Nuno Miranda

Sentinel-1A Product Geolocation Accuracy: Commissioning Phase Results

Sentinel-1A (S1A) is an Earth observation satellite carrying a state-of-the-art Synthetic Aperture Radar (SAR) imaging instrument. It was launched by the European Space Agency (ESA) on 3 April 2014. With the end of the in-orbit commissioning phase having been completed at the end of September 2014, S1A data products are already consistently providing highly accurate geolocation. StripMap (SM) mode products were acquired regularly and tested for geolocation accuracy and consistency during dedicated corner reflector (CR) campaigns. At the completion of this phase, small geometric inconsistencies had been understood and mitigated, with the high quality of the final product geolocation estimates reflecting the mission’s success thus far. This paper describes the measurement campaign, the methods used during geolocation estimation, and presents best estimates of the product Absolute Location Error (ALE) available at the beginning of S1A’s operational phase.

A revised radiometric normalisation standard for SAR

Improved geometric accuracy in SAR sensors implies that more complex models of the Earth may be used not only to geometrically rectify imagery, but also to more robustly calibrate their radiometry. Current beta, sigma, and gamma nought SAR radi-ometry conventions all assume a simple “flat as Kansas” Earth ellipsoid model. We complement these simple models with improved radiometric calibration that accounts for local terrain variations. In the era of ERS-1 and RADARSAT-1, image geolo-cation accuracy was in the order of multiple samples, and tiepoint-free establishment of the relationship between radar and map geometries was not possible. Newer sensors such as ASAR, PALSAR, and TerraSAR-X all support accurate geolocation based on product annotations alone. We show that high geolocation accuracy, combined with availability of high-resolution accurate elevation models, enables a more robust radiometric calibration standard for modern SAR sensors that is based on gamma nought normalised using an Earth terrain-model.

Sentinel-1 mission status

Sentinel-1A has been launched from Kourou on a Soyuz rocket on 3rd April 2014. The second satellite Sentinel-1B is planned to be launched first half of 2016. The In-Orbit Commissioning phase was completed on 23 September 2014, followed by the so-called operational qualification phase (ramp-up). This ramp-up operations phase is a phase during which the capacity of the overall system, including the ground segment operations, is progressively increased, together with the gradual release of the operationally qualified products. The ramp-up phase is being completed at the time of writing this paper (end May 2015); at this stage the routine operations are starting. The Full Operation Capacity (FOC) of the mission will be reached once the In-Orbit Commissioning phase of Sentinel-1B and the subsequent constellation operational qualification phase will have been completed, indicatively by end 2016. The paper provides high-level information on the ongoing mission operations, at the time of the ramp-up phase completion. It described at high level the system operations, incl. ground segment operational activities, as well as some user data access statistics. It presents few examples of mission results achieved during the first year of Sentinel-1A in orbit, in some key application domains.

Sentinel-1A/B Combined Product Geolocation Accuracy

Sentinel-1A and -1B are twin spaceborne synthetic aperture radar (SAR) sensors developed and operated by the European Space Agency under the auspices of the Copernicus Earth observation programme. Launched in April 2014 and April 2016, Sentinel-1A and -1B are currently operating in tandem, in a common orbital configuration to provide an increased revisit frequency. In-orbit commissioning was completed for each unit within months of their respective launches, and level-1 SAR products generated by the operational SAR processor have been geometrically calibrated. In order to compare and monitor the geometric characteristics of the level-1 products from both units, as well as to investigate potential improvements, products from both satellites have been monitored since their respective commissioning phases. In this study, we present geolocation accuracy estimates for both Sentinel-1 units based on the time series of level-1 products collected thus far. While both units were demonstrated to be performing consistently, and providing SAR data products according to the nominal product specifications, a subtle beam- and mode-dependent azimuth bias common to the data from both units was identified. A method for removing the bias is proposed, and the corresponding improvement to the geometric accuracies is demonstrated and quantified.

SPACEBORNE SAR PRODUCT GEOLOCATION ACCURACY: A SENTINEL-1 UPDATE

Sentinel-1A (S1A), launched by the European Space Agency (ESA) on April 3, 2014, is a state-of-the-art spaceborne Synthetic Aperture Radar (SAR) Earth observation platform. S1A products are expected to provide high and consistent geolocation accuracy. As of the end of May, 2014, the satellite has not yet reached its reference orbit. However, estimation of product geolocation accuracy is ongoing, and improvements are continually being made. The results reported here represent the early situation, with continued progress expected.

The Sentinel-1A instrument and operational product performance status

This paper addresses the results of the instrument and product performance verification, radiometric and geometric calibration achieved during the since commissioning and routine phase.

Impact of the antenna stability on the Doppler Centroid frequency

In this paper a method for calculating the antenna contribution to the Doppler Centroid is presented. The Antenna Doppler Contribution (ADC) is originated by the non idealities affecting the TR modules of the active antenna arrays. It is an undesired effect for SAR applications requiring stringent performances on the Doppler Centroid control capability. The first part of this paper introduces the problem and shows how it is possible to predict the ADC by exploiting a suitable Antenna Model. The second part of the paper presents a comparison between the ADC calculated with an Antenna Model and the ADC retrieved from real ASAR data.

Requirements and Tests for Phase Preservation in a SAR Processor

This paper proposes a set of tests that provide necessary and, as far as possible, sufficient conditions for verifying the phase preservation of synthetic aperture radar (SAR) processors. This paper discusses two different test families composed of self-consistency and point-target-based tests. The set of self-consistent tests extends the well-known CEOS-OFFSET to very different acquisition modes, namely, StripMAP, ScanSAR, TopSAR, and SPOTLIGHT, and introduces a new complementary test to check the consistency of the space-variant implementation of a processor. The set of point target tests, based on the analysis of point target simulated data, completes the self-consistent tests by providing a verification of phase accuracy between different bursts and swaths.

Sentinel-1 Ground Segment

This paper provides an overview of the Senitnel-1 Ground Segment and its main operations concepts. It presents the Sentinel-1 Payload data Ground Segment (PDGS), responsible during the Sentinel-1 mission operations for the Sentinel-1 mission planning, X-Band data downlink, data processing, quality performance, data dissemination, archiving activities and user interface, with the main objective to make the core ground segment products available to the Sentinel-1 users within the expected timeliness and quality. The paper describes as well the Sentinel- 1 operational user product family and their characteristics.

The Sentinel-1 data processor and operational products

The Sentinel-1 Instrument Processing Facility (IPF) is the sub-system of the Sentinel-1 Payload Data Ground Segment (PDGS) [1] responsible for the generation of the Level-1 and Level-2 products. This paper provides an overview of the Sentinel-1 IPF architecture, processing algorithms and the output products.