Precise inland surface altimetry (PISA) with nadir specular echoes from Sentinel-3: Algorithm and performance assessment

Abstract In the recent years satellite radar altimetry has evolved from pulse-limited low resolution mode (LRM) to a synthetic aperture radar (SAR) high resolution mode. The SAR mode focusses and coherently sums all radar return echoes within the 2-s time window the target surface area is in the antenna beam. In principle the SAR processing improves along-track resolution. Land contamination has been a major concern for inland waters altimetry and SAR can reduce land interference. This paper shows that the physics of specular echoes from smooth inland waters leads to a very different approach which we call precise inland surface altimetry (PISA). PISA uses only echoes within the specular "flash" period, which is approximately the time the satellite nadir crosses over the water body. The processing is four orders of magnitude less than SAR. Land interference is negligible because specular water echoes are usually >50 dB greater than land. Sentinel-3 SRAL dataset on the salar de Uyuni (Bolivia) is used to evaluate PISA ranging precision. During inundation (wet months), echoes are at the theoretical maximum radar cross section (RCS), σ = 129 dBsm, and ranging precision is ~1 mm. In dry months the echoes are quasi-specular, with σ = 70–100 dBsm, and ranging precision is ~1 cm. The precision assessment is made with variate-differences, with pass-to-pass repeatability, and by comparison with GPS measurements. In addition to the salar de Uyuni analysis we gathered σ statistics on five millions Sentinel-3 SRAL Ku-band altimeter bursts (one burst = 64 contiguous echoes) from 52 passes of Sentinel-3A track 167 over South America. We illustrate specular and quasi-specular waters on lakes, a river, and a fjord. Ranging precision is similar to Uyuni, in the 1 mm-1 cm range. Water surface slopes of 1–3 cm/km are detected. We propose a simple rule-set to distinguish specular waveforms (σ >100 dBsm, sidelobes (with Hamming window) of −37 dB or lower) and quasi-specular (σ >70 dBsm, sidelobes lower than −20 dB), and non-specular (sidelobes> − 20 dB). PISA is appropriate to specular and quasi-specular echoes

Coherent ranging with Envisat radar altimeter: a new perspective in analyzing altimeter data using Doppler Processing

ESA's Envisat mission carried a RA-2 radar altimeter since its launch in 2002 to sense sea state and especially measure sea surface height (SSH). The onboard processing combined multiple echoes incoherently to reduce Speckle noise and benefit from data compression. In fact, according to past literature the amplitudes were generally expected to be independent. Nevertheless, samples of complex data time series of individual echoes (IE) were down-linked and archived since 2004 for research studies. In this note we demonstrate that there is sufficient inter-pulse coherence for Doppler processing and we suggest that the archived data can be re-processed into improved SSH. This is of particular interest in challenging domains (e.g., coastal zone) where coherent processing can mitigate errors from ocean surface backscatter inhomogeneity and nearby land backscatter. A new method called zero-Doppler to process IEs is thus proposed and discussed

ALTICORE: an initiative for coastal altimetry

ALTICORE (value-added ALTImetry for COastal REgions) is an international initiative whose main objective is to encourage the operational use of altimetry over coastal areas, by improving the quality and availability of coastal altimetry data. The ALTICORE proposal has recently been submitted for funding to the INTAS scheme (www.intas.be) by a consortium of partners from Italy, France, UK, Russia and Azerbaijan. ALTICORE is also meant as a contribution to the ongoing International Altimeter Service effort. In this work we will describe the anticipated project stages, namely: 1) improvement of the most widely distributed, 1 Hz, data by analyzing the corrective terms and providing the best solutions, including those derived from appropriate local modelling; 2) development of a set of algorithms to automate quality control and gap-filling functions for the coastal regions; 3) development of testing strategies to ensure a thorough validation of the data. The improved products will be delivered to ALTICORE users via Grid-compliant technology; this makes it easier to integrate the local data holdings, allows access from a range of services, e.g. directly into model assimilation or GIS systems and should therefore facilitate a widespread and complete assessment of the 1Hz data performance and limitations. We will also outline the design and implementation of the Grid-compliant system for efficient access to distributed archives of data; this consists of regional data centres, each having primary responsibility for regional archives, local corrections and quality control, and operating a set of web-services allowing access to the full functionality of data extraction. We will conclude by discussing a follow-on phase of the project; this will investigate further improvements on the processing strategy, including the use of higher frequency (10 or 20 Hz) data. Phenomena happen at smaller spatial scales near the coast, so this approach is necessary to match the required resolution. The whole project will hopefully promote the 15-year sea surface height from altimetry to the rank of operational record for the coastal areas

