compute.Rrs.SAS: Compute the remote sensing reflectance from HyperSAS data

In belasi01/HyperocR: Hyperspectral OCR data processing

Compute the remote sensing reflectance from HyperSAS data

Description

This is the main function of the package. It processes the SAS data for a given data acquisition file.

Arguments

SAS	is a list return by read.horc.L2.SAS.
ID	is the station ID
tilt.max	is the maximum tilt tolerance for the SAS. A default value is 3 degrees is used otherwise.
quantile.prob	is a value (betwwen 0.25 to 1) for the maximum quantile probability for which the surface radiance (Lt) values will be discarded. The quantile probability is the value at which the probability of the random variable being less than or equal to this value. For example, a value of 0.5 means that every Lt value above the 50% quantile probability will be discarded, while a value of 0.8 means that only the values above the 80% quantile probability will be discarded. The rational is to eliminate outliers resulting from sun glitters, foam, sea ice, etc. The default value is 0.5 to eliminate Lt value above the 50% quantile probalibity.
windspeed	is the wind speed in m/s. A default value of 5 m/s is used otherwise.
thetaV	is the viewing sensor zenith angle in degree. A default value of 40 degrees is used otherwise.
Dphi	is the azimuthal difference between the sun and the sensor view angle. A default value of 90 degrees is used otherwise.
NIR.CORRECTION	is the method used to eliminate residual "white reflectance". Such residual is frequent under windy condisitons at sea. Those may be due to sun glint, foam, ocean spray, etc. The "NULL" correction subtracts the residual Rrs value at 800 nm. The "SIMILARITY" correction is prefer in turbid waters. It is based on the similarity spectrum (see Ruddick et al. L&O 2005) in the NIR. Here it assumes a constant Rrs(780)/Rrs(720) ratio of 2.35.
use.COMPASS	is a logical parameter indicating whether or not the azimuth difference between the sun and the view geometry is calculated using the SAS compass data. When FALSE, the azimuth angle difference is taken from the cast.info.dat file. NOTE: The Default is FALSE because compass usually doesn't work on ship.
COPS	is logical parameter to force the water reflectance to pass through the COPS reflectance measurements made a priori. It must be turn on only if COPS data have been processed and validated

Details

This functions is the main part of the HyperSAS data processing.

First it computes the tilt from the Pitch and Roll data recorded by the SAS and stored in SAS$Anc data frame. Then tilt values areassociated to each irradiance (Ed), sky radiance (Li) and surface radiance (Lt) frame found whitin the file (each of them have their specific time integration). Second, the Lt frames are discarded if 1) the tilt is greater than the specified tilt.max; 2) the Lt value around 490 nm is greater than the specified quantile.prob (to remove upper outliers); and 3) the Lt value around 490 nm is below the 10% quantile probalility (to remove lower outliers).

Second, for each valid Lt frame, a corresponding value of Ed and Li is interpolated using a spline function. Mean and standard devivation of valid Lt and interpolated Ed and Li spectra are computed.

Third, a spectral interpolation is performed on the average Ed, Li and Lt spectra from 380 to 800 nm every 5 nm. This step is done using a loess function with a span of 0.05.

Forth, the sky reflectance (Li/Ed) and surface (Lt/Ed) are computed and smoothed using a loess function with a span of 0.75 and 0.1, respectively. This step is necessary to avoid spectral artefact resulting from the individual spectral response of each sensor.

Fift, the specular reflectance of the air-sea interface is determined (i.e., rho_sky). This is based on Ruddick et al. L&0 2005 and Mobley, App. Opt. 1999. Under clear sky, i.e. when the sky reflectance at 750 nm is below 5%, rho_sky is interpolated from a Look-Up-Table provided by Mobley. The rho_sky LUT has 4 dimensions for thetaS, thetaV, Dphi and Windspeed. The current table is not spectrally dependent (so strictly valid for 550 nm). Spectral dependency is therefore not taken into account, which may be significant (see Lee et al, Opt. Exp. 2010)

Author(s)

Simon Bélanger

See Also

process.HyperSAS and read.hocr.SAS

belasi01/HyperocR documentation built on June 11, 2024, 7:21 a.m.