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Sentinel-1 (S-1) has an unparalleled mapping capacity. In Interferometric Wide Swath (IW) mode, three subswaths imaged in the novel Terrain Observation by Progressive Scans (TOPS) SAR mode result in a total swath width of 250 km. Sentinel-1 has become the European workhorse for large area mapping and interferometric monitoring at medium resolution. The interferometric processing of TOPS data requires however special consideration of the signal properties, resulting from the ScanSAR-type burst imaging and the antenna beam steering in azimuth. The high Doppler rate in azimuth sets very stringent coregistration requirements, making the use of Enhanced Spectral Diversity (ESD) necessary to obtain the required fine azimuth coregistration accuracy. Other unique aspects of processing IW data, such as azimuth spectral filtering, image resampling and data deramping and reramping, are reviewed giving a recipe-like description that enables the user community to use Sentinel-1 IW mode repeat-pass SAR data. Interferometric results from Sentinel-1A are provided, demonstrating the mapping capacity of the S-1 system and its interferometric suitability for geophysical applications. An interferometric evaluation of a coherent interferometric pair over Salar de Uyuni, Bolivia is provided, where several aspects related to coregistration, deramping and synchronization are analysed. Additionally, a spatial and temporal evaluation of the along-track shifts, which are directly related to the orbital/instrument timing error, measured from the SAR data is shown that justifies the necessity to refine the azimuth shifts with ESD. The spatial evaluation indicates high stability of the azimuth shifts for several slices of a datatake.

In Sentinel-1 TOPS mode, the antenna sweeps in the azimuth direction for the purpose of illuminating the targets with the entire azimuth antenna pattern (AAP). This azimuth sweeping introduces an extra high-frequency Doppler term into the impulse response function (IRF), which poses a more strict coregistration accuracy for the interferometric purpose. A 1/1000 pixel coregistration accuracy is required for the interferometric phase error to be negligible, and the enhanced spectral diversity (ESD) method is applied for achieving such accuracy. However, since ESD derives miscoregistration from cross-interferometric phase, and phase is always wrapped to [ − π , π ) , an initial coregistration method with enough accuracy is required to resolve the phase ambiguity in ESD. The mainstream for initial coregistration that meets this requirement is the geometrical approach, which accuracy mainly depends on the accuracy of orbits. In this article, the authors propose to investigate the feasibility of using the conventional coregistration approach, namely the cross-correlation-and-rigid-transformation, as the initial coregistration method. The aim is to quantify the coregistration accuracy for cross-correlation-and-rigid-transformation using the Cramér-Rao lower bound (CRLB) and determine whether this method could eventually help to resolve the phase ambiguities of ESD. In addition, we studied the feasibility and robustness of the cross-correlation plus ESD under different conditions. For validation, we checked whether the cross-correlation plus ESD approach could reach the same coregistration accuracy as geometrical plus ESD approach. In general, for large areas with enough coherence and little topography variance, the cross-correlation method could be used as an alternative to the geometrical approach. The interferogram from the two different approaches (with ESD applied afterward) shows a negligible difference under such circumstances.

Terrain observation by progressive scan (TOPS) antenna beam steering is utilized for European Space Agency’s (ESA’s) Sentinel-1 synthetic aperture radar (SAR) sensor for the interferometric wide swath (IW) and extra wide swath (EW) modes. As a consequence of the azimuth steering, the resulting signal characteristics have to be accounted for in SAR interferometric (InSAR) processing. This paper assesses the performance of speckle tracking and spectral diversity (SD) [also referred to as split spectrum or multi-aperture interferometry (MAI)] when applied to TOPS data acquired over nonstationary scenarios, such as glaciers. The characteristics of the TOPS signal, especially the azimuth-variant Doppler centroid, are discussed with particular consideration of along-track surface motion between the interferometric acquisitions. The TOPS specific coregistration requirements are formulated, followed by an analysis of the theoretical estimation accuracy as a function of the estimation window size. A refined adaptive coregistration approach based on D is suggested. Experimental TerraSAR-X TOPS data acquired over the Lambert glacier, Antarctica, are used to validate the proposed speckle tracking and SD methodologies.

The contribution focuses on the current status of the ESA study entitled “InSARAP Sentinel-1 Constellation Study”, which investigates the interferometric performance of the S1A/S1B units. General aspects like the interferometric compatibility in terms of common range and Doppler bandwidth and the burst synchronization are addressed. Besides the first interferometric results with both units, time series results over the pilot sites combining both satellites are also shown, as well as some investigations with fast moving (i.e., glaciers) scenarios.

The orbit for Earth observation satellite synthetic aperture radar (SAR) missions is maintained within an Earth-fixed orbital tube to ensure ground-track coverage repeatability and, consequently, to enable repeat-pass interferometric compatibility between data takes. In this letter, it is shown that the size of the orbital tube may affect the interferometric performance in terms of azimuth spectral decorrelation and azimuth coregistration accuracy under the presence of squint. These effects require special consideration for SAR burst modes, such as ScanSAR or TOPS (i.e., Terrain Observation by Progressive Scans). This letter presents and analyzes these aspects in the frame of the Sentinel-1 mission.

