CALM - Continental waveform ALtimetry Model

Continental waveform ALtimetry Model (CALM) software have been developed in order to simulate the electromagnetic interaction between the wave emitted by the altimeter antenna and the illuminated soil.

Radar Altimetry Backscattering Modeling

The antenna footprint on the soil is sampled using triangular facets in order to be able to take into account any topography. Each facet is characterized by small scale roughness parameters and soil dielectric permittivity. The approach consists in vectorially summing the complex contributions of all the facets at the receiver. The vectorial contribution of each facet is computed assuming that the field distribution over the facet is the same as if it were infinite. Note that the large scale sampling may be adapted to a multi-scale one all the more easily as the computations are very fast.
The Kirchoff model was used to take into account the small-scale roughness. It is used to simulate the electromagnetic (EM) response of a rough surface taking into account rms height (hrms) and the correlation length of the local surface. The macroscopic scale integrates the effect of the surface topography in the altimeter footprint (several to several tenths of km2 depending on the wavelength of the EM wave) through the use of a Digital Elevation Model (DEM).

Reference

  • Software DOI: 10.6096/CTOH_CALM-software_2018_01
  • FATRAS, Christophe, BORDERIES, Pierre, FRAPPART, Frédéric, et al. Impact of Surface Soil Moisture Variations on Radar Altimetry Echoes at Ku and Ka Bands in Semi-Arid Areas. Remote Sensing, 2018, vol. 10, no 4, p. 582.
 

CALM - software access

CALM - software is under GPL licence. It is python3 code module and it is part of The Python Package Index (PyPI) repository. It will be soon available.

Example for the Gourma region, Mali in the west-African Sahel

Location of the five study sites distributed on satellite tracks 302, 373 and 846 of ENVISAT/SARAL over the Gourma region, Mali © Google Earth. The azimuthal direction of the satellite on orbit, the site numbers and the corresponding DEM extraction area are indicated. The water areas limits over each site are marked with black lines.

CALM_site_fig

 

 

Individual (black) and mean (red) waveforms on the five study sites over the observation periods from ENVISAT RA-2 at Ku-band (left of the left panel) and from SARAL/AltiKa at Ka-band (left of the right panel). Comparison over the five study sites between the modeled waveform (in blue) and the mean observed waveform (in red), with the correlation coefficient r indicated from ENVISAT RA-2 at Ku-band (right of the left panel) and from at SARAL/AltiKa at Ka-band (right of the right panel).

 

CALM_Waveform_fig

 

 

 

 

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