Validation of coastal altimetry sea level products against in-situ tide gauge measurements is an essential part of verifying the altimetric sea level observations. The introduction of specialised retrackers, such as Adaptive Leading-Edge SubWaveform (ALES) has improved the estimates of the altimetric sea level close to the coast. One way to assess this improvement is via altimetry-tide gauge comparisons, but this is complicated by the fact that the dominant sea level signals at different tide gauges may have very different length scales. Tide gauges located in regions where signals have relatively long length scales generally have a better agreement with altimetric observations and provide a more accurate assessment of the altimeter's performance. Here, as part of a study conducted within the framework of the ESA Sea Level Climate Change Initiative program, we identify regions of long trend length scales using both the high-resolution NEMO (1/12 degree) global ocean model and XTRACK/ALES atimetry data in order to improve the validation technique for assessing the performance of coastal altimetry products. The tide gauges are grouped according to their decorrelation values. The performance of the coastal altimetry observations is then assessed for each group of tide gauges separately. We report on the methodology and the outcome of these results on three selected regions, North Sea, Mediterranean Sea and West Africa.

We present results of satellite observation and field assessment of internal waves associated with intrusion of freshened water. Studies were conducted in the shelf zone of the eastern part of the Black Sea, adjacent to mouths of the Mzymta, Kodor, Rioni and Chorokh Rivers. During the spring flood, optical characteristics of sea and river waters significantly differ because rivers are loaded with terrigenous matter. It makes them a good tracer of dynamic processes in the coastal zone by satellite data of the visible range. Satellite information was inquired from MSI Sentinel -2A, -2B sensor data; OLCI Sentinel-3A; OLI Landsat-8; and ETM + Landsat 7. We studied the effect of the intensity of river flow during spring flood and after heavy rains on the speed of propagation of the front of freshened water and generation of internal waves. Satellite observations showed that on the border of river plumes, hydrological fronts are developed and emit short-period internal waves moving toward as well parallel to the coast. In spring, the upper quasi-homogeneous warm layer is thin and the thermocline is located close to the surface. This feature of hydrology leaves an imprint on the field of observable internal waves; they move close to the shore. During the autumn period, when river flow is intensified due to heavy rains on some rivers, in particular, on the Kodor and Rioni Rivers, intense generation of internal wave packets occurs directly at the confluence of the river into the sea. In this case, the fronts of internal waves are ring-shaped and propagate from the coast to the open sea.

From April 20 to April 29, 2018, for verification of satellite data, we studied plume characteristics of the Mzymta River from a small vessel using an ADCP RDI WH 300 kHz acoustic current Doppler profiler and a YSI EXO Multiparameter Sonde hydrological probe with additional turbidity and fluorescence sensors. To register the manifestations of internal waves, an moored vertical chain was installed at the outer boundary of the plume based on ten Star Oddi temperature sensors located from the surface to a depth of 35 m. Since 2014, similar experiments have been carried out in the offshore zone of the Black Sea adjacent to mouth of the Kodor River. Both thermistors and ADCP data, in these areas of the coastal zone, intense short-period waves with amplitudes up to 5-6 m and lengths of 40-50 m have widely occur. Satellite observations with high spatial resolution support approximately the same estimations of internal wavelengths.

We compared the manifestations of river plumes on optical and radar images obtained in the studied area with a small time difference. Radar images do not show well defined surface manifestations of internal waves associated with the propagation of the front of freshened water even in the same spatial resolution (10 m) as the data of the visible range. We discuss causes of difference in the appearance of internal waves in satellite images in different bands of the electromagnetic spectrum.

The studies were supported by the Russian Foundation for Basic Research (projects # 17-05-00715 and 17-52-40016 Abkh_a).

