Along-track Interferometric Synthetic Aperture Radar Research Articles

Estimating ocean surface velocity and coherence time using multichannel ATI-SAR systems

Conventional along-track interferometric synthetic aperture radars (ATI-SARs) derive the ocean surface velocity from an estimate of the phase difference between the SAR echoes received from two displaced phase centres with a single time-lag. The authors propose an efficient asymptotic maximum likelihood (ML) algorithm for jointly estimating the ocean surface velocity and ocean coherence time by using multichannel SAR data collected from an array of phase centres with multiple time-lags. The method combines a covariance matching algorithm with the use of the extended invariance principle (EXIP). Simulated results show that the proposed technique provides better estimation accuracy than the conventional two-channel system. Moreover, it provides unambiguous velocity retrieval and flexibility to varying ocean coherence time.

Numerical study on the along-track interferometric radar imaging mechanism of oceanic surface currents

The phase information in along-track interferometric synthetic aperture radar (along-track INSAR, ATI) images is a measure of the Doppler shift of the backscattered signal and thus of the line-of-sight velocity of the scatterers. It can be exploited for oceanic surface current measurements from aircraft or spacecraft. However, as already discussed in previous publications, the mean Doppler frequency of the radar backscatter from the ocean is not exclusively determined by the mean surface current, but it includes contributions associated with surface wave motion. The authors present an efficient new model for the simulation of Doppler spectra and ATI signatures. The model is based on Bragg scattering theory in a composite surface model approach. They show that resulting Doppler spectra are consistent with predictions of an established model based on fundamental electrodynamic expressions, while computation times are reduced by more than one order of magnitude. This can be a key advantage with regard to operational applications of ATI. Based on model calculations for two simple current fields and various wind conditions and radar configurations, they study theoretical possibilities and limitations of oceanic current measurements by ATI. They find that best results can be expected from ATI systems operated at high microwave frequencies like 10 GHz (X band), high incidence angles like 60/spl deg/, low platform altitude/speed ratios, and vertical (VV) polarization. The ATI time lag should be chosen long enough to obtain measurable phase differences, but much shorter than the decorrelation time of the backscattered field.

An exact analytic representation of a regular or interferometric SAR image of ocean swell

The problem of obtaining quantitative data on spatial ocean wave spectra from the images of the ocean surface by either regular SAR or along-track interferometric SAR (INSAR) is studied. The dominant mechanism which allows imaging of ocean waves by SAR/INSAR is the so-called velocity bunching. This mechanism is essentially nonlinear. The theoretical analysis of SAR/INSAR imagery of the ocean surface due to velocity bunching is performed, and nonlinear solutions of the SAR/INSAR images of monochromatic waves and of the spectra of these images are obtained. Analytic expressions are presented which allow for the accurate simulation both SAR and INSAR images of waves with arbitrary lengths, heights and propagation directions. It is demonstrated that a monochromatic wave expands in the SAR/INSAR images into an infinite number of harmonics. In addition to the nonlinearity parameters of SAR which is related to the velocity bunching mechanism, it is shown that for complex INSAR, the degree of nonlinearity depends also on separation time between the two antennas. The results of the present study indicate that in addition to the prevailing practice to consider the phase component of the INSAR image, an analysis of the imaginary part of the complex INSAR map of the ocean surface may provide some supplementary information, beneficial, in particular, for rough sea.

