Azimuth Ambiguity Research Articles

A New Application for PolSAR Imagery in the Field of Moving Target Indication/Ship Detection

Recent analysis of a polarimetric synthetic aperture radar (PolSAR) velocity experiment has shown that azimuth ambiguities can provide useful information for the detection of moving targets with an across-track velocity component in PolSAR images using cross-polarization clutter cancellation. This paper examines this phenomenon using theoretical analysis and simulations and illustrates its application with experimental results. It is shown that, for reciprocal scatterers, the phase difference between HV and VH images is zero, whereas for HV and VH ambiguities, the phase difference is pi. For land applications, clutter cancellation is essential for the detection of moving targets in PolSAR images. For maritime applications, it can be used to distinguish images from ambiguities. However, it is important to note that the underlying physical reasons for the performance of clutter cancellation are entirely different from ground moving target indication along-track interferometric SAR. It is also shown that cross-pol clutter cancellation is related to filtering of a double-rate-sampled single-channel SAR signal.

Several problems in measuring solar vector magnetic fields

Observations of solar photosphere vector magnetic fields with a magnetograph are subjected to many difficulties, such as azimuth ambiguity, magnetic saturation, Faraday rotation, scattered light, polarization crosstalk, and wavelength shift, and so on. Among these issues, Faraday rotation and scattered light are obvious in local-region vector magnetic field measurements with the 35 cm Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS), while polarization crosstalk and wavelength shift are obvious in full-disk vector magnetic field measurements made with a new-developed telescope named Solar Magnetism and Activity Telescope (SMAT). In this paper we put emphasis on these four issues to show how they could affect the measurement accuracy of solar vector magnetic fields. Some preliminary attempts are also made to overcome them.

Fred Fisher and research with acoustic vector sensors; Marine Physical Laboratory’s vertical array of directional acoustic receivers and ocean noise

Fred Fisher had boundless enthusiasm for all topics acoustic. A chance encounter with him in the hallway usually led to a half-hour discussion of the latest research efforts at the lab and recent results he found exciting. In the 1980s, Fred became interested in the problem of identifying the physical phenomena forming the pedestal about the horizontal in vertical directionality measurements of the deep ocean’s low-frequency noise field. Two competing mechanisms had been proposed: downslope conversion of coastal shipping and noise from high latitude winds coupling into the deep sound channel due to the shoaling of the sound channel axis. The relative contributions of these two mechanisms possibly could be separated if the azimuthal ambiguity of a vertical line array of hydrophones somehow could be broken. Therefore, Fred proposed to build a vertical array of ‘‘DIFAR’’ sensors, which led to the design and construction of the Marine Physical Lab’s Vertical ‘‘DIFAR’’ Array. This talk will reminisce a bit about Fred as well as present some results from an ambient noise experiment conducted in 1992 on the continental shelf using the Vertical DIFAR Array co-deployed with MPL’s freely drifting vector sensors, the Swallow floats. [Work supported by ONR and ONT.]

Helicity Measurements from Two Magnetographs

We use 270 pairs of vector magnetograms observed by Haleakala Stokes Polarimeter (HSP) and Solar Magnetic Field Telescope (SMFT) of Huairou Solar Observing Station from 1997 to 2000 to compare current helicity derived by these two instruments. We apply the same approach to both data sets to resolve 180∘ azimuth ambiguity and compute α coefficient of linear force-free field. After careful consideration of various aspects of both data sets, we find that in ≈80% of cases SMFT and HSP data result in the same sign of α, and the Pearson linear correlation coefficient between two data sets is rp = 0.64.

Simulation and graphical representation of the orbit and the imaging parameter of Earth observation satellites

Synthetic aperture radar (SAR) satellites are widely used for Earth observation applications as weather conditions and cloudiness do not affect them. However, in order to be usable, data provided by those satellites need to be processed. This processing requires determination of imaging parameters that are closely linked to the spacecraft position and velocity on its orbit. For example, the difference in velocity of the Earth's surface and the spacecraft produces Doppler shift in the received signal. This decay, called Doppler centroid, is one of the most important parameters required by spaceborne SAR signal processing. As an illustration, the Antarctic anomaly that occurred in ERS data processing induced severe azimuth ambiguities (“ghosting”) in the whole image. Those parameters also vary in relation to the attitude angles of the platform. In order to estimate the Doppler centroid, determination of orbital parameters and their evolution has to be made. In this paper, the method to perform orbital osculating elements calculation and imaging parameters determination for SAR satellites, starting from state vectors or two-line elements files and attitude angles, will be introduced as well as the perturbation due to the Earth flatness, called the J2 perturbation. The near-circular orbit often used by Earth observation satellites makes the perturbation equations of the J2 parameter unstable, and the introduction of new orbital parameters is required in order to be able to determine the evolution of the orbital parameters. Simulations will be performed and the resulting osculating elements will be presented and explained for short-term and long-term variations for different kinds of orbits, such as GEO, SSO, LEO and Molnya-orbit. Full analysis of the imaging parameters will be made for the SAOCOM satellite with particular attention to the Doppler centroid phenomenon: its sensitivity with relation to the attitude angles and the way to cancel it by setting the yaw steering dynamically.

