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The objective of this work is to investigate the Ambiguity Function (AF) characterization of the Iridium L‐band downlink burst transmissions under single burst and multiband multi‐burst mode to demonstrate its suitability and capability as Signal of OPportunity (SoOP) for navigation. The Iridium bursts are collected and examined by demodulating without any prior knowledge for frame verification. The comprehensive analysis and theoretical model of the two mode AFs are presented and verified to evaluate the waveform properties with resolution and sidelobes in both the Doppler and delay domains. Degradation in AF resulting from signal model and multiband channel combination is characterized to exploit the trade‐offs for selection and utilization of the Iridium burst. It has been found that the single burst even with maximum length of 20.32 ms appears available but far from excellent for navigation, while the multiband mode produces significant improvements over the time delay and Doppler resolution as well as a certain degradation on the AF sidelobes. Issues relating to the Iridium bursts utilization have been identified. [ABSTRACT FROM AUTHOR]

Suppressing false alarms without impacting detection rate is an essential issue for search radars. Traditional threshold detection methods identify true targets by simply comparing the energy of target candidates with a threshold. In this work, a new data‐driven false‐alarm suppression approach is proposed based on a random forest model. It discriminates true targets and false alarms in the developed multi‐dimensional ambiguity feature spaces, instead of the time‐frequency features used in previous works, to fulfill short dwell‐time requirement of search radars. Evaluations through field experiments demonstrate that the proposed method can achieve as high as 98% validation accuracy and a significant improvement of detection performance compared to the conventional threshold detection. [ABSTRACT FROM AUTHOR]

Owing to the existence of non-line-of-sight (NLOS) signals in global navigation satellite system (GNSS) challenged environment, the real-time kinematic (RTK) position precision is seriously damaged. In order to improve the position accuracy, the authors propose a robust positioning method which tackles this problem by detecting the NLOS through a decision tree and estimating the double-differenced multipath errors in real time. The NLOS detection performance of the proposed method is 95.64%. The field experiment shows that the ambiguity fixing rate has improved by 43% in the instantaneous mode, and the 3D position accuracy is about 81.77% higher better than that of normal RTK method which is implemented by using RTKLIB software. [ABSTRACT FROM AUTHOR]

In Global Navigation Satellite System (GNSS) community, pseudorange observables, without integer ambiguity, have large measurement noise, while carrier phase observables, with high measurement accuracy, have integer ambiguity, which restricts the direct use either of them for high‐accuracy navigation and positioning. In this regard, fusing pseudorange and ambiguity‐free time‐differenced carrier phase (TDCP) is a popular data processing technique. In this Letter, the least‐squares fusion is studied. A theoretical analysis of the steady performance of this fusion is conducted. A set of formulas of the steady performance is derived, based on which simulations are carried out for different pseudorange‐carrier‐phase precision ratios. Results showed that the steady accuracy of fusion pseudorange is about two orders of magnitude higher than that of the original pseudorange, and is close to that of carrier phase observables. Besides, the convergence speed and the comparison to the Hatch filter are also checked in the simulations. [ABSTRACT FROM AUTHOR]

An iterative adaptive approach (IAA) based algorithm is presented to realise coherent integration for the random pulse repetition interval radar. The proposed method firstly removes the effect of Doppler ambiguity via phase compensation, and then accomplishes target focusing via conducting the azimuth IAA and range inverse fast Fourier transform. Theoretical analysis and numerical experiments have verified the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In Global Navigation Satellite System (GNSS) community, pseudorange observables, without integer ambiguity, have large measurement noise, while carrier phase observables, with high measurement accuracy, have integer ambiguity, which restricts the direct use either of them for high-accuracy navigation and positioning. In this regard, fusing pseudorange and ambiguity-free time-differenced carrier phase (TDCP) is a popular data processing technique. In this Letter, the least-squares fusion is studied. A theoretical analysis of the steady performance of this fusion is conducted. A set of formulas of the steady performance is derived, based on which simulations are carried out for different pseudorange-carrier-phase precision ratios. Results showed that the steady accuracy of fusion pseudorange is about two orders of magnitude higher than that of the original pseudorange, and is close to that of carrier phase observables. Besides, the convergence speed and the comparison to the Hatch filter are also checked in the simulations. [ABSTRACT FROM AUTHOR]

