Coastal Altimetry

Coastal altimetry has achieved important milestones in the recent years. Recent studies revealed that pulse-limited missions, if reprocessed with dedicated algorithms, can provide reliable data up to few km from the coastline. In particular, the ALES retracking algorithm has been validated and applied successfully to sea level research, demonstrating the ability to increase the quality and the quantity of sea level retrievals in coastal areas.

Since more than four years, Cryosat-2 is flying with the first altimeter to allow for Delay-Doppler processing of the echoes ("SAR" altimetry) in parts of the global ocean. This new technique should bring better coastal performances thanks to a reduced along-track footprint and a higher precision.

This study aims at cross-calibrating the first SAR-altimetry dataset from Cryosat-2 (SARvatore) with the ALES-reprocessed Envisat data, in order to test the benefits of the Delay-Doppler processing and to expand the Envisat time series in the coastal ocean. The Indonesian seas are chosen as test areas, since the availability of altimetry data in this region is particularly beneficial due to the lack of in-situ measurements and the importance for the global ocean circulation.

The analysis of the sea level estimations in the Indonesian Seas demonstrates that Cryosat-2 is able to decrease by 0.3 cm the 1-Hz noise of sea level estimations within 50 km of the coast, when compared to the ALES-reprocessed Envisat dataset. 

The cross-calibration at the crossover points proves that in the region of study a sea state bias correction equal to 5\% of the significant wave height is an acceptable approximation for SAR altimetry.

The study of the sea level time series reveals the geographic extent of the semiannual signal caused by the Kelvin waves during the monsoon transitions, the larger amplitudes of the annual signal due to the Java Coastal Current and the impact of the strong La Niña event of 2010 on the rising sea level trends.

With the objective to ensure the complementarity but also the continuity with the SARAL/AltiKa Level-2 products provided in the open ocean, the PEACHI (Prototype for Expertise on AltiKa for Coastal, Hydrology and Ice) project has been set up as an initiative of the French space agency, CNES. The PEACHI prototype is designed to process and fine tune enhanced algorithms dedicated to the assessment of Ka-band parameters, from the instrument processing to geophysical corrections. As a result, end users are routinely provided with the latest available altimeter corrections for scientific applications.

Complementary to the last update of SARAL/AltiKa processing software and the dissemination of the operational Level-2 products, some new or improved algorithms have been developed in the framework of the PEACHI project to better observe the open ocean and achieve SARAL secondary objectives on the study of coastal dynamics, inland waters, polar oceans, or continental and sea ice. The purpose of this work is to provide a global status of the PEACHI project and the reprocessing performed in 2015. We focus on a handful of key algorithm improvements with regard to the operational GDR (Geophysical Data Record) products: new waveform retrackers improving performances over ocean, continental and sea ice, improved 2D and new 3D sea state biases, new tide models, better altimeter wind correction, new editing process over rain and bloom areas or sea ice regions.

We would like to foster all current and prospective users to provide us with independent assessment and feedback in order to determine which parameters should be transferred into the classical GDR, or applied to other missions. Feedback would also be appreciated to define possible innovative algorithms and studies that should be added to future PEACHI versions, keeping in mind the project goal: deliver a long, rich and consistent time series of demonstrative algorithms.

The ocean's mean dynamic topography (MDT) is the surface representation of ocean circulation. It may be determined by the ocean approach, using numerical ocean circulation models, or by the geodetic approach, where MDT is the height of the mean sea surface (MSS), or mean sea level (MSL), above the geoid. Using new geoid models, geodetic MDT profiles based on tide gauges, dedicated coastal altimetry products, and conventional altimetry are compared with six ocean MDT estimates independent of geodetic data.
Emphasis is put on the determination of high-resolution geoid models, combining ESA's fifth release (R5) of GOCE satellite-only global gravity models (GGMs) with a regional geoid model for Norway by a filtering technique.
Differences between MDT profiles along the Norwegian coast together with Taylor diagrams confirm that geodetic and ocean MDTs agree on the ~3-7 cm level at the tide gauges, and on the ~5-11 cm level at the altimetry sites. Some geodetic MDTs correlate more with the best-performing ocean MDT than do other ocean MDTs, suggesting a convergence of the methods. While the GOCE R5 geoids are shown to be more accurate over land, they do not necessarily show the best agreement over the ocean. Pointwise monomission altimetry products give results comparable with the multimission DTU13MSS grid on the ~5 cm level. However, dedicated coastal altimetry products generally do not offer an improvement over conventional altimetry along the Norwegian coast.

