Conference Agenda

Copernicus is the joint European Earth observation program that aims to provide accurate, timely and easily accessible information to improve the management of the environment and security. The Copernicus program comprises a series of dedicated satellite missions i.e. the Sentinels, as well as ground segments for data processing, archiving and dissemination.

The Copernicus Sentinel-1 mission guarantees the continuity of C-band SAR observations for Europe. It is a constellation of two satellites positioned 180 degrees apart in the same orbital plane. The first generation comprises four satellite units developed in two batches, with the first two (A and B) launched and two more planned for launch to ensure continuity of measurement throughout the 2020s. The Sentinel-1 Next Generation [13] aims to provide continuity of measurement beyond 2030 and serves as the future backbone of SAR Earth Observation in Europe.

The mission is characterized by large-scale and repetitive observations, systematic production and free and open data policy. Sentinel-1 data are routinely exploited by Copernicus and many operational services, as well as in the scientific and commercial domain. The routine operations of the constellation are on-going and performed at full mission capacity until the premature failure of Sentinel-1B, which compromises the mission’s ability to ensure global coverage and short revisit , impacting several operational services and applications. The Agency in agreement with the European Commission has put in place several mitigations actions to alleviate the impact on the user side, pending the launch of the Sentinel-1C unit. The Agency, in agreement with the space debris policy is preparing the disposal phase aiming at de-orbiting the spacecraft in safe and responsible manner.

The paper provides a comprehensive status of the Sentinel-1 first generation covering the following aspects:

• Sentinel-1 first generation overview: providing a comprehensive view of the Sentinel-1 first generation development. It provides key elements to understand the Sentinel-1 concept, describes the major achievements of the Sentinel-1 mission and gives a perspective on the evolution of constellation in the coming years bridging with the Copernicus expansion and the Sentinel-1 Next Generation.
• Sentinel-1 routine operations status: giving an overview of the operation concept, providing a status of the routine operations and further describing the adjustments made on the operation plan to support the Copernicus services and the scientific applications following the failure of the S-1B unit. It also provides elements regarding the evolution of the Sentinel-1 Data Access following the PDGS cloud migration.
• Sentinel-1B disposal: Sentinel-1B suffered a major anomaly being unavailable since the 23rd December 2021. The end of mission has officially been announced in August 2022 signifying the end of the exploitation phase. The satellite is parked in orbit pending the detailed definition of the disposal phase. This section describes the activities undertaken by the Agency since the S-1B failure highlights the challenges and describes the approach that will be implemented for a safe re-entry in the atmosphere.
• Sentinel-1C/D improvements: The second batch of units is composed by the Sentinel-1C and D [12] units which will be deployed in a staggered manner, gradually replacing its precursors. The C and D units have been specified to take the maximum benefit of the existing qualified designs and to consider the lessons learnt from the A and B operations to best serve the user community. This results in several improvement in robustness and performance with respect to the currently flying units. In addition, the main characteristics of the new Automatic Identification System (AIS) are discussed.

Sentinel-1 data are routinely used by Copernicus and many operational services, as well as in the scientific and commercial domain. Accordingly, a key aspect of the Copernicus program is the constant provision of quality data, which requires long term engagement to carefully monitor, preserve, and even improve the system performances.

The Sentinel-1 SAR Mission Performance Cluster Service (SAR-MPC) is an international consortium of SAR experts. It oversees 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 done by analyzing the variation of key parameters over time using standard products and/or dedicated auxiliary ones. The MPC is also responsible for the evolution of the L1 and L2 algorithms contributing to the continuous improvement of the quality of S1 products.

The monitoring of both the SAR antenna health status and of the SAR instrument is carried out exploiting the dedicated auxiliary products to ensure that no degradation of SAR data quality is originated by instrument aging or elements failures. The radiometric performance monitoring exploits both the DLR calibration site, hosting transponders and corner reflectors, and uniformly distributed targets, like rainforest, to assess the absolute and relative radiometric accuracy of S-1 products. The geolocation accuracy is monitored using dedicated acquisitions over additional calibration sites. The procedure includes the compensation of known instrument and environmental effects, e.g., propagation through troposphere and ionosphere.

This presentation will provide an overview of the status of the Sentinel-1 instrument and product performance. Moreover, a brief overview of recent algorithm and processor evolution will be shown.

The Copernicus program and particularly Sentinel-1 are among the largest Earth Observation SAR data providers, serving an ever-increasing number of services, users, and applications. A key aspect of the program is the constant provision of quality data, which requires long term engagement to carefully monitor, preserve, and improve the system performances. These tasks are mainly carried out within the Sentinel-1 Mission Performance Cluster (S-1 MPC), an international consortium of SAR experts in charge of the continuous monitoring of the S-1 instruments status and of the L1 and L2 products quality. The S-1 MPC is responsible of detecting any potential issues and implementing the necessary actions to ensure that no data quality degradation occurs for the users.

