Analysis area selection
The sentinels are a series of 5 earth observation missions of ESA satellites. Sentinels are developed on behalf of the joint ESA/European Commission initiative GMES (Global Monitoring for Environment and Security). They aim at providing robust datasets for Copernicus Services.
Sentinel-1 mission is a constellation of two polar-orbiting satellites (S-1 A & S-1 B) performing C-band synthetic Aperture Radar. Sentinel-1 provides data routinely and systematically for maritime and land monitoring, emergency response, climate change and security.
Sentinel-1 A satellite was launched on a Soyuz rocket from Europe's Spaceport in French Guiana on 3 April 2014.
Sentinel-1 carries a single C-band synthetic aperture radar instrument (C-SAR) operating at C-Band (centre frequency of 5.405 GHz). It includes a right-looking active phased array antenna providing fast scanning in elevation and azimuth for TOPS mode. The C-SAR instrument supports operation in dual polarisation : HH+HV, VV+VH. Sentinel-1 operates in four exclusive modes:
- StripMap (SM)
- Interferometric Wide Swath (IW)
- Extra-Wide swath (EW)
- Wave mode (WV)
ESA PDGS processes routinely Sentinel-1 L0 products up to geophysical L2 products: The Level-2 Ocean (OCN) products. This product is composed by 3 sub-products:
- Ocean Wind field (OWI)
- Ocean Swell Spectra (OSW)
- Surface Radial Velocity (RVL)
Data referring to OWI are derived from an internally processed GRD product. Data referring to RVL (and OSW, for SM and WV mode) are derived from an internally processed SLC product. These internal products are not available.
EW and IW do not have OSW component.
Here we focus on Wave mode. WV is built up on the experience gained with the ERS-1/2 active microwave instrument (AMI) and envisat/ASAR to continue and extend open ocean observation with SAR .
Wave mode acquisitions are composed of stripmap imagettes of 20 km by 20 km at 5 m by 5 m resolution, acquired alternately on two different incidence angles (23° and 36.5°). Wave imagettes are acquired every 100 km, with imagettes on the same incidence angle separated by 200 km. Polarization can be either VV or HH. VV is the default polarization mode.
Wave mode imagettes are acquired only over open ocean and aim at providing continuous global directional information on wave field.
Our activity intends to
- Perform a validation of these products
- Provide an upgrade of the algorithms (if required)
- Perform a long term quality assessment.
Validation the Level-2 Ocean products from WV is funded by ESA and performed in the framework of the Mission Performance Center for Sentinel-1 (MPC S-1) project (4000107360/12/I-LG). Data are obtained directly through the MPC S-1. The MPC-S1 is led by CLS.
The main Geophysical parameters to be validated are the :
- wind speed and direction,
- significant wave height of the waves detected by Sentinel-1,
- the main parameters (wavelength, direction and significant wave height) of the different swell systems
- the radial component of the ocean surface velocities.
The validation of geophysical parameters relies on comparisons with reference data that can be split in three main categories:
Model data: Global models are available everywhere with time step of 3 hours. They allow co-locations with all acquired products in Wave mode. Thus, they can be used for statistics based on large dataset. Typically one month of data with global acquisition is able to get a first estimated of the performances. However for case studies they may be insufficient as model cannot always capture the complexity of all situations. ECMWF with a 0.125° spatial resolution is used for ocean surface wind. Wave Watch 3 (WW3) with a 0.5° resolution is used for wave validation and operated at Ifremer by IOWAGA group. An ocean wave spectrum is produced for each Sentinel-1 imagette.
In-situ data: in-situ data are used to get reference measurements for wind, ocean wave spectra and ocean surface current. The limitation is the coverage and a good quality control.
Satellite data: They allow to check the consistency between different sensors measuring the same geophysical parameters. For ocean surface wind, radiometer, scatterometer or altimeter wind can be used. Altimeter also provides good estimate of significant wave height for wave validation and the geostrophic component of the ocean surface current for radial velcoities validation.
In addition we also investigate and monitor radar quantities such as Doppler, Normalized radar cross-section, Normalized variance of the image intensity, signal to noise ratio. They are good indicators to check if the sensor is operating nominally to produce good L2 OCN products. Indeed these radar parameters are used as input of the L2 processing.
Copernicus Program Overview, for general context of the Sentinel missions
Sentinel-1 User Guide for details about Sentine-1 platform, C-SAR instrument and the product family
Integrated Ocean Waves for Geophsyical and other Applications (IOWAGA) website, for details about the WW3 wave model we are using for waves quality assessment. Runs are provided by IOWAGA.
European Center for Medium-range Weather Forecast, for details about the global atmospheric model we are using for wind quality assessment in open ocean.
- OSW Algorithm Theoretical Basis Document
- OWI Algorithm Theoretical Basis Document
- RVL Algorithm Theoretical Basis Document
Publications on Sentinel-1 from the team
Ardhuin Fabrice, Collard Fabrice, Chapron Bertrand, Girard-Ardhuin Fanny, Guitton Gilles, Mouche Alexis, Stopa Justin (2015).
Estimates of ocean wave heights and attenuation in sea ice using the SAR wave mode on Sentinel-1A.
Geophysical Research Letters, 42(7), 2317-2325. http://dx.doi.org/10.1002/2014GL062940
Mouche Alexis and Chapron Bertrand,
Global C-Band Envisat, RADARSAT-2 and Sentinel-1 SAR 1 measurements in co- and cross-polarization.
Accepted for Publications in JGR-Oceans