The Copernicus Sentinel-1 (S-1) mission ensures the continuity of C-band SAR observations over Europe. The routine operations of the constellation are on-going and performed at full mission capacity, with data being acquired with both Sentinel-1 A and Sentinel-1 B units. The mission is characterized by large-scale and repetitive observations, systematic production and free and open data policy. Sentinel-1 data are routinely used by Copernicus and many operational services, as well as in the scientific and commercial domain.
The Sentinel-1 SAR Mission Performance Cluster Service (SAR-MPC) is an international consortium of SAR experts and is in charge of the continuous monitoring of the S-1 instruments status, as well as the monitoring of the quality of the L1 and L2 products. This is typically done by analyzing the variation of key parameters over time using dedicated auxiliary products or standard data available to the public. The MPC is also responsible to implement any actions necessary to ensure that no data quality degradation occurs. This includes the update of processor configuration files and updates of the S-1 Instrument Processing Facility (IPF) algorithms and/or their implementation [1].
The monitoring of both the SAR antenna health status and of the SAR instrument is carried out exploiting the dedicated auxiliary products and ensures that no degradation of SAR data quality is originated by instrument aging or element’s failures. In the case of the antenna health, the analysis is performed using the RF Characterization (RFC) products that allow to assess the status of the 280 TRMs composing the SAR antenna. The monitoring shows that there is no considerable degradation or TRM failures since 2017 for neither Sentinel-1 A or Sentinel-1 B. A small degradation of one element in H pol of S-1A has been observed since January 2021 (loss of about 3 dB gain in Rx and 1 dB gain in Tx), but with no impact in the data quality at the moment. The instrument status is monitored through the internal calibration and noise products, which can be used, for example, to generate time series of the PG product. Currently analysis shows that the overall behavior of both instruments is quite stable, with the slope of the PG gain trend below 0.1 dB/year for both units.
The radiometric and geolocation performance of L1 products is performed using standard S-1A and S-1B data, and are also stable and within specifications. In particular, the DLR calibration site composed of transponders and corner reflectors is used to assess the stability of the radiometry, and current analysis including data from 2017 until 2021 shows a mean value of -0.1 dB and standard deviations below 0.25 dB for both units. In addition to the point-target analysis, gamma measurements over uniformly distributed targets like rainforest are also used to assess a relative radiometric accuracy of S-1 products. Evaluating the flatness of such profiles, updates of the antenna patterns and processing gains are performed in order to ensure the radiometric accuracy. The geolocation accuracy is monitored through the use of dedicated acquisitions over additional calibration sites and includes the compensation of known instrument and environmental effects, e.g., propagation through troposphere and ionosphere [2]. Current analysis of the point targets shows an absolute mean value of less than 25 cm in azimuth and less than 15 cm in range for both units, and standard deviations of less than 50 cm in all cases.
The quality of the L2 products is also continuously monitored by the SAR-MPC (see dedicated presentation in [4]). During the summer of 2021, particular attention was given to this task due the successful introduction of the new SAR Wave Mode beam 2 (WV2) configuration, characterized by a higher incidence angle of beam 2 and new antenna gain settings. Following the update, the MPC performed analysis which confirmed increased SNR in the WV2 products and the corresponding improvement in the L2 products [4].
The IPF is also continuously evolved to improve the data quality and its usability. Main evolutions which have been included in the latest IPF version deployed in November 3rd, 2021 (IPF 3.4) are
• Inclusion of the RFI mitigation module, which contains time and frequency domain approaches to perform detection and removal of RFI signals [3]. Annotation from the RFI detection and correction steps has been included in dedicated fields in the metadata. At the moment, the correction step is deactivated, and we expect it will become active until the end of 2021. A dedicated presentation is planned on this topic in the Sentinel-1 session.
• Inclusion of the burst cycle ID in the product annotation, a number that uniquely identifies a burst cycle within each repeat cycle, providing correspondence between the burst cycle ID of a given sub swath and a region on Earth’s surface. Corresponding IW and EW burst ID maps will be made available to the public in the first quarter of 2022.
