Fernando Rodriguez Gonzalez

Interferometric Processing of Sentinel-1 TOPS Data

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. S-1 has become the European workhorse for large area mapping and interferometric monitoring at medium resolution. The interferometric processing of TOPS data however requires 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 S-1 IW mode repeat-pass SAR data. Interferometric results from S-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 analyzed. Additionally, a spatiotemporal evaluation of the along-track shifts, which are directly related to the orbital/instrument timing error, measured from the SAR data is shown, which 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.

Wide area persistent scatterer interferometry

The persistent scatterer interferometry (PSI) is a well established radar technique to monitor the Earths displacements with millimetre accuracy. It uses men made features typically made of metal (persistent scatterers) given by chance to form interferometric phase time series spanning many years. Actually, its application is limited to urban areas only because of the high density of usable persistent scatters. In the course of ESAs Terrafirma project, a wide area product (WAP) PSI mapping is demonstrated by DLR. Subject is to map countries and continents based on the PSI technique. The WAP is foreseen to be a standard level 1 product for the future Sentinel-1 mission with its TOPS mode acquisition scenario. However, many technical problems need to be solved in order to extend the PSI mapping area from urban areas to rural and even mountainous regions. This paper reports on the wide area product, the technical challenges and their algorithmic solutions. Also, WAP example data are presented.

Wide area Persistent Scatterer Interferometry: Current developments, algorithms and examples

In recent years, Persistent Scatterer Interferometry (PSI) [1], [2] has been widely used for scientific applications and has developed into an operational and commercially rewarding remote sensing technology. Now, ESAs upcoming Sentinel-1 mission allows a continuous, repeated without gap and global mapping of the Earths surface based on Terrain Observation by Progressive Scans (TOPS). The idea at ESA is to also extend PSI processing to such large coverages, mapping countries and continents. This will support users in subsidence monitoring of volcanic and seismogenic areas, of costal lowland, of landslides in mountainous areas and of mining and ground water regulation on a small scale. For this reason, a wide area product (WAP) for PSI monitoring has been developed at DLR. The paper has three specific objectives. The first objective is to list and describe the updated algorithms. In this context, we illustrate the improvement with respective processing examples. Our second objective is to explain the characteristics of the WAP. The last objective is to provide WAP processing results for seismogenic areas. The test cases of Greece and of Turkey demonstrate WAPs potential, its applicability and use.

L1 network inversion for robust outlier rejection in persistent Scatterer Interferometry

Persistent Scatterer Interferometry (PSI) is an established SAR technique for the monitoring of the Earths surface displacement. In the framework of the ESAs Terrafirma project, aWide Area Product (WAP) PSI mapping is demonstrated by DLR. In this paper we address one of the technical difficulties in its development: the robustness of the reference network inversion. Outliers due to both falsely detected PS and the arc estimation process itself must be robustly handled. An approach based on the L1 norm network inversion for outlier rejection is proposed and evaluated.

Ionospheric Phase Screen Compensation for the Sentinel-1 TOPS and ALOS-2 ScanSAR Modes

Variations of the ionosphere can significantly disrupt synthetic aperture radar (SAR) acquisitions and interferometric measurements of ground deformation. In this paper, we show how the ionosphere can also strongly modify C-band interferograms despite its smaller influence at higher frequencies. Thus, ionospheric phase screens should not be neglected: their compensation improves the estimation of ground deformation maps. The split-spectrum method is able to estimate the dispersive ionospheric component of the interferometric phase; we describe the implementation of this method for the burst modes TOPS and ScanSAR to estimate and remove ionospheric phase screens. We present Sentinel-1 interferograms of the 2016 Taiwan earthquake and ALOS-2 interferograms of the 2015 Nepal earthquake, which show strong ionospheric phase gradients, and their corrected versions. Finally, to validate the results and better understand the origin of these ionospheric variations, we compare the estimated differential ionosphere with global Total Electron Content maps and local Global Positioning System measurements.