International audience; InSAR processing usually involves two successive steps: focusing and interferometry. Most public-domain InSAR processing toolboxes are capable of performing both operations with data acquired in the standard Stripmap mode, starting from raw SAR data (level 0). However , the focusing of burst-mode data, such as TOPS and ScanSAR, requires substantial modifications to standard focusing methods due to the particular spectral properties of these data. Anticipating on this potential difficulty for non-expert users, the European Space Agency has chosen to release Sentinel-1 TOPS data in a Single Look Complex format (level 1). The data are already focused using state-of-the-art processing techniques, with phase information preserved. Even so, the focusing method introduces an additional quadratic phase term in the azimuth direction. In case of a small misregistration error between a pair of images, this residual term leads to steep phase ramps in azimuth that are superimposed on the desired in-terferometric phase. Therefore, this quadratic phase term needs to be removed from the SLC data prior to interfer-ogram calculation. Here, a pre-processing method allowing for compensating this phase term and simply feeding the corrected SLC data into a standard InSAR processing chain is described. The method consists of three steps. The first step uses the metadata in order to reconstruct a continuous image in the azimuth direction, accounting for the small overlap between adjacent bursts (" stitching "). In the second step, multiplication of the images by an appropriate phase screen is performed so as to cancel the azimuthal quadratic phase term (" deramp-ing "). The deramping operation uses the metadata, as well as the azimuth time lag between the images deduced from sub-pixel image correlation, in order to determine small misregistration errors. Misregistration errors are compensated using a simple affine relation deduced from least-square fitting of the azimuth offsets. Following this second step, the azimuth phase ramps are significantly reduced in the corrected interferogram. The third step consists in refining the affine coefficients that account for the misregistration error. The refinement is achieved by differencing the backward-and forward-looking interfer-ograms, exploiting the spectral diversity in burst overlap regions (" spectral diversity "). This final step makes it possible to remove residual phase jumps across burst boundaries with the desired level of accuracy. Azimuth spectral properties of burst-mode SAR data, such as ScanSAR [1] or TOPS [2], are significantly different from those of Stripmap. In burst-mode, the system observes a series of sub-swaths by periodically steering of the antenna in the elevation direction and transmitting a short succession of pulses gathered in the so-called " bursts ". As a consequence, a given target is imaged only during a fraction of the equivalent stripmap synthetic aperture duration, thereby reducing its duration of illumination. Therefore, the increased swath width comes at the expense of azimuth resolution (Fig. 1). Furthermore, the Doppler centroid of targets within any burst becomes variable in azimuth. Due to the latter effect, the impulse response function of standard burst-mode focusing methods includes a residual phase term that exhibits a characteristic quadratic variation of the az-imuth phase. This phase term needs to be compensated prior to using the phase for interferometric applications, a process referred to as " deramping " [3, 4]. Figure 1. Schematic representation of the TOPSAR acquisition mode (from [5]). The beam is steered periodically in range, in order to cover several sub-swaths, and progressively in azimuth, aft to fore, in order to increase the azimuth bandwidth, expand the footprint and decrease scalloping.

Image alignment is without doubt the most crucial step in SAR Interferometry. Interferogram formation requires images to be coregistered with an accuracy of better than 1/8 pixel to avoid significant loss of phase coherence. Conventional interferometric precise coregistration methods for full-resolution SAR data (Single-Look Complex imagery, or SLC) are based on the cross-correlation of the SLC data, either in the original complex form or as squared amplitudes. Offset vectors in slant range and azimuth directions are computed on a large number of windows, according to the estimated correlation peaks. Then, a two-dimensional polynomial of a certain degree is usually chosen as warp function and the polynomial parameters are estimated through LMS fit from the shifts measured on the image windows. In case of rough topography and long baselines, the polynomial approximation for the warp function becomes inaccurate, leading to local misregistrations. Moreover, these effects increase with the spatial resolution and then with the sampling frequency of the sensor, as first results on TerraSAR-X interferometry confirm. An improved, DEM-assisted image coregistration procedure can be adopted for providing higher-order prediction of the offset vectors. Instead of estimating the shifts on a limited number of patches and using a polynomial approximation for the transformation, this approach computes pixel by pixel the correspondence between master and slave by using the orbital data and a reference DEM. This study assesses the performance of this approach with respect to the standard procedure. In particular, both analytical relationships and simulations will evaluate the impact of the finite vertical accuracy of the DEM on the final coregistration precision for different radar postings and relative positions of satellites. The two approaches are compared by processing real data at different carrier frequencies and using the interferometric coherence as quality figure.