Albert Garcia-Mondéjar

The Data-Driven Performance Assessment of Altimeters and Radiometers for Next-Generation ESA Altimetry Missions project (DPAAR), presented in this work, establishes a framework with a collection of tools to assess the main performance aspects of a multi-frequency altimeter and an enhanced band microwave radiometer onboard of the future Next Generation Copernicus Sentinel-6 (S6-NG) mission, and provide support for the Phase-0 preliminary design activities, which focus on mission analysis and needs identification constituting the very early assessment stage of the S6-NG mission. The primary strength of this project is not only to rely on accurate simulations and modeling but also on actual measurements. A number of metrics and test cases are implemented to perform both simulation and data-driven evaluation of the altimeter and microwave radiometer, and provide a metrological uncertainty assessment for the different mission characteristics and sizing orbits.

This study proposes a novel altimetry-radiometry synergistic approach for improving SMOS thin sea ice thickness retrievals by incorporating permittivity estimates derived from CryoSat-2 backscatter observations. Since sea ice permittivity remains the dominant source of uncertainty in L-band radiometric thickness retrievals, the sensitivity of the CryoSat-2 backscatter signal to variations in this parameter offers a promising pathway to better constrain it. By integrating these permittivity estimates into a hybrid scheme that includes the inversion of an L-band sea ice emission model using machine learning, the approach aims to enhance the accuracy and robustness of thin sea ice thickness estimates obtained from SMOS. A set of independent in situ datasets is used to validate the proposed methodology and to assess its performance across different ice regimes. The CryoSat-2-derived permittivity values lead to realistic and physically consistent estimates, although its validity is limited to first-year ice. Overall, the synergistic combination of Ku-band altimetry and L-band radiometry yields improved results compared to SMOS-only retrieval methods, which are included as a baseline for reference. This highlights the potential of cross-sensor synergies to advance thin sea ice monitoring, establishing a framework applicable to present and future satellite missions such as CIMR, CRISTAL, and ROSE-L.

The two-dimensional frequency-based omega-K method is known to be a suitable algorithm for Fully-Focused SAR (FF-SAR) radar altimeter processors, as its computational efficiency is much higher than equivalent time-based alternatives without much performance degradation. In this paper we provide a closed-form description of a two-dimensional frequency domain omega-K algorithm specific for instruments such as Poseidon-4 onboard Sentinel-6. The processor is validated with real data from point targets and over open ocean. Applications such as ocean swell retrieval and lead detection are demonstrated, showing the potentiality of the processor for future operational global-scale products.

Coasts are continually changing and remote sensing from satellites has the potential to both map and monitor coastal change at multiple scales. Unlike optical technology, synthetic aperture radar (SAR) is uninfluenced by darkness, clouds, and rain, potentially offering a higher revision period to map shoreline position and change, but this can only be feasible if we have a better interpretation of what shorelines as extracted from SAR imagery represent on the ground. This study aims to assess the application of shorelines extracted from SAR from publicly available satellite imagery to map and capture intra-annual to inter-annual shoreline variability. This is assessed in three tidally controlled coastal study areas that represent sand and gravel beaches with different backshore environments: low-lying dunes and marsh; steep, rocky cliff; and urban environments. We have found that SAR shorelines consistently corresponded to positions above the high-water mark across all three sites. We further discuss the influence of the scene geometry, meteorological and oceanographic conditions, and backshore environment and provide a conceptual interpretation of SAR-derived shorelines. In a low-lying coastal setting, the annual change rate derived through SAR presents a high degree of alignment with the known reference values. The present study contributes to our understanding of the poorly known aspect of using shorelines derived from publicly available SAR satellite missions. It outlines a quantitative approach to automatically assess their quality with a new automatic detection method that is transferable to shoreline evolution assessments worldwide.

The Sentinel-6 mission, launched in November 2020, carries a radar altimeter operating in open burst with a PRF high enough (~9kHz) to perform the focussing of whole target observation echoes in a fully coherent way. Furthermore, such a feature allows improvement of the along-track resolution down to the theoretical limit of around 0.5 m when processing the data with a Fully-Focussed SAR (FFSAR) algorithm. This resolution increment actually represents a revolutionary step with respect to the ~300 m along-track resolution provided by current operational processors based on Unfocussed SAR algorithms, commonly used in radar altimeters with a closed burst chronogram, such as CryoSat-2 and Sentinel-3. In this contribution, we explore new applications over inland water surfaces, such as reservoirs or lakes derived from the new Sentinel-6 FFSAR products. Indeed, a FFSAR Ground Prototype Processor (GPP), developed by isardSAT and based on the backprojection algorithm [1], has been used to process altimetry data and generate FFSAR radargrams of off-nadir inland targets located within certain observation constraints. As a main outcome, we present a methodology to geo-reference and estimate the extension of water bodies located on unambiguous across-track targets and that present strong seasonal extension variability. Validation of the method has been performed by comparing the FFSAR water extent measurements derived from Sentinel-6 against optical (Sentinel-2) measurements and in-situ observations. [1] Egido, Alejandro and Walter H. F. Smith. “Fully Focused SAR Altimetry: Theory and Applications.” IEEE Transactions on Geoscience and Remote Sensing 55 (2017): 392-406.

