Processing Data -- Raw to Reflectance
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What is the digital number (DN)?
The unitless integer that a satellites uses to record radiance, ranging from 0 to 255, where 0 is no radiance and 255 is the maximum amount of radiance that the sensor is sensitive to. Each image pixel has one DN for each band.
What is gain (g)?
The multiplier used to convert DNs to radiance (watts/m2/sr/DN).
What is bias (b)?
The offset added to convert DNs to radiance (watts/m2/sr).
What is irradiance (E)?
The amount of incoming light from the sun either at the ground or at the top of the atmosphere (TOA) (watts/m2).
What is radiance (L)?
The amount of light received at the satellite (watts/m2/sr).
What is transmittance (τ)?
The proportion of light that is transmitted through something (e.g., through the earth's atmosphere), usually given as a fraction.
What is reflectance (ρ)?
The amount of light that reflects off of something divided by the amount of incoming light (often given as a fraction or a percent).
What is exitance?
A measure of the amount of radiant flux leaving a surface per unit area of the surface measured in watts/m-2.
What is the symbol for radiant flux?
What is the equation for radiance leaving a specific projected source area on the ground, in a specific direction, and within a specific solid angle?
Lλ = (Φλ/Ω)/A cos θ, where Ω is the solid angle, A is the source area, and θ the angle of the cone (Ω) to the vertical.
What is total radiance recorded at the sensor (LS)?
LS = gDN + b
What is the equation for total target radiance at the sensor?
LT = (ρλτθvτθoEoλcosθo)/π + LP, where ρλ = reflectance from study area, τθv = transmittance from study area, τθo = transmittance from the sun, Eoλ = solar irradiance, θo is the angle of transmittance from the sun to the vertical, and LP is reflectance from neighboring areas.
What happens during 1G-product rendering?
- Image pixels are converted to units of absolute radiance using 32-bit floating-point calculations. Pixel values are then scaled to byte values prior to media output. The following equation is used to convert DNs in a 1G product back to radiance units:
- Lλ = Grescale * QCAL + Brescale,
- where L = spectral radiance at the sensor's aperture in watts/(m2 * sr * μm).
What is Grescale?
Rescaled gain (the data product "gain" contained in the Level 1 product header or ancillary data record) in watts/(m2 * sr * μm)/DN.
What is Brescale?
Rescaled bias (the data product "offset" contained in the Level 1 product header or ancillary data record) in watts/(m2 * sr * μm).
What is QCAL?
The quantized calibrated pixel value (in DN).
What is the total reflectance formula?
ρλ = LSπ/2τEoλcosθo, where LS = total radiance at the sensor.
What is the formula for the combined surface and atmospheric reflectance of the Earth?
ρλ = Lλπd2/ESUNλ cos θs, where ρλ = unitless planetary reflectance, Lλ = spectral radiance at the sensor's aperture, d = Earth-Sun distance in astronomical units, ESUNλ = mean solar exoatmospheric irradiances, and θs is the solar zenith angle in degrees.
What are the units of radiance?
Watts/m2/sr AND Watts/m2/sr/μm.
What are the units of reflectance?
None; it is given as a percentage.
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