ESA & Sentinel-1 Session

The Copernicus POD (Precise Orbit Determination) Service is part of the Copernicus Processing Data Ground Segment (PDGS) of the Sentinel-1, -2 and -3 missions. A GMV-led consortium is operating the Copernicus POD Service being in charge of generating precise orbital products and auxiliary data files for their use as part of the processing chains of the respective Sentinel PDGS.

The SAR (Synthetic Aperture Radar) mission Sentinel-1 consists of the two satellites Sentinel-1A and Sentinel-1B, launched in April 2014 and 2016, respectively. The precise orbit determination of the satellites is done based on the dual frequency high precision GPS data from the on-board receivers.

Two orbit products are provided for both satellites. The near real-time (NRT) orbit product has a latency of maximum three hours and an accuracy requirement of 10 cm in 2D. The non-time critical (NTC) orbit product has a latency requirement of less the 20 days and an accuracy requirement of 5 cm in 3D. All requirements are fulfilled for both satellites. Quality control of the CPOD orbits is done by validating them with independent orbit solutions provided by the Copernicus POD Quality Working Group. The cross-comparison of orbit solutions from different institutions is essential to monitor and to even improve the orbit accuracy because for Sentinel-1 this is the only possibility to externally assess the quality of the orbits.

This paper presents the Copernicus POD Service in terms of operations and orbital accuracy achieved for Sentinel-1A and -1B. The results of the two satellites are among others compared based on the analysis of the estimated orbit parameters (e.g. drag and solar radiation scale factors). Due to the complex and large satellite body the modelling of the non-gravitational forces acting on the satellite is challenging. The orbit parametrization, models and settings are regularly reviewed to stay state-of-the-art and to improve the orbits even more.

This paper addresses the cross-SAR interferometry (InSAR) performance verification of the Sentinel-1A&B Constellation using data acquired with the novel Interferometric Wide Swath (IW) mode during the Sentinel-1B Commissioning phase.  The IW mode, for the first time, operationally utilizes ScanSAR-type burst imaging with an additional antenna beam steering in azimuth referred to as Terrain Observation with Progressive Scans (TOPS).

The Sentinel-1 mission is implemented through a constellation of identical C-band SAR satellites comprising the current A and B units, which will be eventually replaced by the planned C and D units.

Sentinel-1A was successfully launched on April 3^rd, 2014 followed by the successful launch of Sentinel-1B on April 25^th, 2016. Both satellites fly in the in the same orbital plane with 180 deg. phased positions.

The Sentinel-1 SAR instrument operating at C-band supports four exclusive imaging modes providing different resolution and coverage: Interferometric Wide Swath (IW), Extra Wide Swath (EW), StripMap (SM), and Wave (WV).

In fact, the IW TOPS mode is the main mode of operations for the systematic monitoring of large land and coastal areas. This systematic IW mode data acquisition enables the build-up of long InSAR IW data time series.

In particular, the 6-day repeat orbit interval along with small orbital baselines enables cross-InSAR coherent change detection applications, such as the monitoring of cryosphere dynamics (e.g. glacier flow) and the mapping of surface deformation, caused, for example, by tectonic processes, volcanic activities, landslides or ground subsidence

The generation of high-quality S-1A/S-1B TOPS IW mode cross-interferograms and coherence maps requires an accurate synchronization of the azimuth scanning patterns (i.e. bursts) and a very stable azimuth antenna pointing to achieve the maximum common Doppler bandwidth. (i.e. azimuth spectral alignment).

To achieve highly accurate burst synchronization, i.e. the instrument burst data acquisition from repeat-pass orbits starts at the same time over the same location on the ground, the S-1 mission exploits the novel concept of position-tag commanding using the orbit position angle and its on-board conversion into sensing time along with an orbital point grid. The calculation of the burst synchronization is based upon the use of the orbital state vectors and the annotated azimuth start time.

Using S-1A/S-1B InSAR scene and long data take pairs, as well as multi-temporal stacks, all acquired during the Sentinel-1B Commissioning phase, we measured only very small offsets and variation of the burst synchronization of 3ms. 

The azimuth antenna pointing is very stable for both S-1A and S-1B due the total zero Doppler attitude steering of the spacecraft causing only a very small difference in Doppler centroid of 20Hz when forming S-1A and S-1B, interferograms, respectively. However, during the Commissioning of S-1B, we measured an initial antenna pointing offset (i.e. yaw and pitch) that is equivalent to a mean Doppler centroid difference 200Hz due to a relative star tracker misalignment.

As a result, the common Doppler bandwidth of S-1A/S-1B cross-interferograms was temporarily reduced to 65%. However, at the end of the S-1B Commissioning phase, the S-1B antenna mis-pointing was corrected for to achieve a Doppler centroid frequency of 20Hz, which is equivalent to more than 95% of the common Doppler bandwidth.