Non‐data‐aided (NDA) carrier phase recovery (CPR) is one of simple CPR algorithms which simply rotates received symbols to the nearest constellation point. Unfortunately, when the NDA CPR is used with M‐ary phase shift keying, it suffers inherent M‐fold phase ambiguity and thus the phase ambiguity must be resolved. On the other hand, the generator matrix of polar code G can be obtained with the Kronecker product of a 2×2 lower triangular matrix. Hence, the Hamming weights of all columns of G are given by non‐negative integer powers of 2. Based on this observation, an 180° rotationally invariant polar code suitable for digital communication systems employing binary phase shift keying and NDA CPR is proposed. The proposed polar code offers virtually identical performance under both 0° and 180° carrier phase offsets to that of conventional polar code with perfect CPR with identical encoding and decoding complexities. [ABSTRACT FROM AUTHOR]

The attitude of synthetic aperture radar (SAR) antenna cannot be fully controlled or precisely measured, which causes the ambiguity of Doppler centroid and thus degrades the image quality. A few Doppler ambiguity resolvers (DARs) have been developed by evaluating image quality, which focus on the data with all possible ambiguity numbers until the maximum image contrast or the minimum image entropy is found. Despite of their robustness on various cases, these approaches rely on repeatedly focusing the data, resulting in a heavy computational load. A new DAR that successfully employs the property of range profiles is proposed, and this approach greatly improves the efficiency while keeping robustness. [ABSTRACT FROM AUTHOR]

Fully polarimetric synthetic aperture radar (PolSAR) data are represented by four channels and the coherences between two arbitrary channels are important descriptors for the interpretation of polarimetric scattering mechanisms. However, there exists a scattering ambiguity between oriented urban and natural areas since they both contribute to the cross‐polarised (HV) scattering. A polarimetric correlation coefficient ratio is proposed to overcome this scattering ambiguity in PolSAR imagery. First, the polarimetric coherences of urban areas with various orientation angles were analysed. Subsequently, it was found that the proposed coherence parameter considers the reflection asymmetry of buildings and also the scattering divergence between co‐polarised and cross‐polarised channels, which is beneficial to the scattering analysis in urban areas. The effectiveness of the proposed parameter is demonstrated using the Phased Array type L‐band Synthetic Aperture Radar dataset. [ABSTRACT FROM AUTHOR]

A novel Lq(0 < q ≤ 1) regularisation‐based synthetic aperture radar (SAR) image feature enhancement method via SAR complex image data is presented. Compared with conventional Lq regularisation‐based compressive sensing imaging technique with full‐sampling raw data, the proposed method can achieve the same effect of image feature enhancement while enjoying much less computational cost. Through simulations and experiments, the superiority of the proposed method is demonstrated in terms of sidelobe, azimuth ambiguity, clutter suppression and super‐resolution. [ABSTRACT FROM AUTHOR]

Compared with low‐earth‐orbit synthetic aperture radar (SAR), geosynchronous SAR (GEO SAR) due to the long synthetic aperture time and the sensitive effect of the earth's rotation, has great range migration in its echo and the extension of the receiving time window, resulting in the conventional timing design unusable. To solve this problem, a timing design method for GEO SAR is proposed in which the range migration is taken into consideration to avoid loss of echo. The equations and steps of the timing design are given in detail and in addition the range ambiguity and the azimuth ambiguity for GEO SAR are also discussed. Simulated results have verified the validity of the proposed timing design method for GEO SAR. [ABSTRACT FROM AUTHOR]

To obtain high direction-finding (DF) accuracy in electronic warfare applications, it is necessary to maximise the number of effective data points with a small angle-of-arrival error. Thus, a method to obtain the DF ambiguity probability for the selected set of array spacing is required. In this Letter, a novel method to obtain DF ambiguity probabilities for four- and five-element arrays by using gamma distributions is presented. The DF ambiguity probability obtained by the proposed method for the five-element array was in good agreement with the results of simulations. For the four-element array, the proposed method produced data that showed approximate agreement with the results of simulations. Thus, the proposed method should be useful for future applications. [ABSTRACT FROM AUTHOR]

