A. Theodosiou
Please Note
7 records found
1
The Harmony mission features two bistatic synthetic aperture radar (SAR) companions of Sentinel-1. As with any multistatic system, frequency deviations among the oscillators of the receivers cause a phase error in the phase of the demodulated SAR signal. Given that interferometry will be used to retrieve geophysical parameters from Harmony’s radar instruments, an erroneous phase difference between the SAR signals of the two companions will bias the retrieval. The companions will use a global navigation satellite system (GNSS)-based method to synchronize the phase of the signals. The residual phase that remains after the synchronization is significant enough to make the retrieval of relative sea-surface height (RSSH) impossible. In this article, we present Multisquint with Overlaps (MuSO), a data-driven algorithm to remove the synchronization residual. The algorithm uses the multisquint processing approach, together with the overlap regions of the Terrain Observation by Progressive Scans (TOPSAR) acquisition mode, to estimate the derivative of the residual. After running the algorithm, simulations suggest that the error signal reduces from a standard deviation of 4°–0.01°, allowing the retrieval of RSSH from Harmony data.
Small Scales, Vast Ocean
Submesoscale Ocean Topography with Bistatic Synthetic-aperture Radar Interferometry
Synthetic-aperture radar (SAR) is a unique remote sensing instrument, particularly at C-band, capable of sensing the ocean surface at the submesoscales, with a wide swath, and in nearly all weather conditions. Harmony, the European Space Agency’s 10th Earth Explorer, features two SAR companion satellites. Two, out of a total of five, years of the mission’s life will be spent in a formation where the system will operate as a cross-track interferometer. Cross-track interferometry (XTI) is a technique that estimates the relative height of the surface from two SAR images of the same scene. Thus, Harmony could theoretically retrieve variations of the OST. In other words, the system could operate as a bistatic wide-swath ocean altimeter (WSOA). At the same time, Harmony will retrieve stress-equivalent wind fields, and instantaneous surface currents. Therefore, Harmony has the potential of providing an unprecedented wealth of co-located simultaneous data related to the ocean and the atmosphere. The aim of this thesis is to devise a method to estimate submesoscale ocean surface topography with a bistatic SAR interferometer, such as Harmony.
Assessing the design of a bistatic WSOA requires knowledge of the interferometric sensitivity, and temporal lag. The first obstacle that we encountered was that there is no model or analytical expression for these parameters that apply to a bistatic SAR with a squinted line of sight. The established relations found in the literature assume a zero-squint geometry. Hence, we use the Fourier Diffraction Slice Theorem to derive an analytical expression for the interferometric sensitivity, and the temporal lag. We show that forming an interferogram aligns the regions of support of the two images in the Fourier domain at each resolution cell, and that the temporal lag is the time offset that aligns the two regions. The sensitivity is equal to the vertical component of the aligned wave vectors projected on the elevation direction. We verify our results using simulations and confirm that our analytical expressions agree with the well-established relations for sensitivity and temporal lag for zero-squint systems.
We use the analytical expressions of sensitivity and temporal lag to build an interferometric performance model that computes the standard error of the height estimate for a formation-flying cross-track interferometer. The model considers the following random error sources: temporal decorrelation, thermal noise, spectral shift, volumetric decorrelation, and the effect of removing the phase due to motion of the surface using the individual phase centers of the instruments. Additionally, we derive a relation between the formation parameters that, when satisfied, minimizes the effective temporal lag, while maximizing the interferometric sensitivity. We then proceed to assess the performance over an orbit and along the 250 km-swath of an optimized formation.
Finally, we propose a data-driven algorithm to synchronize the signals of the independent SAR receivers. The algorithm achieves an unbiased root mean square error of 0.010◦ , reducing the phase synchronization error to within the error budget allocation.
Overall, the thesis presents how one can design, analyze, and retrieve relative ocean topography at the submesoscales with a bistatic SAR interferometer. It sets the foundations for an experimental OST product for the Harmony mission. Adding such a product to the mission would offer the first simultaneously acquired observations of wind field, current field, directional wave spectrum, and relative sea-surface height at high resolution and over a 250 km-wide swath. ...
Synthetic-aperture radar (SAR) is a unique remote sensing instrument, particularly at C-band, capable of sensing the ocean surface at the submesoscales, with a wide swath, and in nearly all weather conditions. Harmony, the European Space Agency’s 10th Earth Explorer, features two SAR companion satellites. Two, out of a total of five, years of the mission’s life will be spent in a formation where the system will operate as a cross-track interferometer. Cross-track interferometry (XTI) is a technique that estimates the relative height of the surface from two SAR images of the same scene. Thus, Harmony could theoretically retrieve variations of the OST. In other words, the system could operate as a bistatic wide-swath ocean altimeter (WSOA). At the same time, Harmony will retrieve stress-equivalent wind fields, and instantaneous surface currents. Therefore, Harmony has the potential of providing an unprecedented wealth of co-located simultaneous data related to the ocean and the atmosphere. The aim of this thesis is to devise a method to estimate submesoscale ocean surface topography with a bistatic SAR interferometer, such as Harmony.
