Monitoring of Wave Fields with the SWIM Instrument on CFOSAT
Tison, Céline1; Amiot, Thierry1; Hauser, Danièle2; Castillan, Patrick1; Corcoral, Nathalie1; Rey, Laurent3; Enjolras, Vivien3; Guibert, Sarah1

1. The CFOSAT mission

The Chinese and French Space Agencies propose to jointly carry out an innovative mission, CFOSAT (China France Oceanography Satellite project) devoted to the monitoring of the ocean surface and its related science and applications. CFOSAT will embark both a wind and a wave scatterometers, enabling a simultaneous measure of the wind and the wave vectors with a global coverage for the first time.
Feasibility and Preliminary Design phases (A/B phases) were successfully achieved from 2006 until 2009. The project started Detailed Design phase C beginning of 2011 which will be immediately followed by the Manufacturing Phase D in 2013. The launch and the Assessment Phase completion should lead to deliver a fully validated system on orbit in 2015.

Fig1.: Artist view of the CFOSAT satellite. SWIM antenna is on the left and SCAT antenna on the bottom right. Other antennas on Earth face are for TM/TC. © CNES/ Gekko.

The primary objective of CFOSAT is to monitor, at the global scale, ocean surface winds and waves so as to improve: wind and wave forecast for marine meteorology (including severe events), ocean dynamics modeling and prediction, climate variability knowledge, fundamental knowledge of surface processes, etc. As an opportunity, CFOSAT will also be used to complement other satellite missions for the estimation of land surface parameters (in particular soil moisture and soil roughness), and polar ice sheet characteristics.
The satellite embarks two payloads; both are Ku-band (13.2 to 13.6 GHz) radar scanning around the vertical axis:

  • the wave scatterometer SWIM, a rotating 6-beams radar at small incidence (0 to 10°) [3,4,6],
  • the wind scatterometer SCAT, a fan-beam radar at larger incidence angles (45 and 49°) [5].

    In this paper, after a short presentation of the CFOSAT mission and the associated scientific requirements, a focus on the wave scatterometers SWIM (Surface Wave Investigation and Monitoring) instrument, developed by Thales Alenia Space under CNES contract, will be done.

    2. The SWIM instrument

    The main objective of SWIM is to provide directional wave spectra. SWIM is a Ku-band real aperture radar following the concepts developed in [1, 2]. It illuminates the surface sequentially with 6 incidence angles: 0°, 2°, 4°, 6°, 8° and 10° with an antenna aperture of approximately 2°. In order to acquire data in all azimuth orientations, the antenna is rotating at a speed rate of 5.6 rpm. The six beams enable to measure several geophysical parameters:

  • all beams: estimation of backscattering coefficient profiles from 0° to 10° of each surface,
  • nadir beam (0°): estimation of SWH and wind sea surface, similarly as nadir altimeter,
  • 6°, 8° and 10° (spectrum beams): estimation of the 2D wave spectra.
    The paper [6] provides a detailed overview of SWIM.

    2.1. Backscattering coefficient

    Internal calibration [6] is used to reach high precision on the backscattering estimation. Preliminary budgets show that:

  • the absolute calibration is better than ±0.9 dB,
  • he relative calibration between beams is better than ±0.2 dB.

    2.2. Nadir beam

    The estimation of SWH is done with an accuracy similar to the one obtained on the JASON-2

    2.3. Wave spectra

    This is the most innovative products as no spatial instrument enables yet to get a completely directional wave spectrum. The processing is under development and the first simulated results are promising.
    The wave spectra are derived from the fluctuation of the backscattered power linked to the slopes of the waves. Around 8° of incidence angle, the impact of small scale roughness is null and the only modulation of the backscatter coefficient is due to surface slopes. The wave spectrum is the spectral density function of the fluctuation signal corrected from speckle and thermal effects.

    In order to validate the instrument definition and prepare the processing algorithms, an instrument simulator, SimuSWIM, has been developed [4].

    The following figure is an example of simulation results. In this case, the simulation inputs are Météo France WAM spectra. The study case is the Atlantic ocean close to Galicia, Spain in November 2002, when the Prestige sinking occur. In this study case, there are two sea states: a swell and a wind sea (this last one is becoming a swell during the day). These two sea states are clearly well described in the estimated spectra.

    Fig 2.: Examples of wave spectra simulations (1st line = references coming from WAM models, 2nd line = SWIM simulations) in the case of the storm involved in the Prestige oil tanker sinking in 2002.

    CFOSAT is an innovative mission jointly developed by CNES & CNSA. The unique combination of instruments will allow determining the directional wave spectra of waves in relation with surface winds. The application field sounds promising both for operational and research purposes.

    [1] Jackson, F. "An analysis of short pulse and dual frequency radar techniques for measuring ocean wave spectra from satellites", Radio Science, 16(6) :1385-1400, 1981

    [2] Hauser, D., Soussi, E., Thouvenot, E., and Rey, L., "SWIMSAT: a real aperture radar to measure directional spectra of ocean waves from space - main characteristics and performance simulation". Jour. Atmos. Oceanic Tech, , 18, 2001

    [3] Enjolras V. Rey L., Amiot T., Tison C., Castillan P., SWIM, the first ever spaceborne waves scatterometer radar, now under development, IGARSS'11, July 2011

    [4] C. Tison, C. Manent, T. Amiot, V. Enjolras, D. Hauser, L. Rey, P. Castillan, Estimation of Wave spectra with SWIM on CFOSAT - Illustration on a real case, IGARSS'11, Vancouver (Canada), July 2011

    [5] X. Dong, D. Zhu, W. Lin, H. Liu, J. Jiang, A Ku-band rotating fan-beam scatterometer : design and performance simulations, IGARSS'10, pp1081-1084, 2010

    [6] V. Enjolras, L. Rey, T. Amiot, C. Tison, P. Castillan, SWIM, the first ever spaceborne waves scatterometer radar, now under development, 20 Years of Progress in Radar Altimetry, September 2012