Lakes and reservoirs are considered sentinels of climate and anthropogenic changes. Lakes and reservoirs surface water storage is an essential hydrological variable but poorly known as this information is scarce. Earth Observation data are a reliable source of information to overcome this scarcity. Among these, the combined use of satellite images, to derive water extent, and radar altimetry, which enables to estimate water levels, provides valuable information on water storage changes. Here, we used Synthetic Aperture Radar images from Sentinel-1 and radar altimetry data from Sentinel-3 to monitor the water volume changes of Lake Bracciano from 2016 to 2021. This lake was affected by a water crisis in 2017 and the water supply to the city of Rome (Italy) was interrupted September 2017 to preserve its ecosystem. Hence, we demonstrate how Sentinel-1 and Sentinel-3 data can be useful to monitor water extent and level, which can be profoundly changed by the climate crisis.

ALtimetry for BIOMass (ALBIOM) is a Permanent Open Call Project funded by the European Space Agency (ESA) to explore the possibility of forest biomass retrieval by using Copernicus Sentinel-3 (S-3) Synthetic Aperture Radar Altimeter (SRAL) in low- and high-resolution mode at Ku- and C-bands. It represents an original work in the research of new techniques for vegetation observation using altimetry data. Because of the complexity of the land surfaces, no algorithm has been developed for a specific retracking of the altimetric land waveform. This calls for the development of a model able to reproduce the acquisition system and the target scattering phenomena to simulate the interaction of the radar pulse with the land. In this first work we present the electromagnetic simulator of S-3 SRAL altimeter measurements over bare soil scenarios realized through a modification of the SAVERS (Soil And Vegetation Reflection Simulator) simulator developed by the team for GNSS-R reflectometry over land. The impact of topography has been also taken into account. We demonstrate that SAVERS for S-3 SRAL proved its capability to reproduce altimeter waveforms’ main attributes for both flat and topography scenarios.

Forests play a crucial role in the climate change mitigation by acting as sinks for carbon and, consequently, reducing the CO2 concentration in the atmosphere and slowing global warming. For this reason, above ground biomass (AGB) estimation is essential for effectively monitoring forest health around the globe. Although remote sensing-based forest AGB quantification can be pursued in different ways, in this work, we discuss a new technique for vegetation observation through the use of altimetry data that have been introduced by the ESA-funded ALtimetry for BIOMass (ALBIOM) project. ALBIOM investigates the possibility of retrieving forest biomass through Copernicus Sentinel-3 Synthetic Aperture Radar Altimeter (SRAL) measurements at the Ku- and C-bands in low- and high-resolution modes. To reach this goal, a simulator able to reproduce the altimeter acquisition system and the scattering phenomena that occur in the interaction of the radar altimeter pulse with vegetated surfaces has been developed. The Tor Vergata Vegetation Scattering Model (TOVSM) developed at Tor Vergata University has been exploited to simulate the contribution from the vegetation volume via the modeling of the backscattering of forest canopy through a discrete scatterer representation. A modification of the Soil And Vegetation Reflection Simulator (SAVERS) developed by the team for Global Navigation Satellite System Reflectometry over land has also been taken into account to simulate the soil contribution.

ALtimetry for BIOMass (ALBIOM) is a research project funded by the European Space Agency to study the possibility of estimating above ground biomass by means of low- and high-resolution Sentinel-3 altimetry data. We present preliminary results of a sensitivity analysis, developed to assess in what extent waveforms and altimetry observables are affected by the presence of forests. Ku- and C-band altimetry data have been collected and processed to properly select well-tracked waveforms over land, which are then related to collocated biomass data. A statistical analysis has been performed, highlighting a sensitivity of the estimated normalized radar cross section with respect to forest biomass, even though it is strongly disturbed by the soil topography within the radar footprint. Additionally, the inaccurate positioning of the time-tracking window limits the number of useful data.