Multibaseline gradient ambiguity resolution to support Minimum Cost Flow Phase Unwrapping

The TanDEM-X Mission has as primary objective to generate a high resolution global Digital Elevation Model. This paper proposes a new method for multibaseline Phase Unwrapping which is the critical point of this generation. We propose to combine both Minimum Cost Flow (MCF) and Maximum a Posteriori (MAP) estimation. The latter is used to solve phase gradient ambiguities. The problem is posed as an energy minimization one and solved using Belief Propagation (BP) which is an iterative process. Nevertheless, although very good results are obtained on loopy graphs, it is not guaranteed to converge. Thus, phase unwrapping of the most accurate interferogram is finally performed with the MCF algorithm and takes as input the unwrapped gradients.

Thermal dilation monitoring of complex urban infrastructure using high resolution SAR data

Monitoring the deformation of urban infrastructure is crucial for improving safety and reducing economic loss. Ground deformation (caused by construction, groundwater extraction etc.) can be measured with millimeter accuracy using advanced stacking techniques in differential SAR interferometry. However, deformation might also occur due to thermal dilation of complex urban objects such as buildings and bridges, resulting in structural stress and thus, a need for monitoring. High resolution SAR missions e.g. TerraSAR-X provide a unique opportunity to measure such small seasonal displacements. This paper investigates different techniques such as Persistent Scatterer Interferometry (PSI), Distributed Scatterer Interferometry (DSI), Stable Point Network (SPN) and Small Baseline Subset Algorithm (SBAS) for thermal dilation monitoring of complex urban infrastructure using X-band SAR data.

An advanced co-registration method for TOPSAR interferometry

This paper presents a robust mis-registration estimator based on ESD method for TOPSAR interferometry. For the in-terferometric applications of TOPSAR mode data, a master and slave SLC pair requires a co-registration accuracy in the milli-pixel order due to the large Doppler centroid frequency variation. Although ESD method estimates the mis-registration with the required sensitivity, a prior fine co-registration step is a prerequisite. This paper presents an advanced co-registration method that relaxes the fine co-registration requirement of the ESD and improves the estimation sensitivity. Experimental results with Sentinel-1A data are shown to validate the proposed method.

Four level least squares adjustment in Persistent Scatterer Interferometry for the Wide Area Product

Persistent Scatterer Interferometry (PSI) is a well established SAR technique to monitor the Earths surface displacements with millimeter accuracy [1]. In the framework of the ESAs Terrafirma project, a PSI based Wide Area Product (WAP) is developed and demonstrated by DLR [2]. The PSI technique is well applicable for the processing of urban areas, because many persistent scatterers (PSs) are available due to man-made features. However, the mapping of areas with an extension of more hundreds of kilometres is the objective of the WAP. Thus rural and mountainous regions also need to be handled by the processor. In such areas, the distribution of PSs is very inhomogeneous and, typically, the point density is very low. In this paper we address two technical difficulties in the development of the WAP. Firstly, we report on the determination and estimation of a reliable reference network of the processing area in order to estimate and compensate the atmospheric phase screen (APS). Secondly, the mosaiking of independent reference networks into a global consistent data set is described. As basic method, the conventional least squares adjustment is used on four hierarchic levels. Consequently, the error propagation through all these levels can be analyzed and evaluated.

Mitigation of Tropospheric Delay in SAR and InSAR Using NWP Data: Its Validation and Application Examples

The neutral atmospheric delay has a great impact on SAR absolute ranging and on differential interferometry. In this paper, we demonstrate its effective mitigation by means of the direction integration method using two products from the European Centre for Medium-Range Weather Forecast: ERA-Interim and operational data. Firstly, we shortly review the modeling of the neutral atmospheric delay for the direct integration method, focusing on the different refractivity models and constant coefficients available. Secondly, a thorough validation of the method is performed using two approaches. In the first approach, NWP derived Zenith Path Delay (ZPD) is validated against ZPD from permanent GNSS stations on a global scale, demonstrating a mean accuracy of 14.5 mm for ERA-Interim. Local analysis shows a 1 mm improvement using operational data. In the second approach, NWP derived Slant Path Delay (SPD) is validated against SAR SPD measured on Corner Reflectors in more than 300 TerraSAR-X High Resolution SpotLight acquisitions, demonstrating an accuracy in the centimeter-range for both ERA-Interim and operational data. Finally, the application of this accurate delay estimate for the mitigation of the impact of the neutral atmosphere on SAR absolute ranging and on differential interferometry, both for individual interferograms and multi-temporal processing, is demonstrated.