The accurate determination of the water surface height from satellite radar altimetry, either in ocean or continental waters, depends on the accuracy of all terms involved in its computation, namely the Wet Tropospheric Correction (WTC) or its symmetric value, the Wet Path Delay (WPD). With a high spatial-temporal variation, WPD is best determined from on-board Microwave Radiometers (MWR) measurements. Satellite altimetry has been used over coastal and inland waters, however these measurements become invalid and cannot be used over these regions. Alternative WPD sources for these zones are the Global Navigation Satellite Systems (GNSS) and the Numerical Weather Models (NWM), e.g. those from European Centre for Medium-Range Weather Forecasts (ECMWF).
Designed for applications over the ocean, altimetric missions are mainly focused on the sea surface. Thus, MWR-derived WPD measurements refer to the sea level, while WPD derived from an NWM are computed at the level of its orography (usually a smoothed representation of a digital elevation model). The corresponding path delays from GNSS are computed at each station height. Due to the differences between these three data types, the modelling of the height dependence of the WPD is a crucial step to combine these different data (MWR, NWM and GNSS) for altimetry application over coastal and inland waters. Over coastal zones, all measurements must refer to the sea level, while over continental waters they must refer to the level of the corresponding water body.
Therefore, an expression to reduce the WPD from GNSS station height and orography level to sea level (over coastal zones) and to water body height (over inland waters) is required. At present, there is an expression for the altitude modelling of the WPD developed by Kouba (2008), however due to the complex 4D variation of the WPD it has some limitations, since it assumes that the altitude dependence of the WPD is the same over the whole globe.
The focus of this analysis is the modelling of the altitude dependence of the WPD, aiming to derive improved expressions to account for its complex 4D variation, required for regions of interest, as coastal and continental waters. For this purpose, the latest reanalysis model from ECMWF, ERA5, which provides hourly atmospheric parameters at 0.25°x0.25° spatial sampling on 137 vertical levels (from the surface up to around 80 km), is used.
This study is performed in two main steps. First, the errors introduced when using the Kouba expression are assessed. This provides a quantification of the magnitude of the errors and their spatial distribution. Using the knowledge acquired in the first step, improved expressions for the vertical variation of the WPD are determined in the second step.
For use in the first step, two types of WPD are derived from ERA5 at a given level of interest: i) from single level (orography) parameters and then reduced to the level of interest with the Kouba expression mentioned above (the so-called 2D approach); ii) from numerical integration of temperature and specific humidity vertical parameters, from the top of atmosphere down to the orography and then interpolated to the level of interest (the so-called 3D approach).
An assessment of the Kouba expression is performed globally at each vertical profile. WPD computed with ERA5 single-layer parameters and then reduced to each level of the vertical profile is compared with WPD from the vertical profile derived from 3D parameters. This comparison shows WPD differences at altitudes lower than 3 km with RMS values larger than 4 cm in some coastal regions of the globe. The origin of these large differences is mainly attributed to errors in the height reduction performed using the Kouba formula.
An independent assessment is performed at GNSS sites by comparing GNSS-derived WPD at station height and those derived from ERA5 using different methodologies (2D and 3D atmospheric parameters) at the same height. Absolute mean and standard deviation values of the WPD differences between GNSS and 3D approach are smaller than the corresponding values of the differences between GNSS and 2D approach. When compared with the 2D approach, the 3D approach can reduce the mean differences between GNSS and ERA5 by a few centimeters.
Secondly, an improved expression was developed in this study (UP expression), from WPD at ERA5 vertical levels, considering regional dependence coefficients. This new expression has errors smaller than the Kouba expression. The maximum RMS for the Kouba expression is larger than 4 cm, while for the UP expression the same value is smaller than 2 cm. In order to establish the best modelling, the analysis also considers temporal dependence of the modelled coefficients and piecewise functions at different altitude intervals. The last step of this study is the validation (using e.g. radiosondes and GNSS data) and the application of the new expressions in regions of interest (coastal and inland waters), thus assessing the impact of the new expressions in the derivation of accurate absolute water levels in these regions.

The latest altimetry missions (Jason-3, Sentinel-3A & 3B) or for the future ones (SWOT) aim to deliver a measurement of the topography at a finer spatial resolution, a higher temporal rate and over heterogeneous surfaces, open ocean but also coastal regions, hydrological targets and over ice and sea-ice. In this perspective, the role played by the wet tropospheric correction (WTC) is critical, due to its large temporal and spatial variability and its crucial weight in the final budget error.

Current retrieval methods are valid over open ocean only, where a model of the emissivity is available. The performances are then degraded wherever the instrumental measurements are contaminated by other surfaces (land, ice, sea-ice); solutions exist to correct for this contamination but will not be able to satisfy the future constraints on the retrieval errors over coastal regions or hydrological surfaces.

We present here a method based from the one-dimensional variational (1D-VAR) approach to retrieve the WTC at the scale of a high resolution numerical weather prediction (NWP) model.

The classical approach for the 1DVAR consists on the optimization of a unique triplet of atmospheric profiles (temperature, humidity, cloud liquid water content) that best explains the brightness temperatures observations provided by the radiometer.

In the frame of a study funded by CNES, strategies are defined to apply multiple retrievals for each grid cells of a high-resolution NWP below the surface covered by the MWR field of view. The strategy differs whether the observations are performed over open ocean or over coastal areas. For this latter case, emissivity atlas allow to decontaminate the observations from the land contamination which paves the way to a better accuracy of the WTC over transitional regions (ocean/land, ocean/sea ice) and hydrological targets.

