Signal processing simulations of the SWIM wave data

Céline Tison (CNES, France)


Danièle Hauser (CNRS INSU LATMOS, France); Lauriane Delaye (CNRS INSU LATMOS, France); Thierry Koleck (CNES, France); Milena Planells (CNES, France); Nicolas Lamquin (ACRI, France); Lofti Aouf (Météo France, France); Patrick Castillan (CNES, France)

Event: 2014 Ocean Surface Topography Science Team Meeting

Session: Near Real Time Products and Applications and Multi-Mission, Multi-Sensor Observations

Presentation type: Type Poster

The SWIM (Surface Wave Instrument Monitoring) is a wave conical scanning scatterometer in Ku-band with 6 incidence angles (from 0° to 10°) which will be embarked on the CFOSAT mission [1]. CFOSAT is a Chinese-French oceanographic mission for the joint observation of the wave and the wind vectors at the oceanic surface. This mission will provide a very accurate description of the sea surface, which can greatly benefit to altimetry studies (e.g. modeling of electromagnetic bias).

SWIM will provide the directional wave spectra at a global scale with the 6°, 8° and 10° beams (named spectrum beams). The nadir beam will provide the significant wave height and the wind speed similarly to a conventional altimeter. The six beams are used to get the backscattering coefficient profiles from 0 to 10° and for all azimuths.
The definition of the ground processing of the SWIM data is now under progress, with software implementations and data simulations. The different products have been defined as follow:
- L1a: georeferenced calibrated waveforms (including the nadir waveform),
- L1b: (for spectrum beams only) modulation spectrum,
- L2:
. nadir: SWH, wind speed,
. 6°, 8°, 10°: 2D wave spectra,
. all: backscattering coefficient profiles with respect to incidence and azimuth angles.
These levels should be processed in near-real time.

The SWIM raw data are simulated on orbit parts with the SWIM simulator, SimuSWIM [2]. The sea state is described through numerical wave prediction models forced by analyzed winds delivered by Météo France. The instrument evolves along the expected orbit and the sea state condition changes with a resolution of 0.25°x0.25° in latitude and longitude. Regarding the nadir beam, some extensive studies have been performed to check the consistency with the conventional altimetry simulations and measures.

The raw data are firstly inversed to retrieve the backscattering coefficients in each range gate of the six beams. In addition, the geographical position of the gates is computed.
Then, the inversion is carried out on the spectrum beams to estimate the modulation spectrum from the modulations of signal linked to the long slopes of the surface (i.e. the waves) [1]. This implies the correction of the speckle perturbation, which is performed through several methods under test.
Finally, the modulation transfer function (MTF) of the spectrum is estimated to obtain the wave height spectrum from the modulation spectrum. An automatic partitioning of the 2D spectra is added to get the geophysical parameters of the waves (wavelength, direction, energy) associated to up to 3 different wave trains.

This paper will illustrate all the processing steps with results and analysis of their quality.

[1] Hauser D., C. Tison, J.-M. Lefevre, J. Lambin, T.Amiot, L. Aouf, F. Collard, and P. Castillan, Measuring ocean waves from space: Objectives and characteristics of the China-France Oceanography SATellite (CFOSAT), Proceedings of the ASME 2010 29th International Conference on Ocean, Offshore and Arctic engineering, 2010
[2] Tison C., Amiot T., Enjolras V., Hauser D., Rey L., Souyris J.C., Castillan P., Performance status of the wave scatterometer SWIM, IGARSS'10, 2010
Céline Tison