Genotyping of the G1138A mutation of the FGFR3 gene in patients with achondroplasia using high-resolution melting analysis

[[abstract]]Objectives: The fibroblast growth factor receptor 3 gene (FGFR3) plays a critical role in cartilage growth-plate differentiation and bony development. It has been shown that 97% of patients with achondroplasia have a G to A transition mutation at position 1138 (c.1138 G>A) of codon 380 of the FGFR3 gene. Design and methods: Exon 8 of the FGFR3 gene was analyzed in 40 patients with achondroplasia, as well as in 50 control individuals for the presence of the c.1138G>A variant using melting curve analysis with a high-resolution melting instrument (HR-1). Results: The high-resolution melting curve analysis successfully genotyped the c.1138G>A mutation in exon 8 of the FGFR3 gene in all 40 patients with achondroplasia without the need of further assays. The technique had a sensitivity and specificity of 100%. Conclusion: High-resolution melting analysis is a simple, rapid, and sensitive one tube assay for genotyping the FGFR3 gene. The technique is a low cost high-throughput FGFR3 screening assay. (c) 2007 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved

Comparison of the X-TRACK altimetry estimated currents with moored ADCP and HF radar observations on the West Florida Shelf

The performance of coastal altimetry over a wide continental shelf is assessed using multiple-year ocean current observations by moored Acoustic Doppler Current Profilers (ADCP) and high frequency (HF) radar on the West Florida Shelf. Across track, surface geostrophic velocity anomalies, derived from the XTRACK along-track sea level anomalies are compared with the near surface current vector components from moored ADCP observations at mid shelf. The altimeter derived velocity anomalies are also directly compared with the HF radar surface current vector radial components that are aligned perpendicular to the satellite track. Preliminary results indicate the potential usefulness of the along-track altimetry data in contributing to descriptions of the surface circulation on the West Florida Shelf and the challenges of such applications. On subtidal time scales, the root mean square difference (rmsd) between the estimated and the observed near surface velocity component anomalies is 8 11 cms-1, which is about the same magnitude as the standard deviations of the velocity components themselves. Adding a wind-driven Ekman velocity component generally helps to reduce the rmsd values

Validation of Sentinel-3a Sral Coastal Sea Level Data at High Posting Rate: 80Hz

Altimetry data of two and a half years (June 2016-November 2018) of Sentinel 3A SRAL were validated at the sampling frequency of 80 Hz. The study areas are three coastal sites in Spain: Huelva (HU) (Gulf of Cadiz), Barcelona (BA), and Bilbao (BI). Two tracks were selected in each site: one ascending and one descending. Data were validated using in situ tide gauge (TG) data provided by the Spanish Puertos del Estado. In the 5 to 20 km segment, the results were 6-8 cm (rmse) and 0.7-0.8 (r) for all the tracks. The 0 to 5 km segment was also analyzed in detail to study the land effect on the altimetry data quality. The results showed that the track orientation, the angle of intersection with the coast, and the land topography concur to determine the nearest distance to the coast at which the data retain a similar level of accuracy than in the 5 to 20 km segment. This distance of good quality to shore reaches a minimum of 3 km for the tracks at HU and the descending track at BA

Coastal altimetry products in the strait of Gibraltar

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official SGDRs and the outputs of a coastal waveform retracker, the ALES retracker; in addition, SLAs at 1 Hz were obtained from CTOH, France. For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES

Modeling Envisat RA-2 waveforms in the coastal zone: Case study of calm water contamination

This letter examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10-km 2 island 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

Wind-induced Cross-Strait Sea Level Variability in the Strait of Gibraltar using Coastal Altimetry and In-Situ Measurements

In this work, we retracked altimeter waveforms of ESA satellites. ERS2 RA and Envisat RA2 from descending track 0360 over the eastern side of the Strait of Gibraltar using the Adaptive Leading Edge Subwaveform ALES retracker