Rayleigh scattering correction
1 Overview
The eotools.rayleigh module provides a generic look-up table (LUT) based Rayleigh correction method. The correction LUT, instead of being calculated for each sensor, relies on a generic “Rayleigh optical depth” dimension. The Rayleigh optical depth is calculated dynamically, for each pixel and each sensor band (characterized either by a spectral response function (SRF) or by a central wavelength), using the rod() function based on the analytical formula from (Bodhaine et al. 1999). Then, the Rayleigh LUT is interpolated pixel by pixel based on the observation geometry, wind speed, and Rayleigh optical depth.
The correction is implemented in the RayleighCorrection class.
2 LUT structure ("polymer_legacy")
The lookup table used by RayleighCorrection is "polymer_legacy", a LUT calculated with the SOS (successive order of scattering) code. It contains the following data variables:
| Variable | Dimensions | Description |
|---|---|---|
rho_r |
(mu_s, raa, mu_v, odr) |
Rayleigh reflectance (no glint, black surface) |
rho_rg |
(mu_s, raa, mu_v, odr, wind_speed) |
Rayleigh reflectance with sun glint |
t_d |
(mu_s, odr, wind_speed, mu_v) |
Total Rayleigh transmittance (downward × upward, direct + diffuse) |
It has the following grid structure:
| Dimension | Size | Min | Max | Description |
|---|---|---|---|---|
odr |
201 | 0.000 | 0.400 | Rayleigh optical depth. odr=0.4 allows covering \(\lambda > 390nm\) |
mu_s |
25 | 0.032 | 1.000 | Solar zenith cosine (0-88°) |
mu_v |
25 | 0.032 | 1.000 | Viewing zenith cosine (0-88°) |
raa |
37 | 0° | 180° | Relative azimuth angle (5° spacing) |
wind_speed |
5 | 0 m/s | 16 m/s | Horizontal wind speed (values: 0, 2, 4, 8, 16 m/s) |
The RAA convention follows the standard definition: \(\text{raa} = \text{saa} - \text{vaa}\) (solar azimuth minus viewing azimuth).
3 Output variables
After Rayleigh correction, the following variables are added to the dataset:
| Variable | Description |
|---|---|
rho_rc |
Rayleigh-corrected reflectance (depending on options: with or without glint correction, with or without Rayleigh transmittance correction) |
rho_r |
Rayleigh reflectance (without glint) |
rho_rg |
Rayleigh + sun glint reflectance |
t_d |
Total atmospheric transmittance (downward × upward, direct + diffuse) |
odr |
Rayleigh optical depth (computed intermediate) |