[1] Sentinel-1 Annual Performance Report 2020, on-line document, https://sentinels.copernicus.eu/web/sentinel/user-guides/sentinel-1-sar/document-library
[2] R. Piantanida et al., "Accurate Geometric Calibration of Sentinel-1 Data," EUSAR 2018; 12th European Conference on Synthetic Aperture Radar, 2018
[3] Franceschi et al., “Operational RFI Mitigation Approach in Sentinel-1 IPF”, submitted to EUSAR 2022
[4] A. Benchaabane,” Sentinel 1 Level 2 Ocean Products Performance Monitoring: current status and evolutions”, submitted to the LPS2022
Acknowledgement:
The results presented here are outcome of the ESA contract Sentinel-1 / SAR Mission Performance Cluster Service 4000135998/21/I BG, funded by the EU and ESA.
The views expressed herein can in no way be taken to reflect the official opinion of the European Space Agency or the European Union.
Introduction
Copernicus Sentinel-1is a constellation of two C-band Synthetic Aperture Radars (SAR) operated by the European Space Agency, launched in March 2014 for S1A, flying in tandem with S1B since 2016. Their Level-1 products consist of high-resolution Radar images distributed either in GRD (Ground-Detected) or SLC (Single-Look Complex) processing. Over the ocean, these measurements depend on the surface roughness, the presence of waves and currents. The knowledge acquired since decades on the SAR data allows for the generation of Level-2 products of surface wind vectors, wave spectra and radial velocity, referred to as OCN products (ocean).
L2 OCN Data quality is constantly evolving thanks to radiometric and algorithmic improvements. This work aims at presenting the strategy, current performances and evolutions of the so-called Sentinel-1 L2 OCN products from the Mission Performance Cluster Service (S-1 MPC). This activity both depends on and benefits from evolutions of the Level-1 products quality (in a dedicated presentation)
Ocean Wind
The estimation of wind vectors is made possible by the knowledge of GMFs (Geophysical Model Function) relating the calibrated NRCS to the wind in a statistical way. These functions are precisely known and have been evolving for decades. Their knowledge allows us to estimate a SAR wind vector at a resolution of 1 km from a Bayesian inversion.
The validation strategy relies on massive comparisons with atmospheric model forecasts at various resolutions: ECMWF 10 km 3-hourly forecast, NCEP 10km 3-hourly forecast, AROME and ARPEGE. Numerous statistical diagnostics are developed that monitor both the product performances on wind speed and direction. Strategies are being developed for the incorporation of in-situ data in the Sentinel-1 CAL/VAL chain, especially, weather buoy data, other satellite missions as well as opportunity data sets (LIDARs for example).
On the algorithmic side, the planned improvements concern
strong winds and the flagging of non-wind related features treatment that can otherwise affect the wind estimates.
Increase of update frequency of the ECMWF wind forecast used as a first guess in the ocean wind measurement process
Activation of RFI mitigation applied on input Level 1 products (dedicated presentation on RFI mitigation is planned during the symposium).
Ocean Surface Waves
The Sentinel-1 derived wave measurements provide 2-D ocean swell spectra (2-D wave energy distribution, function of wavelength and direction) as well as classical integrated parameters such as significant wave height for the observed swell partition, dominant wavelength, and direction.
Several dedicated methodologies have been set up for validation. (i) For each Sentinel-1 Ocean swell spectrum measurement, a directional spectrum is systematically produced with co-located WAVEWATCH III numerical wave model (ii) As very few acquisitions are available in coastal areas where in-situ buoys are deployed, we perform a dynamical co-location. This method allows propagating wave measurements acquired by Sentinel-1A in open ocean up to the closest in-situ buoy for comparison. Such methods will be particularly interesting for cross-comparison and inter-calibration of Sentinel-1A and B swell measurements. (iii) More classical methods such as co-location against altimeters are also used and will be presented as well.
We will present here the calibration and validation methodology, the main improvements put in place in the last years and the planned improvements in the coming months.