Assessing the design of a bistatic WSOA requires knowledge of the interferometric sensitivity, and temporal lag. The first obstacle that we encountered was that there is no model or analytical expression for these parameters that apply to a bistatic SAR with a squinted line of sight. The established relations found in the literature assume a zero-squint geometry. Hence, we use the Fourier Diffraction Slice Theorem to derive an analytical expression for the interferometric sensitivity, and the temporal lag. We show that forming an interferogram aligns the regions of support of the two images in the Fourier domain at each resolution cell, and that the temporal lag is the time offset that aligns the two regions. The sensitivity is equal to the vertical component of the aligned wave vectors projected on the elevation direction. We verify our results using simulations and confirm that our analytical expressions agree with the well-established relations for sensitivity and temporal lag for zero-squint systems.
We use the analytical expressions of sensitivity and temporal lag to build an interferometric performance model that computes the standard error of the height estimate for a formation-flying cross-track interferometer. The model considers the following random error sources: temporal decorrelation, thermal noise, spectral shift, volumetric decorrelation, and the effect of removing the phase due to motion of the surface using the individual phase centers of the instruments. Additionally, we derive a relation between the formation parameters that, when satisfied, minimizes the effective temporal lag, while maximizing the interferometric sensitivity. We then proceed to assess the performance over an orbit and along the 250 km-swath of an optimized formation.
Finally, we propose a data-driven algorithm to synchronize the signals of the independent SAR receivers. The algorithm achieves an unbiased root mean square error of 0.010◦ , reducing the phase synchronization error to within the error budget allocation.
Overall, the thesis presents how one can design, analyze, and retrieve relative ocean topography at the submesoscales with a bistatic SAR interferometer. It sets the foundations for an experimental OST product for the Harmony mission. Adding such a product to the mission would offer the first simultaneously acquired observations of wind field, current field, directional wave spectrum, and relative sea-surface height at high resolution and over a 250 km-wide swath.
Estimating sea surface height using cross-track interferometry (XTI) requires high sensitivity because the ocean surface signal is in the order of 10 cm. In addition, the interferometer requires a temporal delay of a few milliseconds to ensure the coherency of the moving ocean surface. We show that a squinted line of sight (LoS), in combination with a helix satellite formation, allows optimizing the effective perpendicular and along-track baselines to satisfy these conditions. This article presents a model to estimate the performance of a formation-flying cross-track interferometer with a squinted LoS. The tenth Earth Explorer, Harmony, which features two bistatic synthetic aperture radar (SAR) companions, and a theoretical system with one monostatic and one bistatic SAR are used as case studies. The standard deviation of the height estimate is 1-10 cm between 29° and 41° and increases to 30 cm at the far range (46°) at a wind speed of 5 ms-1. The power spectral density of the elevation shows that spatial scales of 47 km can be resolved. The performance improves at higher wind speeds due to higher backscattering. At a wind speed of 15 ms-1, the wavelengths from 27 to 11 km can be resolved, depending on the elevation spectrum. The performance over a 250-km swath enables the instantaneous estimation of the surface elevation at the submesoscales for the first time.
This article describes the observation techniques and suggests processing methods to estimate dynamical sea-ice parameters from data of the Earth Explorer 10 candidate Harmony. The two Harmony satellites will fly in a reconfigurable formation with Sentinel-1D. Both will be equipped with a multi-angle thermal infrared sensor and a passive radar receiver, which receives the reflected Sentinel-1D signals using two antennas. During the lifetime of the mission, two different formations will be flown. In the stereo formation, the Harmony satellites will fly approximately 300km in front and behind Sentinel-1, which allows for the estimation of instantaneous sea-ice drift vectors. We demonstrate that the addition of instantaneous sea-ice drift estimates on top of the daily integrated values from feature tracking have benefits in terms of interpretation, sampling and resolution. The wide-swath instantaneous drift observations of Harmony also help to put high-temporal-resolution instantaneous buoy observations into a spatial context. Additionally, it allows for the extraction of deformation parameters, such as shear and divergence. As a result, Harmony's data will help to improve sea-ice statistics and parametrizations to constrain sea-ice models. In the cross-track interferometry (XTI) mode, Harmony's satellites will fly in close formation with an XTI baseline to be able to estimate surface elevations. This will allow for improved estimates of sea-ice volume and also enables the retrieval of full, two-dimensional swell-wave spectra in sea-ice-covered regions without any gaps. In stereo formation, the line-of-sight diversity allows the inference of swell properties in both directions using traditional velocity bunching approaches. In XTI mode, Harmony's phase differences are only sensitive to the ground-range direction swell. To fully recover two-dimensional swell-wave spectra, a synergy between XTI height spectra and intensity spectra is required. If selected, the Harmony mission will be launched in 2028.
Wide-Swath Ocean Topography using Formation Flying Under Squinted Geometries
The Harmony Mission Case
The paper investigates the polarimetry of bistatic Synthetic Aperture Radar (SAR) acquisitions over rough surfaces, with focus on the rotation of the scattered wave orientation at the companion antenna axes and on the optimal linear polarization in transmission. This latter is defined as the polarization achieving the maximum radar cross section and will be herewith recalled as principal polarization. The paper outlines a geometrical framework for the interpretation and the estimation of the principal polarizations. It is shown that the theoretical formulation provides a good agreement with the second-order analytical approach in [1]. The paper finally postulates that a bistatic illumination in the traditional H and V linear modes can be considered equivalent to a compact φ-pol, i.e. with the transmittion in a linear polarization rotated by φ. For long baselines, such those as those envisioned by the ESA Harmony EE10 candidate, and for steep incidence angles, an equivalent π/4-pol might be possible for rough surfaces.