Unlike previous altimetric missions, the CryoSat-2 altimeter (SIRAL) features a novel Synthetic Aperture Radar (SAR) mode that allows higher resolution and more accurate altimeter-derived parameters in the coastal zone, thanks to the reduced along-track footprint. The scope of this study is a regional analysis and inter-comparison of the CryoSat-2 SAR altimeter products against in-situ data and regional model results at distances to coast smaller than 10 km. The in-situ data are from a network of tide gauges and GNSS stations. The validated geophysical altimeter parameters are the sea surface height above the ellipsoid (SSH), the significant sea wave height (SWH) and wind speed (U10), all estimated at 20 Hz.

We have carried out, from CryoSat-2 FBR (L1a) data, a Delay-Doppler processing and waveform retracking tailored specifically for coastal zone by applying Hamming Window and Zero-Padding, using an extended vertical swath window in order to minimize tracker errors and a dedicated SAMOSA-based coastal retracker (named SAMOSA+). SAMOSA+ accepts mean square slope as free parameter and the epoch’s first guess fitting value is decided according to the peak in correlation between 20 consecutive waveforms (in order to reduce land off-ranging effect).

Since the highest remaining uncertainties in the altimeter parameters derived in coastal shallow waters arise from residual errors in the applied corrections we use regional ocean tide and high resolution geoid and mean sea surface models (as TPXO8 for tides, EGM 2008 or EIGEN-6C4 for the geoid and DTU13 for the mean sea surface). We also apply a regional improved wet tropospheric correction computed from the GNSS-derived Path Delay Plus (GPD+) algorithm at the University of Porto. Hence for the in-situ validation, errors in corrections are expected to contribute less then in previous analysis on sea level differences measured by altimeter and in-situ data.

In parallel with SAR measurements, in order to quantify the improvement with respect to pulse-limited altimetry, we build 20 Hz PLRM (pseudo-LRM) data from FBR and retrack them with numerical convolutional Brown-based retracker. Hence, here, PLRM is used as a proxy for real pulse-limited products (LRM), since there is no direct comparison of SAR and LRM possible otherwise. The L2 SAR ocean data products are generated and extracted from ESA-ESRIN GPOD service (named SARvatore) while the PLRM data are built and retracked by Technical University of Darmstadt (TUDa). The region of interest is the German Bight and West Baltic Sea (being a very challenging area for radar altimetry due to its complex coastal morphology and its high tide dynamics) while the time of interest is the complete the mission duration (5 years).

The analysis exploits both geometric parameters, as the distance-to-coast parameter and the sea floor bathymetry with resolution of 300 m (from the MERIS water mask and TPXO8 Atlas) and waveform quality parameters, as the misfit between the SAMOSA model waveform and the received echo, the waveform entropy (an high value of waveform entropy is an index of land contamination) and the equivalent number of Looks (ENL, a very low value of ENL is an index of  heavy data dispersion and hence land contamination).

Considering the almost five year long analysis, the final objective is to verify the ability of SAR Altimetry to measure accurately in coastal zone the sea level annual cycle and the sea level trend.

he wet tropospheric correction (WTC) is a major source of uncertainty in altimetry budget error, due to its large spatial and temporal variability: this is why the main altimetry missions include a microwave radiometer (MR) The commonly agreed requirement on WTC for current missions is to retrieve WTC with an error better than 1cm rms.

With the introduction of the along-track synthetic aperture processing, first implemented in CryoSat-2, and now in the upcoming operational altimetry missions such as Sentinel-3 and Jason-CS, more accurate altimetry data are anticipated for coastal and inland waters. Nevertheless, the quality of data in those areas are expected to be degraded with respect to those of the open oceans due to the rather wide field of view of the MR (-3 dB beam-width of ~20 km). As a matter of fact, the MR observations over those waters are subject to contamination by land brightness temperatures which fall within the MR footprint.

The present team has been selected by ESA/ESTEC to work on a MR instrument design for future operational radar altimetry missions. Such a design shall include the classical MR channels for ensuring observation continuity, augmented by a set of high frequency channels for enabling accurate altimetry over coastal and inland waters.

In this study team, the extensive systems and radiometric engineering experience of JCR Systems is complemented by a significant expertise of LOCEAN, CLS and CNRM in water vapour retrievals, a considerable experience in design and development of microwave and millimetre wave radiometer front ends from RAL and a substantial knowledge of SMT Consultancies in microwave and millimetre-wave antenna design.

We will present the results of the study, covering design aspects, requirements on accuracy, sensitivity, long and short-term stability, spatial resolution and pointing but also optimal processing of the observations, based on a 1D-var approach for the wet tropospheric correction. Expected performances will be presented as well.