In this paper, we discuss the cross-InSAR performance considering the effects of burst synchronization and SAR antenna pointing on the achievable common Doppler bandwidth. In addition, we show analysis results of the Sentinel-1 ground-track repeatability (i.e. orbital tube) performance and the resulting cross-InSAR baselines.

Results of differential cross-interferograms are presented showing the coseismic surface displacement caused by the central Italy earthquake. The high quality of these interferograms (i.e. no phase jumps at burst edges) demonstrates the excellent compatibility and stability of the radar instruments on both satellites, as well as the accurate orbit control of both spacecraft.

The SENTINEL-1 Toolbox is an open source software for scientific learning, research and exploitation of the large archives of SENTINEL and heritage missions. The Toolbox consists of a collection of processing tools, data product readers and writers and a display and analysis application.  A common architecture for all Sentinel Toolboxes is being jointly developed by Brockmann Consult, Array Systems Computing and C-S called the Sentinel Application Platform (SNAP). The SNAP architecture is ideal for Earth Observation processing and analysis due the following technological innovations: Extensibility, Portability, Modular Rich Client Platform, Generic EO Data Abstraction, Tiled Memory Management, and a Graph Processing Framework.

The SNAP platform provides a graph processing framework (GPF) to efficiently process large amounts of image data through directed, acyclic processing graphs whose nodes are pluggable processing operators. Tiles are processed in parallel according to the number of available cores. Graphs can be processed in the GUI or via the command line interface.

The Toolbox includes reading, calibration and de-noising, slice product assembling, TOPSAR deburst and sub-swath merging, terrain flattening radiometric normalization, terrain correction, mosaicking, supervised and unsupervised classification, change detection and visualization for L2 OCN products. The Toolbox also provides tools for exploitation of polarimetric data including speckle filters, decompositions, and classifiers.

The project has developed new tools for working with SENTINEL-1 data in particular for working with the new Interferometric TOPSAR mode. TOPSAR Complex Coregistration and a complete Interferometric processing chain has been implemented for SENTINEL-1 TOPSAR data. To accomplish this, a coregistration following the Enhanced Spectral Diversity (ESD) method has been developed as well as special burst handling in the coherence, interferogram and other InSAR operators.

Recently, new tools for interferometry have been added including an export for PSI processing with StaMPS, DEM-assisted coregistration, offset/speckle tracking, integer interferogram combination and a baseline/time plot.

The Toolbox  not only supports Sentinel-1 but also most other civillian SAR missions including RADARSAT-2, TerraSAR-X/TanDEM-X, ALOS-1&2, Cosmo-Skymed, Kompsat-5, ERS-1&2 and ENVISAT.

The project is funded through ESA's Scientific Exploitation of Operational Missions (SEOM). The SENTINEL-1 Toolbox will strive to serve the SEOM mandate by providing leading-edge software to the science and application users in support of ESA's operational SAR mission as well as by educating and growing a SAR user community.

The Copernicus environment-monitoring programme with its fleet of Sentinel satellites forming the heart of the programme’s space component, entered its operational phase in 2014 with the launch of the first dedicated satellite, Sentinel-1 A.

In the meantime four more spacecraft have been launched in the years 2015 to 2017 and other two will be launched within a year. To complete the family of Sentinel satellites, eight more spacecraft and 5 additional Sentinel instruments, embarked on European meteorological satellites, will be put in orbit and will cover all environmental domains.

The data are distributed free of charge as part of a European policy seeking to stimulate downstream value-added Earth observation-related business. With this space configuration, an uninterrupted data delivery to users is guaranteed until at least 2030. Over 80.000 users world-wide are accessing Sentinel data from several data access hubs developed by ESA. Over 24 PB of data have been already downloaded with an average of several TB of data products downloaded every day, making Copernicus a Big Data challenge and requiring new solutions for data dissemination. These figures will grow as new satellites will be put in orbit and new applications will operationally implement Copernicus data.

In the meantime, and thinking of a near term future, new priorities have been introduced in the EU policies and new societal needs and challenges have arisen requiring new observations. This led to the so-called Sentinels’ expansion and will finally result in the Next Generation of Sentinel Missions.

The expansion of the Sentinel family is a joint EU-ESA endeavour which just started concerning the investigation of new domains/techniques for future satellite missions like greenhouse gas monitoring, polar ice/ocean interferometric altimetry, thermal Infrared, and hyper-spectral land imaging.

The presentation will therefore give an overview of the current status of the space component and corresponding data access, and some hints on the future perspectives of the Copernicus space component.

ESA Session