During the summer of 2021, particular attention was given to this task due the successful introduction of the new SAR Wave Mode beam 2 (WV2) configuration, characterised by a higher incidence angle of beam 2 and new antenna gain settings. The S-1 MPC analysis which followed the update has confirmed:
Increased SNR of about 5 dB in WV2 imagettes,
Reduced bias for L2 Ocean Swell Spectra (OSW) wind speed and significant wave height estimates,
Radial Velocity
The so-called radial velocity (RVL) is related to the velocity of the scatters in the line-of-sight of the SAR antenna. Over ocean, a strong dependence to surface current and winds-waves is expected. Unfortunately, the Sentinel-1 Level 2 RVL measurements are currently coloured by the Doppler frequency (DC) derived from AOCS and the antenna DC bias (electronic mispointing). This prevents the current version of the Level 2 processor to provide calibrated RVL estimates. We will present here the status of performances achieved from Sentinel-1 measurements in the OCN products and the foreseen way forward.
The overall status is that the calibration of RVL measurement suffers for insufficient spacecraft attitude knowledge now of the generation of the OCN products. Analysis of ways to collect the attitude information with proper accuracy are ongoing.
Few improvements were applied in 2021 by the introduction of a new SAR Wave mode Beam 2 (WV2) configuration, enabling to achieve better correlation between Doppler Centroid estimates and ECMWF range wind speed.
Acknowledgement:
The results presented here are outcome of the ESA contract Sentinel-1 / SAR Mission Performance Cluster Service 4000135998/21/I BG. Copernicus Sentinel-1 mission is funded by the EU and ESA.
The views expressed herein can in no way be taken to reflect the official opinion of the European Space Agency or the European Union.
The Radio Frequency Interferences (RFI) disturbance is affecting more and more spaceborne SAR missions due to the increasing number of ground (or even space) emitters transmitting in the frequency band allocated for the Earth Observation. Operative L-band SAR missions such ALOS and SAOCOM already foresee RFI mitigation strategies at processing level. Many cases of RFI contamination have been observed by Sentinel-1 users as well.
The paper focuses on two aspects:
• The global monitoring of RFI based on Sentinel-1 acquisitions
• The introduction, in the Senntinel-1 operational processor (IPF) of the RFI detection and mitigation functionality
The strategy for the global mapping of C-band RFIs is based on the use of the so-called rank echoes, the first signal free echoes of each TopSAR burst. A statistical analysis of the spectrum of the rank echoes allows to detect and characterize RFI disturbance [8]. The routine analysis of the S-1 noise products allows to create databases including all the RFI occurrence of a given S-1 cycle (12 days). The analysis of the databases allows to create global RFI contamination probability maps and to assess the temporal evolution of the contamination. The RFI databases and the probability maps are regularly published on a dedicated website https://s1rfimap.aresys.it/.
The monitoring of RFI contamination performed in the framework of the Sentinel-1/SAR Mission Performance Cluster (MPC) can trigger, in case of particular RFI contamination events, dedicated analyses to characterize the source responsible of the contamination, being it a ground-based device or another space emitter working in the same band of Sentinel-1.
The observed increasing level of contamination triggered an evolution of the S-1 IPF (the operational S-1 processor) aimed at introducing the capability of automatically detecting and mitigating RFI signals. The mitigation strategy implemented in the S-1 IPF is based on the time and frequency domain analysis of the raw data. Statistical outliers identified in one of the two domains are marked as RFI signals and removed from the raw data to reduce the quality degradation of the focused data. An example of the mitigation results is reported in the figure for a Seninel-1A image acquired over Dubai on 22nd September 2021. The image on the left shows the artifacts corresponding to RFI contamination. The same artifacts are not visible in the image on the right after reprocessing with the RFI mitigation feature.
The results of the mitigation step have been included in the new S-1 products format, providing information about the performed detection and mitigation.
The S-1 IPF version (v340) implementing the new RFI mitigation strategy was made operational on the 3rd November 2021. For the time being, only the detection feature is active in order to collect statistics on the number of products and bursts affected by RFI contamination.
Acknowledgement
The results presented here are outcome of the ESA contract Sentinel-1 / SAR Mission Performance Cluster Service 4000135998/21/I BG, funded by the EU and ESA.