Validation of along-track interferometric SAR measurements of ocean surface waves

C-band HH-polarized along-track interferometric synthetic aperture radar (ATInSAR) images of ocean waves from the Canada Centre for Remote Sensing (CCRS) CV-580 SAR are used to study ATInSAR wave imaging fidelity. ATInSAR phase spectra are derived using cross spectra between the interferogram phase generated from individual look complex SAR images. The resulting observed phase spectra are quantitatively compared with forward-mapped in situ directional wave spectra collocated with the ATInSAR observations. The forward mapping models include two quasilinear forward transforms that include varying degrees of velocity bunching nonlinearity, both of which include an azimuth null in the modulation transfer function, and an implementation of the fully nonlinear forward transform. The null arises from the cancellation of upward and downward orbital velocity components within a resolution cell. The second of the two quasilinear transforms and the nonlinear forward transform are shown to have agreement with the observed velocity spectra, as measured by correlation coefficients between the observed spectra or forward-mapped spectra and the ATInSAR spectra. ATInSAR can allow a direct measurement of the orbital velocity spectrum. There is, however, an added layer of complexity caused by the cancellation of upward and downward orbital velocity components within a resolution cell. Furthermore, the SAR complication of velocity bunching nonlinearity is not avoided by using an ATInSAR.

A new nonlinear integral transform relating ocean wave spectra to phase image spectra of an along-track interferometric synthetic aperture radar

The along-track interferometric synthetic aperture radar (AT-INSAR) has the potential to measure ocean surface currents and ocean surface waves. The authors have derived a new nonlinear integral transform that relates the ocean wave spectrum to the AT-INSAR phase image spectrum. This integral transform is obtained from an approximation of the mapping integral that includes the radial component of the velocity of the scatter elements and velocity bunching. However, it neglects the modulation of the normalized radar cross section by the long ocean waves, which has a small effect on the AT-INSAR phase. Under the assumption that the imaging is only weakly nonlinear, the nonlinear integral transform can be further simplified. In this case, the relationship between the AT-INSAR phase image spectrum and the ocean wave spectrum can be described by a linear transfer function.

Radar‐derived interferometric surface currents and their relationship to subsurface current structure

Radar‐derived ocean surface currents are analyzed in conjunction with in situ acoustic Doppler current profiler (ADCP) measurements. The interferometric measurements were collected by an X‐band imaging Doppler radar in a manner analogous to those of along‐track interferometric synthetic aperture radar (ATI‐SAR). While the advent of ATI‐SAR has provided a new, potentially powerful technique for current mapping, the relationship between surface currents and interferometric velocity measurements is not yet clearly understood. This paper presents comparisons between radar‐derived and in situ current measurements. To develop a precise method for estimating the surface current from interferometric measurements, the influence of long wave orbital velocities and the influence of Bragg resonant waves are studied. We find that coupling between the orbital velocity and backscattered power (i.e., the modulation transfer function) can bias surface current estimates, potentially by up to 20 cm s−1 in an upwind viewing orientation. Furthermore, experimental observations verify a cos2n (θ/2) analytical model for the directional spreading of Bragg resonant waves. Extending our analysis to include subsurface currents, case studies are presented under varying environmental conditions for which the vertical current structure changes considerably. Analysis of radar imagery yields both radial surface currents and vector subsurface current estimates derived from long wave dispersion characteristics. Combining these with coincident ADCP measurements yields a vertical profile of current. Using these measurement techniques, we make several observations within the upper meter of the ocean. These profiles reveal the sensitivity of X‐band interferometric measurements to wind‐drift and the near‐surface current structure.

Simulation of ocean waves imaging by an along-track interferometric synthetic aperture radar