A test case is defined for the Sentinel-3A altimetry mission using the 1.3 km scale of AROME.

The downscaled WTC is compared to the reference GDR product, specific product as Fernandes GPD+, in-situ measurements or coincident alternative observations (water vapour from SLSTR or OLCI on-board Sentinel-3A). The improvement of the altimetry system, especially with the altimeter SAR-mode, is assessed using traditional cal/val metrics.

Multibistatic radar systems for enhanced observation of the ocean
Davide Comite and Nazzareno Pierdicca
Department of Information Engineering, Electronics and Telecommunications, Sapienza University of Rome, Italy.

In the last decade, there has been increasing interest in the design of companion satellite missions enhancing interferometric applications and enabling tomographic capabilities of established monostatic SAR systems. The introduction of a passive receiver aboard of small satellites, flying in formation with an active one to constitute a bistatic synthetic aperture radar (SAR), has been the object of broad research and industrial activities [1], [2]. Bistatic systems made by large baselines, achievable by means of so-called formation flying, have been analysed and considered as most suitable to provide the desired performance [2], [3]. On the other hand, passive SAR systems have been demonstrated as a valuable method to achieve high-resolution estimation of wind fields in coastal regions (see, e.g., [5], [6] and refs. therein). In this frame, enhancing the capabilities of conventional scatterometers for mapping the wind in coastal regions is highly desirable.
In this frame, our research activity, mainly developed in the framework of the SAOCOM-CS satellite mission (a small satellite under design by ESA to be associated with the Argentinian SAOCOM 1B satellite [1]), is devoted to the understanding of the potential improvement provided by the use of arbitrary bistatic geometries, i.e., over variable along-track (AT) and cross-track baselines, determined by the introduction of one or more passive systems associated to the active one. The outcome of our research can be of specific interest in the frame of the scientific activity for the design of future bistatic spatial missions, such as SESAME passive radar [3] (presently STEROID), which comprises two-receive only C-band SAR satellites flying in formation with the active system of the European Sentinel-1 satellite.
We have considered state-of-art electromagnetic modelling to analyse L and C-band co- and cross-polarized bistatic normalized radar cross section for the analysis of the bistatic scattering from the rough ocean under different conditions and illuminations [6]. A detailed analysis on the capabilities of both multistatic (i.e., one transmitter and two receivers, one of the two being in the same position of the transmitter) and multibistatic (i.e., one transmitter and n receivers) observations of estimating wind fields over the ocean have been carried out. With this approach, valuable information and some general guidelines can be provided for the design of future bistatic missions, enabling real aperture methods and, more importantly enhanced resolution given by synthetic aperture implementations. The more promising configuration parameters (i.e., direction of observation, polarization, and number and positions of the passive receivers) have been analyzed comparing the expected estimation accuracy with that provided by a fixed-beam monostatic system equipped with more antennas. For each of the considered cases, the expected lower bound of the error standard deviation of the estimation has been quantified by means of the Cramer-Rao lower bound.
Overall, the results indicate that the estimation accuracy of the wind field provided by multibistatic radar observations can achieve good performance implementing large baseline satellite constellations. Particular attention has been devoted to configurations involving one or two passive satellites aligned along the direction defined by the active system, which can be of great interest for the assessment of the performance provided by future bistatic SAR missions for the coastal observation of the ocean. We have found that, by designing a configuration involving two passive receivers on the same orbital plane of the active system, following and preceding the active, with AT baseline of the order of 250-300 km, the wind speed (in the range 5-25 m/s) retrieval accuracy can be better of that one of a monostatic radar performing three backscattering measurements, especially for wind directions where the monostatic system presents poor performance [6].
Other investigations are in progress contributing to the assessment of the unique opportunities and the scientific potential enabled by bistatic companion satellite configurations.