The views expressed herein can in no way be taken to reflect the official opinion of the European Space Agency or the European Union.
SAR images benefit from excellent geometric accuracy due to accurate time measurements in range and precise orbit determination in azimuth. Moreover, the interferometric phase of each single pixel can be exploited to achieve differential range measurements for the reconstruction of topography and the observation of Earth surface deformation and surface motions. But these measurements are influenced by the spatial and temporal variability of the atmospheric conditions, by Earth dynamics, and by SAR processor approximations, which may lead to spurious displacements shifts of up to several meters. These effects become visible in various SAR applications including the retrieval of surface deformation applying offset tracking and various InSAR applications, which might require several post-processing steps and external information for correction.
To facilitate straightforward correction of the perturbing signals in the Sentinel-1 SAR data, the Extended Timing Annotation Dataset (ETAD) was developed in a joint effort by ESA and DLR based on research results and processor prototypes available at DLR. ETAD is a novel and flexible product for correcting the SAR range and azimuth time annotations in standard Sentinel-1 interferometric wide-swath and stripmap products. It accounts for the most relevant effects, including tropospheric delays based on 3D ECMWF operational analysis data, ionospheric delays based on total electron content (TEC) maps inferred from GNSS, solid Earth tides calculated following geodetic conventions, and corrections of SAR processor approximations. The effects are converted to range and azimuth time corrections with an accuracy at a global level of at least 0.2 m, and are provided as 200m resolution grids matching the swath and burst structure of Sentinel-1 SAR data. First experimental evaluations show that an even better accuracy of a few centimeters can be attained when applying the ETAD corrections.
The ETAD product is planned to become an operational Sentinel-1 product by the end of 2022. Currently, the processing software is undergoing pre-operation evaluation at ESA. In addition, it was integrated into the Geohazards Thematic Exploitation Platform (G-TEP) where users can participate in pilot studies and can generate ETAD products for dedicated applications. First successful usage of ETAD corrections could be demonstrated in ice velocity tracking and InSAR applications. ESA has now formed several study groups making use of the G-TEP to perform further evaluations. Our presentation will summarize the ETAD product and report on the status of operational integration. Moreover, results from the pilot studies with the G-TEP will be presented.
The current family of Copernicus Sentinel-1 (European Space Agency / European Commission) products primarily contains Level-1 Single Look Complex (SLC) and Ground Range Detected (GRD) products [1], which inherit their definition from the European heritage Synthetic Aperture Radar (SAR) satellite missions ERS-1, -2 and ENVISAT. These products have over the years proven to be reliable high-quality data sources since the start of Sentinel-1A in 2014 and users largely benefit from the open and free data policy of the Copernicus program. This has led to Sentinel-1 products being routinely used in several operational applications as well as enlarging the user base of SAR data in general.
However, the rapid increase of data volume is presenting a challenge to many users who still want to exploit this wealth of information but lack the resources for the processing needed to convert these Level-1 products to interoperable geoinformation. Cloud exploitation of data offers opportunities for accelerated data exploitation but requires new strategies of data management and provision.
In this context, the term Analysis Ready Data (ARD) has been coined and several activities have indicated the potential of enlarging the Copernicus product family by such ARD products. However, no agreement has yet been reached on the definition of ARD as different user communities perform very different analyses and thus have different understandings of analysis readiness.
With the aim to standardize different categories of ARD, the Committee on Earth Observation Satellites (CEOS) has set up the CEOS Analysis Ready Data for Land initiative (CARD4L) [2]. Different product specifications are currently being defined to provide guidelines on how to best process and organize data to serve as many use cases as possible with the respective products. One such product is Normalized Radar Backscatter (NRB) [3].
However, the CARD4L NRB specification can also be thought of as a guideline rather than a specification. It gives recommendations on what information should be included in a dataset but gives no concrete specifications on how this information should be created. For this reason, data providers are required to translate these guidelines into actual products with the software of choice to then reach out to CEOS for assessment of CARD4L compliance.
