In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the "Green" Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments' development and satellite missions' evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion.
<p>The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 and from an altitude of just over 700 km and reaching latitudes of 88 degrees, monitors precise changes in the thickness of terrestrial ice sheets and marine ice. The aim of the CryoSat-2 mission is to determine variations in the thickness of the Earth's marine ice cover and understand the extent to which the Antarctic and Greenland ice sheets are contributing global sea level rise. In its 10 years of operations, CryoSat has achieved its mission objectives and has provided high-quality of data for a number of Earth science applications and opened up new research streams and triggered new scientific questions which have emerged from the previous phases.&#160;The purpose of this paper is to provide a general overview of the mission status and provide programmatic highlights in its new extended phase until 2021.&#160;It will also provide an overview of CryoSat data products covering both Ocean and Ice processing chains, presenting also the main evolutions and improvements that have implemented to the processors and anticipating evolutions for the future.&#160;</p><p>&#160;</p>
Abstract. Sea ice thickness is essential for climate studies and numerical weather prediction. Radar altimetry has provided sea ice thickness measurement since the launch of ERS-1 and currently through CryoSat-2, Sentinel-3 and Altika but uncertainty in the scattering horizon used to retrieve sea ice thickness arises from interactions between the emitted signal and snow cover on the ice surface. Therefore, modelling the scattering of the electromagnetic waves with the snowpack and ice is necessary to retrieve the sea ice thickness accurately. The Snow Microwave Radiative Transfer (SMRT) model was used to simulate the low resolution altimeter waveform echo from the snow-covered sea ice, using in-situ measurements as input. Measurement from four field campaigns were used: Cambridge Bay, Eureka Sound and near Alert, Nunavut, Canada in April 2022 in the cold and later winter condition when snow and ice thickness are neat their seasonal maxima prior to melt. In-situ measurements included snow temperature, salinity, density, specific surface area, microstructure from X-ray tomography and surface roughness measurements using structure from motion photogrammetry. Evaluation of SMRT in altimeter mode was performed against CryoSat-2 waveform data in pseudo-low-resolution mode. Simulated and observed waveforms showed good agreement, although it was necessary to adjust sea ice roughness. The retrieved roughness (root-mean-square height) in Cambridge Bay was 2.1 mm and 1.6 mm in Eureka, which was close to the observed value of 1.4 mm for flat sea ice. In addition, simulations of backscatter in preparation for the European Space Agency's CRISTAL mission demonstrated the dominance of scattering from the snow surface at Ku and Ka-band. However, these findings depend on the parameterisation of the roughness. The scattering from the snow surface dominates when roughness is high, but the interface return dominates if the roughness is low ( < 2.5 mm). This is the first study to consider scattering within the snow and demonstrate the origin of CryoSat-2 signals. This work paved the way to a new physical retracker using SMRT to retrieve snow depth and sea ice thickness for radar altimeter missions.
Abstract. The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 to monitor the precise changes in the thickness of terrestrial ice sheets and marine floating ice. For that, CryoSat orbits the planet at an altitude of around 720 km with a retrograde orbit inclination of 92° and a quasi repeat cycle of 369 days (30 days sub-cycle). To reach the mission goals, the CryoSat products have to meet the highest quality standards to date, achieved through continual improvements of the operational processing chains. The new CryoSat Ice Baseline-D, in operation since 27th May 2019, represents a major processor upgrade with respect to the previous Ice Baseline-C. Over land ice the new Baseline-D provides better results with respect to previous baseline when comparing the data to a reference elevation model over the Austfonna ice cap region, improving the ascending and descending crossover statistics from 1.9 m to 0.1 m. The improved processing of the star tracker measurements implemented in Baseline-D has led to a reduction of the standard deviation of the point-to-point comparison with the previous star tracker processing method implemented in Baseline-C from 3.8 m to 3.7 m. Over sea ice, the Baseline-D improves the quality of the retrieved heights in areas up to ~ 12 km inside the Synthetic Aperture Radar Interferometric (SARIn or SIN) acquisition mask, which is beneficial not only for freeboard retrieval, but for any application that exploits the phase information from SARIn Level-1 (L1) products. In addition, scatter comparisons with the Beaufort Gyre Exploration Project (BGEP, https://www.whoi.edu/beaufortgyre) and Operation IceBridge (OIB, Kurtz et al., 2013) in-situ measurements confirm the improvements in the Baseline-D freeboard product quality. Relative to OIB, the Baseline-D freeboard mean bias is reduced by about 8 cm, which roughly corresponds to a 60 % decrease with respect to Baseline-C. The BGEP data indicate a similar tendency with a mean draft bias lowered from 0.85 m to −0.14 m. For the two in-situ datasets, the Root Mean Square Deviation (RMSD) is also well reduced from 14 cm to 11 cm for OIB and with a factor 2 for BGEP. Observations over inland waters, show a slight increase in the percentage of good observations in Baseline-D, generally around 5–10 % for most lakes. This paper provides an overview of the new Level-1 and Level-2 (L2) CryoSat ice Baseline-D evolutions and related data quality assessment, based on results obtained from analysing the 6-month Baseline-D test dataset released to CryoSat expert users prior the final transfer to operations.