References

[1] Scipal, K. and Davidson, M. The SAOCOM-CS mission: ESA's first bistatic and tomographic L-band mission. In Proc. IEEE International Geoscience and Remote Sensing Symposium (IGARSS), pp. 123-124, July 2017.
[2] López-Dekker, P., Prats, P., Rodríguez-Cassola, M. and Domínguez, B. C. Companion SAR missions: Scientific rationale and technical challenges. In Proc. IEEE International Geoscience and Remote Sensing Symposium (IGARSS), pp. 141-144, July 2017.
[3] Rott, H., López-Dekker, P., Solberg, S., Ulander, L., Nagler, T., Krieger, G., Prats, P., Rodriguez, M., Zonno, M. and Moreira, A. SESAME: A single-pass interferometric Sentinel-1 companion SAR mission for monitoring GEO-and biosphere dynamics. In Proc. IEEE International Geoscience and Remote Sensing Symposium (IGARSS), pp. 107-110, July 2017.
[4] Zecchetto, S., De Biasio, F., della Valle, A., Quattrocchi, G., Cadau, E. and Cucco, A. Wind fields from C-and X-band SAR images at VV polarization in coastal area (Gulf of Oristano, Italy). IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 9, no. 6, pp.2643-2650, June 2016.
[5] Martin, A.C., Gommenginger, C.P. and Quilfen, Y. Simultaneous ocean surface current and wind vectors retrieval with squinted SAR interferometry: Geophysical inversion and performance assessment. Remote Sensing of Environment, vol. 216, Oct. 2018.
[6] Comite, D., and Pierdicca, N., Bistatic Radar Systems at Large Baselines for Ocean Observation. IEEE Transactions on Geoscience and Remote Sensing, vol. 56, no. 3, pp.1816-1828, Mar. 2018.

Coastal areas are key areas for sustainable economy, biodiversity protection and population development. Nonetheless, they are continuously and rapidly changing due to urbanization, deforestation, erosions, etc. Natural hazards as earthquakes, landslides and floods also threaten coastal stability and safety. Hence, a continuous, accurate and updated monitoring of coastal areas and the time/space evolution of coastal profiles is needed. It was proven that remote sensing tool as Unmanned Aerial Vehicle (UAV) and Synthetic Aperture Radar (SAR) systems are valuable source of information that allow coastal monitoring in order to support local authorities in the management of coastal risk [1-4].
In this study, the time/space coastal evolution of the embayed natural beaches of Serapo and S. Agostino located along the Tyrrhenian coast of Italy (Lazio region) is analyzed in order to achieve reliable vulnerability assessment. Attention must be paid on those microtidal beaches since, in recent years, they were affected by intense erosion/accretion processes that non-uniformly affected the shoreline. Beach rotation due to both cross-shore sediment exchange and along-shore sediment exchange between the beaches’ extremities was found to be a response to natural forcing from waves of various incident directions [5-6].
The dataset consists of a time-series of 17 C-band Sentinel-1 SAR satellite scenes collected in dual-polarimetric VH/VV Interferometric Wide swath mode over the study area in the period 2014- 2018. The coastline extraction algorithm adopted for the analysis takes full benefits of the polarimetric information provided by Sentinel-1 SAR imagery and is characterized by satisfactory performance in terms of both accuracy and computational effort [7]. Additional information on sea state provided by the national buoys network, topographic surveys obtained from aerial UAV Differential Global Positioning Systems (DGPS) operating in Real-Time Kinematic (RTK) mode and ground coastline measurements collected during an in-situ GPS campaign are also exploited for the analysis.

[1] F. Nunziata, M. Migliaccio, X. Li and X. Ding, “Coastline extraction using dual-polarimetric COSMO-Skymed Pingpong mode SAR data,” IEEE Geosci. Remote Sens. Lett., vol. 11, no. 1, pp. 104–108, 2014.

[2] A. Buono, F. Nunziata, L. Mascolo and M. Migliaccio, “A multipolarization analysis of coastline extraction using X-band COSMO-Skymed SAR data,” IEEE J. Sel. Top. Appl. Earth Obs. Remote Sens., vol. 7, no. 7, pp. 2811–2820, 2014.

[3] G. Benassai, D. Di Luccio, V. Corcione, F. Nunziata and M. Migliaccio, “Marine spatial planning using high-resolution synthetic aperture radar measurements,” IEEE J. Ocean. Eng., 2018.

[4] Y. Matsuba and S. Sato, “Nearshore bathymetry estimation uinsg UAV”, Int. J. Remote Sens., vol. 60, no. 1, pp. 51-59, 2018.

[5] A. H. da Fontoura Klein, L. Benedet Filho and D. H. Schumacher, “Short-term beach rotation processes in distinct headland bay beach systems,” J. Coast. Res., pp. 442–458, 2002.

[6] E. Ojeda and J. Guillèn, “Shoreline dynamics and beach rotation of artificial embayed beaches,” Mar. Geol., vol. 253, no. 1-2, pp. 51– 62, 2008.

[7] F. Nunziata, A. Buono, M. Migliaccio and G. Benassai, “Dual-polarimetric C-and X-band SAR data for coastline extraction,” IEEE J. Sel. Topics Appl. Earth Obs.. Remote Sens., vol. 9, pp. 4921–4928, 2016.