18 research outputs found
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
Validación con datos in-situ de alturas de ola obtenidas mediante radar altimétrico
Los rádares altimétricos a bordo de satélites artificiales, han sido diseñados para dar información precisa de la altura del nivel del mar, la altura de ola significante y la velocidad del viento sobre la superficie del mar. En los estudios, tanto anteriores como actuales, se observan problemas en las regiones costeras, donde las medidas del altímetro tienen menor precisión y una mayor dificultad a la hora de interpretar estos datos. Estos inconvenientes son debidos a dos factores principales. En primer lugar, la contaminación de la señal radar debido a la cercanía de la costa. En segundo lugar, inexactitudes en las correcciones de marea y troposférica húmeda. A estos problemas se añade la complejidad de la zona de estudio, al ser una región con una amplia gama de procesos hidrodinámicos con diversas escalas espacio-temporales. Tener acceso a información exacta en la costa, con las condiciones que conlleva, tiene una gran importancia debido al enorme interés económico-estratégico de la zona litoral. Este interés hace que surjan nuevas estrategias para generar productos altimétricos optimizados para tales condiciones.
Las medidas de la altura de ola significante y su variabilidad en las áreas costeras son usadas para muchos propósitos (por ejemplo, análisis del transporte de sedimento, setup de la ola y tormentas de marea), y para la validación/calibración de modelos (pronóstico de oleaje, circulación oceánica). Estas aplicaciones sirven para un amplio rango de propósitos sociales relevantes, tales como el diseño de estructuras de ingeniería en alta mar, la protección de las zonas costeras, rutas para los buques y la planificación de las operaciones en el océano. Muchos estudios se han dedicado a la validación de los datos de la altura de ola significante dados por el radar altimétrico a bordo de satélites artificiales, utilizando observaciones in-situ.
El objetivo de esta ponencia es exponer una metodología para validar los datos de altura de oleaje proporcionados por el radar altimétrico RA-2 a bordo del satélite ENVISAT, a partir de datos in-situ medidos por una boya, y su aplicación a un caso particular
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
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
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
Variability in storm climate along the Gulf of Cadiz: the role of large scale atmospheric forcing and implications to coastal hazards
In the context of increased coastal hazards due
to variability in storminess patterns, the danger of coastal
damages and/or morphological changes is related to the
sum of sea level conditions, storm surge, maximum wave
height and run up values. In order to better understand the
physical processes that cause the variability of the above
parameters a 44 years reanalysis record (HIPOCAS) was
used. The HIPOCAS time-series was validated with real
wave and sea-level data using linear and vector correlation
methods. In the present work changes in the magnitude,
duration, frequency and approach direction of the Atlantic
storms over the Gulf of Cadiz (SW Iberian Peninsula)
were identified by computing various storm characteristics
such as maximum wave height, total energy per storm wave
direction and storm duration. The obtained time-series were
compared with large-scale atmospheric indices such as the
North Atlantic Oscillation (NAO) and the East Atlantic pattern.
The results show a good correlation between negative
NAO values and increased storminess over the entire Gulf of Cadiz. Furthermore, negative NAO values were correlated
with high residual sea level values. Finally, a joint
probability analysis of storm and sea level analysis resulted
in increased probabilities of the two events happening at
the same time indicating higher vulnerability of the coast
and increased coastal risks. The above results were compared
with coastal inundation events that took place over
the last winter seasons in the province of Cadiz.info:eu-repo/semantics/publishedVersio
NSC32251
For early satellite altimeters, the retrieval of geophysical information (e.g., range, significant wave height) from altimeter ocean waveforms was performed on board the satellite, but this was restricted by computational constraints that limited how much processing could be performed. Today, ground-based retracking of averaged waveforms transmitted to the earth is less restrictive, especially with respect to assumptions about the statistics of ocean waves. In this paper, a theoretical maximum likelihood estimation (MLE) ocean waveform retracker is applied to the Envisat Radar Altimeter system (RA-2) 18-Hz averaged waveforms under both linear (Gaussian) and nonlinear ocean wave statistics assumptions, to determine whether ocean wave skewness can be sensibly retrieved from Envisat RA-2 waveforms. Results from the MLE retracker used in nonlinear mode provide the first estimates of global ocean wave skewness based on RA-2 Envisat averaged waveforms. These results show for the first time geographically coherent skewness fields and confirm the notion that large values of skewness occur primarily in regions of large significant wave height. Results from the MLE retracker run in linear and nonlinear modes are compared with each other and with the RA-2 Level 2 Sensor Geophysical Data Records (SGDR) products to evaluate the impact of retrieving skewness on other geophysical parameters. Good agreement is obtained between the linear and nonlinear MLE results for both significant wave height and epoch (range), except in areas of high-wave-height conditions.<br/