CryoSat-2 (CS2) is the first mission carrying on board an altimeter instrument able to operate in Synthetic Aperture Radar Interferometric (SARIn) mode. CS2 SARIn acquisitions have been exploited for different scientific applications that take advantage of the capability to determine the across-track angle of the first return and, in particular, they have been proved to reduce the uncertainty of sea ice freeboard retrievals. Nonetheless, the analysis of pan-Arctic freeboard obtained by processing CS2 Baseline C L1b products has shown large negative freeboard estimates in correspondence of the beginning of SARIn acquisitions. Throughout this letter, the SARIn waveforms are analyzed to identify the cause of this behavior. An improvement of the CS2 L1b processor is then prototyped and used to obtain a pan-Arctic freeboard data set where the percentage of negative freeboard is successfully minimized.
<p>This presentation provides an update on the ESA radar altimetry processing services portfolio, known as SARvatore, for the exploitation of CryoSat-2 (CS-2) and Sentinel-3 (S-3) data from L1A (FBR) data products up to SAR/SARin L2 geophysical data products. The following on-line & on-demand services compose the portfolio, now hosted in the ESA Altimetry Virtual Lab at the EarthConsole&#174; (https://earthconsole.eu):</p> <ul> <li>The ESA-ESRIN SARvatore (SAR Versatile Altimetric Toolkit for Research & Exploitation) for CS-2 and S-3 services. These processor prototypes allow the users to customize the processing at L1b & L2 by setting a list of configurable options, including those not available in the operational processing chains (e.g., SAMOSA+ and ALES+ SAR retrackers).</li> <li>The TUDaBo SAR-RDSAR (TU Darmstadt &#8211; U Bonn SAR-Reduced SAR) for CS-2 and S-3 service. It allows users to generate reduced SAR, unfocused SAR & LRMC data. Several configurable L1b & L2 processing options and retrackers (BMLE3, SINC2, TALES, SINCS, SINCS OV) are available.</li> <li>The TU M&#252;nchen ALES+ SAR for CS-2 and S-3 service. It allows users to process L1b data applying the empirical ALES+ SAR subwaveform retracker, including a dedicated SSB solution.</li> <li>The Aresys FF-SAR (Fully-Focused SAR) for CS-2 & S-3 service. It provides the capability to produce L1b products with several configurable options and with the possibility of appending the ALES+ FFSAR output to the L1b products. In the future, the service will be extended to process Sentinel-6 data.</li> </ul> <p>The following new services will be made available: the CLS SMAP S-3 FF-SAR processor (s-3-smap, http://doi.org/10.5270/esa-cnes.sentinel-3.smap) and the ESA-ESTEC/isardSAT L1 Sentinel-6 Ground Prototype Processor.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; &#160;&#160;&#160;&#160;</p> <p>All output data products are generated in standard netCDF format and are therefore also compatible with the multi-mission &#8220;Broadview Radar Altimetry Toolbox&#8221; (BRAT, http://www.altimetry.info).</p> <p>The Altimetry Virtual Lab is a community space for simplified processing services and knowledge-sharing, hosted on the EarthConsole&#174;, a powerful EO data processing platform now on the ESA Network of Resources. This enables SARvatore Services to remain open for worldwide scientific applications, including for R&D studies on the retrieval of radar altimetry measured variables contributing to Inland Water monitoring (write to altimetry.info@esa.int for further information).</p>
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.
Introduction HYDROCOASTAL is a two year project funded by ESA, with the objective to maximise exploitation of SAR and SARin altimeter measurements in the coastal zone and inland waters, by evaluating and implementing new approaches to process SAR and SARin data from CryoSat-2, and SAR altimeter data from Sentinel-3A and Sentinel-3B. Optical data from Sentinel-2 MSI and Sentinel-3 OLCI instruments will also be used in generating River Discharge products. New SAR and SARin processing algorithms for the coastal zone and inland waters will be developed and implemented and evaluated through an initial Test Data Set for selected regions. From the results of this evaluation a processing scheme will be implemented to generate global coastal zone and river discharge data sets. A series of case studies will assess these products in terms of their scientific impacts. All the produced data sets will be available on request to external researchers, and full descriptions of the processing algorithms will be provided Objectives The scientific objectives of HYDROCOASTAL are to enhance our understanding of interactions between the inland water and coastal zone, between the coastal zone and the open ocean, and the small scale processes that govern these interactions. Also the project aims to improve our capability to characterize the variation at different time scales of inland water storage, exchanges with the ocean and the impact on regional sea-level changes The technical objectives are to develop and evaluate new SAR and SARin altimetry processing techniques in support of the scientific objectives, including stack processing, and filtering, and retracking. Also an improved Wet Troposphere Correction will be developed and evaluated. Presentation The presentation will describe the different SAR altimeter processing algorithms that are being evaluated in the first phase of the project, and present results from the evaluation of the initial test data set focusing on performance at the coast. It will also present the results of a study assessing regional tidal models.