Adaptive removal of azimuth ambiguities in SAR images

We introduce an innovative algorithm that is capable of suppression of strong azimuth ambiguities in single-look complex (SLC) synthetic aperture radar images, to be used both for detected products and for interferometric surveys. The algorithm exploits a band-pass filter to select that portion of the azimuth spectrum that is less influenced by aliasing, the one that corresponds to the nulls in the replicated azimuth antenna pattern. The selectivity of this filter adapts locally depending on the estimated ambiguity intensity: the filter is more selective in the presence of strong ambiguities and becomes all-pass in absence of ambiguities. The design of such filter frames in the Wiener approach with two different normalization options, depending on the use of the SLC image, either for getting multilook averaged detected products or for interferometric applications. Preliminary results achieved by processing ENVISAT Advanced Synthetic Aperture Radar sensor data are provided.

Evaluation of JERS-1 SAR Images From a Coastal Wind Retrieval Point of View

Wind retrieval from Japanese Earth Resources Satellite-1 (JERS-1) synthetic aperture radar (SAR) using an L-band model function in coastal regions is evaluated. It is known that JERS-1 SAR has excessive ambiguities. This paper also gives a quantitative evaluation of excessive ambiguities in coastal scenes of JERS-1 SAR. First, focusing on the cases where wind blows from the shore in Sagami Bay, we investigate phenomena of wind speed increase with offshore distance using European Remote sensing Satellite-1 (ERS-1) SAR-derived wind speeds. The relation between wind speed and offshore distance is well formulated, which indicates the transition of the atmospheric boundary layer from land to sea surface. Wind speeds derived from JERS-1 SAR should be overestimated due to the excessive ambiguity. Then, for observation time of each JERS-1 SAR capturing the cases that wind blows from the shore in Sagami Bay, the expected wind speed growth profile is derived from the wind speed growth formula and an in situ wind observation of Hiratsuka Experiment Station. We convert the wind-speed profile into the sigma-0 profile by an L-band model function. Finally, the profiles of JERS-1 SAR-observed and the estimated sigma-0 are compared, and the excessive ambiguity is estimated as the difference between them. As a result, the dynamic range of first azimuth ambiguity is as large as that of the wind-relating signal from the ocean surface. Moreover, higher order azimuth ambiguities and range ambiguity also may have a significant impact on near-shore wind retrieval.

On the Resolution of the Azimuthal Ambiguity in Vector Magnetograms of Solar Active Regions

We introduce a technique to resolve the azimuthal ambiguity of 180°, intrinsic in solar vector magnetic field measurements. We resolve the 180° ambiguity by minimizing the inhomogeneities of the magnetic field strength perpendicular to the magnetic field vector. This relates to a minimization of the sheath currents that envelope the solar magnetic flux tubes, thus allowing for more space-filling and less complex magnetic fields. Structure minimization proceeds in two steps: First, it derives a local solution analytically, by means of a structure minimization function. Second, it reaches a global solution numerically, assuming smoothness of the magnetic field vector. Structure minimization (i) is disentangled from any use of potential or linear force-free extrapolations and (ii) eliminates pixel-to-pixel dependencies, thus reducing exponentially the required computations. We apply structure minimization to four active regions, located at various distances from disk center. The structure solution for each case is compared with the minimum energy solution obtained by the slower simulated annealing algorithm. We find correlation coefficients ranging from significant to excellent. Moreover, structure minimization provides an ambiguity-free vertical gradient of the magnetic field strength that reveals the variation of the magnetic field with height. The simplicity and speed of the method allow a near real-time processing of solar vector magnetograms. This task was not possible in the past and may be of interest to both existing and future solar missions and ground-based magnetographs.

Analysis of multistatic configurations for spaceborne SAR interferometry

Spaceborne bistatic and multistatic SAR configurations are an attractive approach to acquire along-track and cross-track interferograms on a global scale. An efficient realisation of such systems may be achieved by a set of low-cost, passive receivers onboard a constellation of microsatellites which simultaneously record the backscattered signals transmitted by a conventional spaceborne radar. The authors introduce several multistatic SAR configurations suitable for global single-pass interferometry and discuss their advantages and limitations. The achievable interferometric performance is analysed in detail, taking into account thermal noise, block adaptive quantisation, range and azimuth ambiguities, and geometric decorrelation for both surface and random volume scatterers. Based on the estimated interferometric phase errors, the relative height accuracies for three illuminators (PALSAR, ASAR, TerraSAR-X) are derived. The achievable height accuracies are of the order of 2 m for PALSAR and ASAR, and of the order of 1 m for TerraSAR-X. However, it turns out that for vegetated areas, volume decorrelation may become a limiting factor for configurations with large interferometric baselines. These restrictions can be overcome by making the interferometric configuration fully polarimetric and/or increasing the number of available baselines.