The SRAL Radar Altimeter, on board of the ESA Mission Sentinel-3 (S-3), has the capacity to operate either in the Pulse-Limited Mode (also known as LRM) or in the novel Synthetic Aperture Radar (SAR) mode. Thanks to the initial results from SAR Altimetry obtained exploiting CryoSat-2 data, lately the interest by the scientific community in this new technology has significantly increased and consequently the definition of accurate processing methodologies (along with validation strategies) has now assumed a capital importance. In this paper, we present the algorithm proposed to retrieve from S-3 STM SAR return waveforms the standard ocean geophysical parameters (ocean topography, wave height and sigma nought) and the validation results that have been so far achieved exploiting the CryoSat-2 data as well as the simulated data. The inversion method (retracking) to extract from the return waveform the geophysical information is a curve best-fitting scheme based on the bounded Levenberg-Marquardt Least-Squares Estimation Method (LEVMAR-LSE). The S-3 STM SAR Ocean retracking algorithm adopts, as return waveform's model, the “SAMOSA” model [Ray et al, 2014], named after the R&D project SAMOSA (led by Satoc and funded by ESA), in which it has been initially developed. The SAMOSA model is a physically-based model that offers a complete description of a SAR Altimeter return waveform from ocean surface, expressed in the form of maps of reflected power in Delay-Doppler space (also known as stack) or expressed as multilooked echoes. SAMOSA is able to account for an elliptical antenna pattern, mispointing errors in roll and yaw, surface scattering pattern, non-linear ocean wave statistics and spherical Earth surface effects. In spite of its truly comprehensive character, the SAMOSA model comes with a compact analytical formulation expressed in term of Modified Bessel functions. The specifications of the retracking algorithm have been gathered in a technical document (DPM) and delivered as baseline for industrial implementation. For operational needs, thanks to the fine tuning of the fitting library parameters and the usage of look-up table for Bessel functions computation, the CPU execution time was accelerated over 100 times and made the execution in par with real time. In the course of the ESA-funded project CryoSat+ for Ocean (CP4O), new technical evolutions for the algorithm have been proposed (as usage of PTR width look up table and application of a stack masking). One of the main outcomes of the CP4O project was that, with these latest evolutions, the SAMOSA SAR retracking was giving equivalent results to CNES CPP retracking prototype, which was built with a totally different approach, which enforces the validation results. Work actually is underway to align the industrial implementation with the last new evolutions. Further, in order to test the algorithm with a dataset as realistic as possible, a set of simulated Test Data Set (generated by S-3 STM End-to-End Simulator) has been created by CLS following the specifications as described in a test data set requirements document drafted by ESA. In this work, we will show the baseline algorithm details, the evolutions, the impact of the evolutions and the results obtained processing the CryoSat-2 data and the simulated test data set.

The backscatters power single-look waveform recorded by a synthetic aperture radar altimeter is approximated in a closed-form model. The model, being expressed in terms of parameterless functions, allows for efficient computation of the waveform and a clear understanding of how the various sea state and instrument parameters affect the waveform.

Satellite radar altimetry has been used for many years to monitor the world's largest inland waterbodies [1]. As the global in-situ network of gauge stations progressively falls out of use, whilst demands on water resources escalate [2], the importance of a remote measurement capability continues to increase. This paper presents an analysis of the Ku band echoes over inland water, with a global analysis of the contribution of the EnviSat RA-2 to inland water height measurement, showing that 15,067 height time series have successfully been retrieved. Analysis of the samples of unaveraged echoes that are available at 1800Hz demonstrates that the higher Pulse Repetition Frequency allows successful measurement of smaller water surfaces, illustrating the future potential of Sentinel-3 for inland water measurement.

Satellite radar altimeters have an established and increasingly vital role in monitoring the Earth's surface inland water resources. This paper analyzes two years of EnviSat burst echoes (which have 3.9-m nominal along-track separation) over rivers, lakes, and ephemeral water globally to assess the increase in monitoring potential afforded by the higher pulse repetition frequency (PRF) of the next generation of synthetic aperture radar altimeters and investigate spatial burst correlation. Burst echoes at 1800 Hz are successfully retracked with no waveform averaging. The conclusion is that the higher PRF allows detection and measurement of water bodies on a far finer spatial scale because water is a very bright reflector at Ku-band and land in general is relatively poor, with pools of water a few tens of meters across being successfully identified.

Radar altimeters have so far had limited use in the coastal zone, the area with most societal impact. This is due to both lack of, or insufficient accuracy in the necessary corrections, and more complicated altimeter signals. This letter examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10-km2island in the northwestern Mediterranean). Forty-six repeat passes were analyzed, with most showing a reduction in signal upon passing over the island, with weak early returns corresponding to the reflections from land. Intriguingly, one third of cases showed an anomalously bright hyperbolic feature. This feature may be due to extremely calm waters in the Golfo della Botte (northern side of the island), but the cause of its intermittency is not clear. The modeling of waveforms in such a complex land/sea environment demonstrates the potential for sea surface height retrievals much closer to the coast than is achieved by routine processing. The long-term development of altimetric records in the coastal zone will not only improve the calibration of altimetric data with coastal tide gauges but also greatly enhance the study of storm surges and other coastal phenomena.