In this context an ARD product for Sentinel-1 was defined: the Sentinel-1 Normalized Radar Backscatter Product (S1-NRB). The aim is to be fully aligned with the CARD4L certification, offering a high-quality radiometrically terrain corrected SAR backscatter product with all relevant general metadata and per-pixel metadata (ancillary data layers). It is intended as a global and consistently processed product achieving the highest possible quality. It is designed to be complete to satisfy requirements of users with a prioritized focus on the Copernicus Services. Ultimately, it intends to significantly lower the effort for SAR processing for the SAR user community. Core characteristics are the structuring into the Military Grid Reference System (MGRS) tile grid for interoperability with Sentinel-2 ARD products, storage in Cloud Optimized GeoTIFFs and the provision of Spatiotemporal Asset Catalogue (STAC) metadata.
We present the characteristics of this S1-NRB product, summarize the results from several use cases conducted to ensure a high data quality, and give an outlook on the future availability.
This type of product is now baseline in the frame of the Copernicus Expansion and Sentinel Next Generation and is hence planned for further future missions.
References
[1] CLS, Sentinel-1 Product Specification, Version 3.9, 2021, https://sentinel.esa.int/documents/247904/1877131/Sentinel-1-Product-Specification-18052021.pdf/c2f9d58d-217f-e21d-548d-97a2cbd71e2b?t=1621347421421.
[2] A. Lewis, J. Lacey, S. Mecklenburg, J. Ross, A. Siqueira, B. Killough, Z. Szantoi, T. Tadono, A. Rosenqvist, P. Goryl, N. Miranda, and S. Hosford, "CEOS Analysis Ready Data for Land (CARD4L) Overview." pp. 7407-7410, IGARSS, Valencia, Spain, 2018, https://doi.org/10.1109/IGARSS.2018.8519255.
[3] CEOS, Analysis Ready Data For Land: Normalized Radar Backscatter, Version 5.5, 2021, https://ceos.org/ard/files/PFS/NRB/v5.5/CARD4L-PFS_NRB_v5.5.pdf.
The Sentinel 1 C & D Mission
The Sentinel 1 C & D (S1C and S1D) satellite program, whose Thales Alenia Space is the Prime Contractor, aims at supplying two identical observation satellites to ESA.
The Sentinel 1 C & D (S1C and S1D) satellites are part of the important Copernicus long term program founded by the European Union’s Earth Observation Programme, whose objective is the deployment in orbit a several satellites, equipped with high performance instruments and aiming at looking at the Earth and environment monitoring.
The Sentinel 1 mission is based upon a constellation of polar-orbiting radar satellites embarking a C-Band Synthetic Aperture Radars, imaging the same ground track night and day, regardless weather conditions in a combined six-days period.
The two satellites S1 C and S1 D are the “twin young brothers” of the of the already in orbit operating satellites Sentinel 1 A & B, which were launched on April 3rd 2014 and April 25th 2016, respectively. S1 C and D will perform the follow-on mission of S1 A and B.
The S1C and S1D satellites, even if they embark the same C-Band Synthetic Aperture Radars and the OCP payload, actually in use with S1 A and B, they are not fully identical to S1A and S1B. In fact S1 C and D are now equipped with the additional AIS (Automatic Identification System) payload, which will allow monitoring of the naval ships traffic.
Furthermore some performance upgrading’s have been implemented, in particular the on-board computer SMU (Satellite Management Unit) and the mass memory units DSHA (Data Storage and Handling Assembly) performance have been enhanced.
The S1 C/D project involves the contribution of several European industries, among which the main industrial partners are Airbus-De (supplier of SAR) and TESAT (supplier of OCP - Optical Communication Payload).
The first satellite S1D has recently achieved the Pre Storage Review (PSR) milestone and it is in storage in a dedicated clean room area at TAS-in-Italy Rome facility.
The second S1C satellite is presently undergoing the assembly and integration phase and its delivery is planned before the end of the 2022.
The two satellites are compatible with both Soyouz and VEGA-C launchers of Arianespace.
The paper will describe the Sentinel 1 C & D mission, as well as their design features and characteristics of both satellites.