Use of Principal Component Analysis to Determine Down-hole Tool Orientation and Enhance SH-Waves

A common problem in down-hole shear wave surveys is the determination of the tool rotation relative to the seismic source polarization direction. This paper reports an algorithm which has been developed to solve for this angle in the horizontal plane. The method employs Principal Component Analysis (PCA) to determine the angle from the large motion of the SH-wave (not first motion). Once the angle is known, the horizontal component data may be rotated so that one component is aligned with the source polarization, and the other orthogonal to it. Significant findings include the following. First, subtraction of oppositely polarized source efforts improves the PCA formulation by enhancing the S-waves. Second, the Swave enhancement is improved by scaling the oppositely polarized source efforts by the vertical (not horizontal) component data, as this provides a better attenuation of non-Swave energy. Third, observation of the tool orientation as it exits the borehole is helpful in resolving the [Formula: see text] azimuthal ambiguity in the PCA approach. Finally, the polarization of the source radiation may drift relative to the axis of the source during a survey, and should not be assumed invariant.

Helicity computation using observations from two different polarimetric instruments

We compare vector magnetograms of active region NOAA 5747 observed by two very different polarimetric instruments: the imaging vector magnetograph of Huairou Solar Observing Station (HSOS) and the Haleakala Stokes Polarimeter of Mees Solar Observatory (MSO). Unlike previous comparative studies, we concentrate our attention on differences in observations and data reduction techniques that can affect the helicity computation. Overall, we find a qualitative agreement between the HSOS and MSO vector magnetograms. The HSOS data show slightly higher field strength, but the distribution of inclination angles is similar in measurements from the two instruments. There is a systematic difference (up to ∼20°) in the azimuths of transverse fields, which is roughly proportional to the longitudinal field strength. We estimate that Faraday rotation in the HSOS magnetograms contributes ∼12° in the azimuth difference if possible sources of error are taken into account. Next, we apply two independent methods to both data sets to resolve 180° azimuth ambiguity and to compute two helicity measures – the force-free field parameter αbest and the current helicity fractional imbalance ρh. The methods agree reasonably well in sign and value of the helicity measures, but the HSOS magnetograms show systematically smaller values of ρh and αbest in agreement with an expected contribution of Faraday rotation. Finally, we discuss the role of Faraday rotation in computation of αbest and ρh and conclude that it does not affect the strength of the hemispheric helicity rule. The strength of the rule appears to be related to a helicity parameter: αbest shows weaker hemispheric asymmetry than ρh.

Wide-swath space-borne SAR using a quad-element array

The authors describe an array-based approach to increasing the swath width of space-borne synthetic aperture radar beyond the limit normally imposed by ambiguity. The nulls of a quad-element rectangular array are used to suppress returns from range and azimuth ambiguities, allowing the swath to be increased by a factor of approximately four. A conceptual design is presented, based on ERS-1, that gives a swath extending from 250 km off nadir to over 700 km, with a blind region centred at 500 km. Some adaptive techniques for steering the nulls are discussed.

Coherent and incoherent broadband matched-field processing with SWellEX-96 data

Several approaches to passive broadband matched-field processing have been proposed. In most cases, an incoherent average over a narrow band is quite successful in driving down unwanted sidelobes. The ability of coherent broadband techniques to out perform this simple approach may require additional phase information about the source that, in general, may not be available. In this paper, a fully coherent method that can out perform an incoherent average is derived, although its practical application is problematic. In other words, the simple incoherent average may be the most efficient approach. These ideas are demonstrated using data from the SWellEx-96 experiment. Given the environmental conditions under which these data were taken, the new coherent approach has some physical advantages over other broadband methods since it more efficiently uses variations of range dependence exhibited by the different frequency components of the signal. The net result is that the range, depth, and azimuth ambiguities have comparable spatial extent.

ScanSAR processing and simulation for ASAR using ERS-1 raw data

Abstract This paper presents results from a study on scanSAR processing and simulation. ERS-1 raw data in both normal mode and roll tilt mode are used for Doppler centroid estimation, processing and simulation. Azimuth gain variations and azimuth ambiguity ratios are evaluated for different scanning strategies and as a function of Doppler centroid errors. A scanning strategy with two scan cycles in each imaging cycle has been selected and a processing scheme with three range looks and two azimuth looks has been implemented. SPECAN is considered very favourable for azimuth processing because or its efficiency. The degradation in image quality is investigated by imposing individual (differential) beam pointing errors on real ERS-1 data. It is demonstrated in this work that the individual azimuth beam pointing errors should be less than 0004° to avoid visible degradations in homogeneous regions in the scanSAR image. Using many bursts and all subswaths for Doppler centroid estimation, it is assumed that Doppl...