A NEW SET OF TOOLS FOR ALL ALTIMETRY USERS The field of satellite radar altimetry has matured to a point where it is now time to encourage a multimission approach and conceive an "all-altimeter" toolbox including a tutorial. Such an integrated approach and view is vital not only for assessing the current status of what offers altimeter products but also to show the system and consistency with the past. The Basic Radar Altimetry Toolbox (BRAT) is a collection of tools, tutorials and documents designed to facilitate the use of radar altimetry data for altimetry users, experienced as well as beginners, and particularly the users of the upcoming CryoSat mission. It is able to read most distributed radar altimetry data, from ERS-1 & 2, TOPEX/Poseidon, Geosat Follow-on, JASON-1, Envisat, and the future Cryosat missions, to perform some processing, data editing and statistic and to visualise the results. A version 2 is being developed with additional visualisation features such as waveform viewing. Also, a release for the MacOS is planned. As part of the Toolbox, a Radar Altimetry Tutorial gives general information about altimetry, the technique involved and its applications, as well as an overview of past, present and future missions, including information on how to access data and additional software and documentation. It also presents a series of data use cases, covering all uses of altimetry over ocean, cryosphere and land, showing the basic methods for some of the most frequent manners of using altimetry data.

This paper provides a brief overview of the objectives and methodology of the ESA funded project SAMOSA: "Development of SAR Altimetry Studies and Applications over Ocean, Coastal zones and Inland waters", and mainly concentrates on the development of a theoretical model for the mean return echo from a synthetic aperture radar (SAR) altimeter (also know as a delay doppler altimeter or DDA) observations over water surfaces, in the same spirit set by conventional altimeters.

The Envisat RA-2 burst echoes are being gathered throughout the mission; however, these data are only now being made generally available by the European space agency. Considerable work has been necessary to turn these engineering-level data into a useable altimeter product. This paper documents the processing steps undertaken to generate usable data and presents the first extensive analysis of this unique dataset, using over 75 000 burst sequences with a global distribution. The results show that the burst echo data from Envisat are of extremely low noise, which is particularly evident over non-ocean surfaces, and contain a wealth of detailed information from both land and ocean surfaces. Examples illustrate the complexity of surface response from targets such as inland water and rough terrain. These unique data clearly have the potential to inform future instrument design as well as to improve the understanding of existing altimeter datasets.

We have investigated the effect of the radar-altimeter antenna polarization on European Remote-sensing Satellite and Envisat observations of the media penetrable by a radar microwave such as ice sheets. This effect is due to the complex interaction between the radar wave, the subsurface backscatter, and the antenna polarization direction. It is modulated by the angle between the antenna polarization and the direction of the anisotropy of the target. Thus, it depends on both the anisotropy direction and the interaction between the radar wave and the reflecting surface. This effect leads to one of the most complex and least understood errors of radar altimetry over ice sheets and can be clearly identified when looking at the crossover differences between ascending and descending satellite tracks. The crossover differences are as large as a few decibels for a backscattering coefficient and a few meters for height, and affect more strongly the Ku-band than the S-band. This causes limitations and difficulties for the processing of altimetric observations, for instance when comparing time series from different satellites whose polarization geometry differs. This will be the case when a new altimeter will fly on a different orbit, as planned for CryoSat. Nevertheless, the ability of both the roughness anisotropy direction and the subsurface modulation to be inverted with satisfactory precision by using simultaneous observations at crossover points between two different satellites is demonstrated here. Thus, it offers a unique way of describing this error accurately to correct for it

ESA launched its environmental research satellite, ENVISAT, on 1 March 2002. It carries a suite of 10 instruments offering opportunities for a broad range of scientific research and applications. In particular, ENVISAT carries a radar altimeter (RA-2), a microwave radiometer (MWR), and a precise tracking system (DORIS). The evaluation of the quality of the ENVISAT altimetry geophysical data products and the cross-calibration on ERS-2 and other flying altimeters has been performed during the commissioning phase within a team of scientists drawn up from the pre-launch announcement of opportunity. The results obtained from this teamwork are presented here.