130 research outputs found
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), Ï = 129 dBsm, and ranging precision is ~1 mm. In dry months the echoes are quasi-specular, with Ï = 70â100 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 Ï 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 1â3 cm/km are detected. We propose a simple rule-set to distinguish specular waveforms (Ï >100 dBsm, sidelobes (with Hamming window) of â37 dB or lower) and quasi-specular (Ï >70 dBsm, sidelobes lower than â20 dB), and non-specular (sidelobes> â 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
The Potential Use of SARAL ALTIKA Towards CoastalRegions - A Case Study on East Coast of India
Altimeter data provides a precise measurement of wave parameters especially over open ocean. Now, several attempts were made to retrieve wind and wave information towards coastal regions. SARAL/Altika is a first of its kind to operate at such a high frequency to provide better observation of coastal regions. The present study is especially important for the Indian coasts because most of the in-situ observations are either coastally located or at deep water
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