Unambiguous noise and reverberation measurements from a dual line horizontal array

Single horizontal line arrays are often utilized to resolve the azimuthal character of the low‐frequency noise dominated by long‐range shipping sources. However, such arrays are subject to an azimuthal ambiguity on conjugate bearings. Coherent beamforming of an array composed of two parallel horizontal lines was recently employed to eliminate this left/right ambiguity using a minimum variance adaptive algorithm with single‐point main response constraints under an assumed horizontally isotropic noise field. The ability of this array and beamformer to resolve the directional characteristics of the acoustic noise field was demonstrated in the Atlantic and the Mediterranean. Median left/right rejection of 20 dB or more was observed over a wide frequency and azimuthal band. The measured beam noise exhibits temporal and spatial variability that could not have been quantified with a single‐line system. Median noise gain improvements of up to 7 dB above that of a single line were measured. Distant low‐frequency reverberation ambiguity resolution was also measured using the reverberation from signals underwater sound (SUS) explosive charges deployed in the Atlantic. Reverberation left/right rejection of 20 dB was achieved despite imperfect, nonparallel line tow conditions.

On the use of the ideal filter concept for improving SAR image quality

The conventional matched filter forms the basis for SAR image generation. Although the impulse response function obtained by the matched filter approach has the best achievable signal-to-noise ratio, high sidelobes, paired echoes and azimuth ambiguities will appear in the impulse response function (IRF) for severe phase and amplitude errors. This paper considers two types of error: 1) SAR transceiver chain errors and instabilities which influence the range IRF and 2) azimuth signal aliasing which leads to azimuth ambiguities. In order to suppress these errors and to improve the SAR image quality, an ideal filter concept is used which provides, in addition to the matched filtering, the deconvolution of the phase and amplitude errors. Several simulation results are presented concerning the improvement of the range and azimuth IRFs. In the case of range compression, the sidelobes can efficiently be suppressed if the replica of the transmitted signal is known with sufficient accuracy. For azimuth processing, ...

Design of a monopulse SAR system for the determination of elevation angles

Terrain height variations in mountainous areas cause problems in the radiometric correction of SAR images. The authors propose a novel SAR system which exploits the monopulse principle to determine the elevation angle and thus the height at the different parts of the image. From the ratios of images radiometrically modulated by the difference and sum antenna pattern in cross-track direction, the authors can calculate the appropriate elevation angle at any point in the image. In this paper, they present design considerations for an array antenna for DLR's airborne X-band SAR system and give estimates of the error due to system noise and azimuth ambiguities as well as the expected performance and precision in topographic mapping.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Suppressing the azimuth ambiguities in synthetic aperture radar images

A method for removing the azimuth ambiguities from synthetic aperture radar (SAR) images is proposed. The basic idea is to generate a two-dimensional reference function for SAR processing which provides, in addition to the matched filtering for the unaliased part of the received signal, the deconvolution of the azimuth ambiguities. This approach corresponds to an ideal filter concept, where an ideal impulse response function is obtained even in the presence of several phase and amplitude errors. Modeling the sampled azimuth signal shows that the absolute phase value of the ambiguities cannot easily be determined due to their undersampling. The concept of the ideal filter is then extended to accommodate the undefined phase of the ambiguities and also the fading of the azimuth signal. Raw data from the E-SAR system have been used to verify the improvement in image quality obtained by the new method. It has a substantial advantage in enabling the pulse-repetition frequency (PRF) constraints in the SAR system design to be relaxed and also for improving SAR image quality and interpretation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A spectroscopic method for the solution of the 180-deg azimuth ambiguity in magnetograms

A spectroscopic method is proposed for removing the azimuth ambiguity intrinsically present in vector magnetograms. The method is based on the idea that the polarization observed in resonance lines (typically formed at the chromospheric level) depends not only on the magnetic field vector, but also on the anisotropy of the radiation field that is illuminating the scattering atoms. An observational scheme is proposed.

A source reconstruction method for multiple scatter Compton cameras

A new method for finding the direction of gamma rays incident on a multiple scatter Compton camera has been proposed. The method uses the polarization dependence of double Compton scattering to reduce the azimuthal ambiguity normally associated with interpreting Compton scattering events. The CYLTRAN photon Monte Carlo code was modified to include the polarization effects of gamma rays undergoing Compton scattering in the 150-1000-keV energy range and used to test the method. Results for 150-keV gamma rays incident upon a multiple Compton scatter imaging camera are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>