Tomography SAR (TomoSAR) imaging can achieve threedimensional (3D) imaging, which is significant for urban 3D mapping. When the baseline of TomoSAR is sparse, the scattering centres of several different altitudes in the same resolution unit are aliased together, which affects the accuracy of TomoSAR imaging. To solve this problem, an altitude ambiguity suppression method based on dual-frequency interfering and multi-frequency averaging is proposed. Firstly, dual-frequency interfering is used to obtain the radar cross-section (RCS) of targets and the sum of the results of the cross interference between targets and ambiguities in different positions. Then multi-frequency averaging is used to suppress the power of the cross-interference, and finally the RCS of target is accurately obtained. Simulation results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Recovering a distance from ambiguous phase measurements observed at multiple signal frequencies involves the estimation of both integer and scalar quantities simultaneously. The problem is optimally addressed by existing methods only when the measurement noise is IID. A noise 'whitening' procedure is derived with a least‐square estimation structure inherent to the Lattice method proposed previously which thus extends this Lattice‐based algorithm to cope with coloured phase measurement noise in an optimal manner demonstrated via simulation. [ABSTRACT FROM AUTHOR]

Theoretical imaging performance for forward-looking synthetic aperture radar (SAR) with simultaneous cross-track transceiver translation is characterised. Cross-range resolution as a function of transceiver transverse velocity and synthetic aperture length is analysed. Then the transverse velocity needed to avoid left-right ambiguity is derived. Lastly, imaging performance is demonstrated by focusing simulated data. [ABSTRACT FROM AUTHOR]

A new Doppler ambiguity resolver (DAR) is proposed for ground moving targets of synthetic aperture radar. After range migration correction in range–Doppler domain based on a static target, it is found that the absolute value and the sign of Doppler ambiguity integer can be determined according to the range blur and the inclination direction of a moving target signal response, respectively. The range blur based DAR is computationally efficient and works better in many scenarios compared with the existing Radon transform based DAR in 2D time domain. Finally, some numerical experimental results are provided to demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

An improved decision mechanism to the previously advertised correlation power analysis (CPA) type of attack which is applied on the Montgomery ladder exponentiation steps of the Application-Specific Integrated Circuit (ASIC) RSA implementation is proposed. As the nature of this previous attack, for each m-bit sized windows of the key bits, ratios of the correlation values of the correct type vector with all the other candidate vectors must also exist between the peak value of the correlation curve of the correct type vector and of the other candidate vectors. As a novelty, proposed method uses this property; namely, it searches the closest matching between these ratios to decide the correct bit type vector. It is experimentally and theoretically shown that this decision mechanism requires lesser number of traces to extract the correct key bit types compared with the previous CPA type attack. Also this decision methodology can be used to improve the effectiveness of other possible CPA type of attacks which construct power leakage models for more than one bit. [ABSTRACT FROM AUTHOR]

The issue of ambiguity of objective image quality assessment is addressed. The necessity of considering perceptual ambiguity of two objective quality scores is justified first. Then, an approach to obtain an ambiguity interval defining the quality score range is developed within which quality difference is not perceptually distinguishable. Experimental results demonstrate the validity of the developed approach and examine various aspects of the ambiguity of objective quality assessment in viewpoints of the type of image distortion, objective quality metric, image content etc. [ABSTRACT FROM AUTHOR]

Localisation error due to sensitivity to position ambiguity, with respect to some neighbouring nodes, is a major problem in wireless localisation. To solve this problem, the tangent linear approximation (TLA) for the nonlinear localisation model is derived, contributing to incorporating the effect of the neighbours' position ambiguity into belief propagation (BP) localisation. Theoretical analysis and numerical results indicate that the TLA‐BP algorithm can acquire higher accuracy and convergence probability compared with other typical methods. [ABSTRACT FROM AUTHOR]