a high PRF can be mitigated by inter-pulse coding. The approach suggested here should be attractive for close and mid range applications of radar, ground penetrating radar, ultrasound imaging, and more.

Reduced-rank STAP for high PRF radar

Due to the range ambiguity of high pulse-repetition frequency (HPRF) radars, echoes from far-range fold over near-range returns. This effect may cause low Doppler targets to compete with near-range strong clutter. Another consequence of the range ambiguity is that the sample support for estimating the array covariance matrix is reduced, leading to degraded performance. It is shown that space-time adaptive processing (STAP) techniques are required to reject the clutter in HPRF radar. Four STAP methods are studied in the context of the HPRF radar problem: low rank approximation sample matrix inversion (SMI), diagonally loaded SMI, eigencanceler, and element-space post-Doppler. These three methods are evaluated in typical HPRF radar scenarios and for various training conditions, including when the target is present in the training data.

Medium PRF Performance Analysis

A discussion of various types of x-band airborne radars is presented together with their systematic development through the years to the present time. Starting with simple, low pulse-repetition frequency (PRF) radars for measuring radar-target range, airborne radar development proceeded with more sophisticated high PRF Doppler radars where radar-target range and range rate were measured simultaneously. The use of Doppler (frequency) in signal processing allowed the separation of moving from nonmoving targets (ground), enabling the detection of moving targets in the presence of ground clutter. More recent developments in waveform generation and selection has resulted in the development of medium PRF radars, whereby a greater degree of tactical flexibility in target detection is achieved by combining the desirable features of both low and high PRF radars. Part of the available literature gives an overview, together with a specific example of the design and performance of an airborne medium PRF radar. Here, however, the systematic evolution of these radars is emphasized and the necessary theoretical background is developed for their performance calculations. Modern day airborne radars may be equipped with all three modes of operation, low, medium, and high PRF, allowing the operator to utilize the mode best suited for the tactical encounter. Low PRF and high PRF radars have been described elsewhere and are given here primarily for the sake of completeness and for the necessary background for developing medium PRF radar equations. They are also needed for developing the reasons why medium PRF radars came into being.

Reduction of Eclipsing Loss in High PRF Radars

In order to reduce the eclipsing loss in high pulse-repetition frequency radars, one can use random pulse-repetition interval (PRI) waveforms. Random PRI waveforms are also desirable because they reduce the probability of intercept by unwanted listeners. On the other hand, since randomizing the PRI results in smearing the spectrum of the transmitted signal, the amount of clutter power (most seriously, main-lobe clutter) returned to the receiver will be enhanced over what it would have been had the PRI been uniform. Therefore, it is of interest to determine the exact form of the new power spectrum and further to determine the effects of that power spectrum on the radar system.