The Sentinel-6 mission, launched in November 2020 is a satellite mission carrying an altimeter operating in open-burst, the Poseidon-4 altimeter. This altimeter has a PRF of approximately 9 kHz, able to perform focussing of whole target observation echoes in a totally coherent way. This means that not only we are obtaining measurements with very reduced contaminant contributions coming from along-track replicas, but also the along-track resolution can therefore be narrowed down to its theoretical limit (∼0.5 m) when processing the data with a Fully-Focussed SAR (FF-SAR) algorithm. The latter is what is of interest, especially when compared to the ∼300 m along-track resolution provided by other operational processors based on Unfocussed SAR algorithms, widely used in the bast majority of satellite missions with radar altimeters that operate with closed-burst (e.g. Sentinel-3).In this study, we apply this algorithm to perform measurements of the water surface height (WSH) over a series of inland targets including relatively small reservoirs and lakes, with typical sizes between 0.1 and 10 km. To do so, a FF-SAR Ground Prototype Processor (GPP) developed by isardSAT and based on the back projection algorithm [1] has been used to process the Sentinel-6’s altimetry data and generate FF-SAR L1B records of the nadir targets being evaluated. This study will present the methodology defined to obtain the WSH measurements using the FF-SAR products alongside the validation process, based on comparison of results with in situ water height measurements. [1] Egido, Alejandro and Walter H. F. Smith. “Fully Focused SAR Altimetry: Theory and Applications.” IEEE Transactions on Geoscience and Remote Sensing 55 (2017): 392-406.

A trihedral corner reflector has been used to evaluate the capability of passive reflectors to calibrate radar altimeters such as the Poseidon-4 altimeter on board Sentinel-6A. The reflector location, placed on the top of a mountain ridge and about 4-km off-nadir of the Sentinel-6A subsatellite track, allows capturing echoed signals with a Signal-to-Clutter Ratio around 40 dB when processing the received data with a Fully-Focussed SAR algorithm. Results obtained show a range bias of 33.9 mm with 8.5 mm of standard deviation and datation bias of -2.3 μs with standard deviation of 1.8 μs for the measurement campaign between September 2021 and April 2022. Such values are comparable to what is currently achieved by means of active transponders and therefore it is demonstrated that passive reflectors may be of interest to support radar altimeter regular calibration.

CryoSat-2 is the first satellite mission carrying a high pulse repetition frequency radar altimeter with interferometric capability on board. Across track interferometry allows the angle to the point of closest approach to be determined by combining echoes received by two antennas and knowledge of their orientation. Accurate information of the platform mispointing angles, in particular of the roll, is crucial to determine the angle of arrival in the across-track direction with sufficient accuracy. As a consequence, different methods were designed in the CryoSat-2 calibration plan in order to estimate interferometer performance along with the mission and to assess the roll’s contribution to the accuracy of the angle of arrival. In this paper, we present the comprehensive approach used in the CryoSat-2 Mission to calibrate the roll mispointing angle, combining analysis from external calibration of both man-made targets, i.e., transponder and natural targets. The roll calibration approach for CryoSat-2 is proven to guarantee that the interferometric measurements are exceeding the expected performance.

The Sentinel-3 Mission Performance Centre (S3MPC) is tasked by the European Space Agency (ESA) to monitor the health of the Copernicus Sentinel-3 satellites and ensure a high data quality to the users. This paper deals exclusively with the effort devoted to the altimeter and microwave radiometer, both components of the Surface Topography Mission (STM). The altimeters on Sentinel-3A and -3B are the first to operate in delay-Doppler or SAR mode over all Earth surfaces, which enables better spatial resolution of the signal in the along-track direction and improved noise reduction through multi-looking, whilst the radiometer is a two-channel nadir-viewing system. There are regular routine assessments of the instruments through investigation of telemetered housekeeping data, calibrations over selected sites and comparisons of geophysical retrievals with models, in situ data and other satellite systems. These are performed both to monitor the daily production, assessing the uncertainties and errors on the estimates, and also to characterize the long-term performance for climate science applications. This is critical because an undetected drift in performance could be misconstrued as a climate variation. As the data are used by the Copernicus Services (e.g., CMEMS, Global Land Monitoring Services) and by the research community over open ocean, coastal waters, sea ice, land ice, rivers and lakes, the validation activities encompass all these domains, with regular reports openly available. The S3MPC is also in charge of preparing improvements to the processing, and of the development and tuning of algorithms to improve their accuracy. This paper is thus the first refereed publication to bring together the analysis of SAR altimetry across all these different domains to highlight the benefits and existing challenges.

Swath mode processing of CryoSat-2 Synthetic Aperture Radar Interferometric (SARIn) mode has been used to monitor elevation of areas with complex topography such as over ice sheet and ice cap margins. Swath processing relies on an accurate measure of the angle of arrival of the measured echo and, therefore, requires custom strategies in order to resolve the ambiguous phase measurement. In mountainous regions of complex terrain, it may be necessary to apply different phase ambiguities across a waveform record when returns come from different scatters distributed perpendicularly to the CryoSat-2 ground tracks. In this letter, we present modifications to the conventional swath processing method whereby a multisurface retracker is first applied to the record in order to identify potential different scattering surfaces. Phase ambiguity is then independently resolved for each of these subsurfaces. The improvements with this new method over the Karakoram glaciers are a 10% increase in the number of measurements with improvements of almost 50% for individual glaciers and a reduction in the median absolute deviation of the elevations from 20.18 to 14.69 m.