The end of mission for S-1B has officially been announced after it suffered an anomaly resulting in its unavailability since the 23rd of December 2021. Henceforth, this contribution focuses on the monitoring of S-1A acquisitions, which repeat after a cycle of 12 days.

An important part of the S-1 MPC monitoring concerns the analysis of the S-1A interferometric parameters. This happens via the evaluation of the time series of the burst synchronization time between cycles, the interferometric baseline between passes, and the instrument Doppler pointing.

To support the users of Sentinel-1 data, the Instrument Processing Facility (IPF) has introduced since version 3.4 the annotation of burst cycle ID numbers for the TOPS (Terrain Observation with Progressive Scans) modes. Specifically, each burst in a sub-swath is labelled by an absolute and a relative burst ID, which allow to identify it unambiguously since the beginning of the mission and the beginning of its 12-day cycle, respectively. Both IDs are integers increasing monotonically from one burst to the next one, while the relative burst ID resets at the start of a new 12-day cycle. The bursts which belong to the same burst cycle (three bursts for IW, five bursts for EW) share the same relative burst ID. As S-1 bursts are synchronized from one pass to the other, it is possible to create a univocal correspondence between the burst cycle ID (for a certain sub swath) and a region on Earth’s surface. The annotation of the burst cycle ID was then introduced to simplify the search of a specific ROI over time and to aid the creation of interferometric stacks.
To further develop this goal, the S-1 MPC has published a set of Burst ID maps. A Burst ID map associates along a full 12-day cycle each relative burst ID with a geolocated polygon that delimitates the burst footprint. The polygon is delimitated by six points, three along the ground range axis (start, middle, and far range) at the burst start, and three at the burst end. Two different maps are provided for the IW and EW TOPSAR acquisition modes and are provided both as sqlite3 databases (one per mode) and KMZ files (one for each mode and relative orbit number). For each burst id and sub-swath, they provide information on its relative orbit number within the 12-day cycle (ranging from 1 to 175), on the orbit direction (ascending or descending), and on the nominal time at which the burst starts. The maps are global, i.e., they provide information also where no SAR data is acquired. The maps were generated by means of geocoding along the orbits of cycle number 213 (starting on 9^th of October 2020), in the EPSG:4326 Coordinate Reference System (CRS) using the WGS84 ellipsoid as horizontal datum and assuming zero height for each point. The maps have been validated with respect to cycle 240 (starting on 25^th of February 2022), evaluating the distances between the corners of the same burst footprints in the two cycles. The analysis showed an average absolute discrepancy of 960 ± 553 m for IW mode and 996 ± 465 m for EW mode.

This contribution will present:

• An overview on the S-1A monitoring in 2022-2023, especially the time series of the interferometric parameters
• The description of the Burst IDs annotation, showing their definition and formula
• The description of the Burst ID maps definition, generation, and validation

Acknowledgement

The SAR Mission Performance Cluster (MPC) Service is financed by the European Union, through the Copernicus Program implemented by ESA. 

Views and opinion expressed are however those of the author(s) only and the European Commission and/or ESA cannot be held responsible for any use which may be made of the information contained therein. 

Abstract

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 implemented RFI mitigation strategies at processing level since the begin. Many cases of RFI contamination have been observed by Sentinel-1 users as well.

The RFI contamination in L1 data is observed as very bright areas in the data, due to the fact that the energy of the received RFI signal (that can be much higher than the received SAR) is spread in azimuth and range by the focusing kernel. The result is that part of the SAR image is useless for radiometric and interferometric applications since the signal is overwhelmed by the RFI disturbance.

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. The RFI mitigation capability was introduced with S-1 IPF version (v340) on the 3rd November 2021. The feature was operationally activated on the 23rd March 2022, after properly verifying that no quality degradation affected the L1 data after RFI mitigation. 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 implemented RFI mitigation strategy is able to almost completely remove the RFI disturbance from L1 products by filtering a relatively small number of pixels or frequency bands in the raw data. This results in a quite negligible data quality reduction w.r.t. the one introduced by the RFI contamination.

The proposed contribution focuses on two aspects:

• The description of the RFI mitigation technique and of the products evolution, with sample results of the performed RFI mitigation

• The description of the verification of the interferometric SAR data quality after RFI mitigation with exempla of the improvement in interferometric quality related to the mitigation of RFI in S-1 products

Acknowledgement

The SAR Mission Performance Cluster (MPC) Service is financed by the European Union, through the Copernicus Programme implemented by ESA. 

Views and opinion expressed are however those of the author(s) only and the European Commission and/or ESA cannot be held responsible for any use which may be made of the information contained therein. 

Discussion