A two-dimensional (2D) model for describing the imaging of ocean waves by an along-track interferometric synthetic aperture radar (AT-INSAR) is derived. It includes the modulation of the normalized radar cross section by the long waves, velocity bunching, and azimuthal image smear due to orbital acceleration associated with long waves and due to the orbital velocity spread within the AT-INSAR resolution cell (parameterized by the scene coherence time). By applying the Monte-Carlo method, AT-INSAR amplitude and phase image spectra are calculated for different sea states and radar configurations. The Monte-Carlo simulations show that velocity bunching affects the AT-INSAR imaging mechanism of ocean waves, and that a unimodal ocean wave spectrum may be mapped into a bimodal AT-INSAR phase image spectrum due to an interference between the velocity term and the velocity bunching term in the AT-INSAR imaging model. It is shown that the AT-INSAR imaging mechanism of ocean waves depends on the ratio of the scene coherence time and the time separation between the observations by the two antennas. If this ratio is larger than one, the AT-INSAR phase image spectra are distorted. Furthermore, the simulations show that the AT-INSAR phase image spectrum is quite insensitive to the ocean wave-radar modulation transfer function. Comparing AT-INSAR with conventional SAR imaging of ocean waves, the authors find that the azimuthal cut-off in AT-INSAR phase image spectra is shifted toward higher wavenumbers than in conventional SAR image spectra. This implies that AT-INSAR can resolve shorter azimuthal wavenumbers than conventional SAR. Thus the authors conclude that AT-INSAR phase images are better suited for measuring ocean waves spectra than conventional SAR images.

Ocean Surface Feature Detection with the CCRS Along-Track InSAR

RÉSUMÉDe 1993 à 1996, le Centre canadien de télédétection (CCT) a possédé et opéré un capteur radar à synthèse d'ouverture (RSO) aéroporté en bande C (l = 5.656 cm) doté de capacités interférométriques azimutales. Le capteur interférométrique InSAR était équipé d'une antenne à microruban montée du côté droit de l'avion pouvant être divisée électroniquement en deux sous-antennes avec des centres de phase séparés de 0,46 mètres, le long de la ligne de vol. En juin 1994, le capteur InSAR du CCT a réalisé des survols au-dessus de la baie de Fundy au cours d'une série de missions effectuées dans le cadre d'une expérience sur les dunes menée par le navire du Service hydrographique canadien, le “NSC Frederick G. Creed”. La capacité du capteur InSAR à mesurer les vélocités des courants de surface et de pouvoir utiliser ces données pour améliorer la détection des caractéristiques de la sous-surface a été analysée et comparée avec les résultats des données de sondage par sonar acquises par le navire et des images RSO du satellite ERS-1. Les expériences se sont poursuivies en 1995 visant également à déterminer la capacité du capteur InSAR à détecter des navires en mouvement et des signatures de houle. Les résultats indiquent que le capteur interférométrique InSAR aéroporté du CCT est capable, dans des conditions favorables, de détecter des changements dans la topographie du fond, de fournir un cliché haute résolution des courants de surface pour de vastes étendues et enfin, d'améliorer la détection de cibles en mouvement à la surface de l'océan.

Simulation of ship wakes image by an along-track interferometric SAR

Abstract A mathematical model that allows simulations of the image of waves in ship wakes by either regular or interferometric airborne Synthetic Aperture Radar (SAR or INSAR) is described. The three-component velocity field induced at the ocean surface by a moving ship serves as the input to the model. The simulations take into account the effect of temporal variations of the wave field in the wake on the INSAR imaging by the velocity bunching mechanism. The model also accounts for the scanning distortion of the image. The developed algorithm allows study of the visibility of the ship wave wake by a regular SAR or by INSAR for arbitrary imaging parameters, as well as for different ship sizes and ship velocity vectors relative to the platform flight track. Various patterns of ship wake images obtained by numerical simulations are presented.

An analytical presentation of the monochromatic ocean wave image by a regular or an interferometric synthetic aperture radar

An asymptotic expansion of the theoretical model for simulation of a monochromatic ocean wave imaging by either regular synthetic aperture radar (SAR), or by along-track interferometric SAR (INSAR), and its analytical approximation are considered. The model takes into account the so-called velocity bunching imaging mechanism. The domains of validity of the simplified asymptotic and analytical expressions for both SAR and INSAR imaging of the ocean waves are established by comparing those solutions with the corresponding full numerical simulations. The analytical presentations of the SAR and the INSAR wave imaging obtained in the present study are used to understand better the limitations and the relative advantages of both techniques and to define the dimensionless parameters which govern the imaging process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>