Radiometric Corrections Atmospheric Corrections Atmospheric Effects on EMR

Kodiak Island, AK – Volcanic ash and clouds (MODIS image)

Farther away = more blue light mixed in.

Learning Objectives What are the main ways that the atmosphere affects light traveling through it? What are the different types of scattering, and what causes them? How can we correct satellite data to remove the effects of scattering? What causes differences in atmospheric transmittance? How can we correct satellite data for differences in transmittance?

Learning Objectives (cont.) When do we have to atmospherically correct imagery, and when does it not necessarily matter? What is temporal compositing for cloud removal?

Atmospheric Effects EMR from the sun passes through the atmosphere TWICE before it reaches a satellite The atmosphere is made up of molecules that interact with EMR Many (not all) atmospheric effects are wavelength dependent!

Atmospheric Effects What are the 2 primary effects of the atmosphere on EMR? EMR can also be refracted (bent) by the atmosphere (relevant for target location applications)

The Green Flash!

What factors affect the amount of radiance the satellite measures? Hint: What is the equation for at-satellite radiance??

Diagram from U. of Illinois Dept. of Atmos. Sci. Scattering Amount of atmospheric scattering is affected by: Wavelength of EMR Size of atmospheric particles Density of atmospheric particles Length of travel path (optical depth) Diagram from U. of Illinois Dept. of Atmos. Sci.

Atmospheric Scattering Lλ = (Etmr/𝜋) + Lp Where Lp is called the path radiance (atmospheric scattering TOWARDS the satellite that increases measured radiance)

Top of Atmosphere Irradiance Path Radiance Radiance Top of Atmosphere Irradiance Surface Irradiance

Types of Scattering Rayleigh Mie Nonselective Affects blue wavelengths most strongly Blue sky during day Contributes to red sunsets Mie Caused by larger particles with diameter comparable to or larger than light wavelengths. Not very wavelength dependent – e.g., white clouds Nonselective Not wavelength dependent

What are atmospheric corrections? Atmospheric “corrections” are methods used to convert satellite DNs to numbers that represent radiance leaving the earth’s surface. Required if you want to calculate surface reflectance.

Atmospheric Corrections So…if you want to remove path radiance from the satellite DNs, how would you do it? (Again, what’s the equation for satellite radiance?)

Atmospheric Corrections (cont.) Each band must be corrected separately! Green radiation is scattered by atmosphere 4x more than near-infrared In general, atmospheric effects are much stronger in visible part of the spectrum than in the IR

Correcting for Path Radiance (Scattering) Techniques include Dark pixel subtraction (a.k.a. the histogram minimum method) Regression of short wavelength band against long (unscattered) wavelength bands.

Dark Pixel Subtraction Assume that the darkest objects in the image (the minimum value in the histogram) should have a DN of 0 (little or no reflectance) Not always a correct assumption! Find the true minimum pixel value from each band (using histograms or dark areas) Subtract that value from all of the pixels in the band

Histogram

Lλ (satellite radiance) - Lp = (Etmr/𝜋 + Lp) - Lp = Ground radiance corrected for scattering Does not account for absorption (transmittance) by the atmosphere!

Regression Technique Assumes that long-wavelength bands are not scattered Plot the DNs from a shorter wavelength band on the x-axis against long wavelength DNs on the y-axis. y-intercept should be at 0. If not, the difference is Lp.

Band 7 (for example) DN Path radiance (Lp) Band 1 (for example) DN

Atmospheric Absorption (or transmittance) The atmosphere absorbs some light for all wavelengths The atmosphere absorbs some wavelengths more than others due to specific atmospheric constituents (e.g., water vapor, CO2, ozone)

Thermal IR – Greenhouse Effect Absorption Ozone Hole Thermal IR – Greenhouse Effect

Correcting for Atmospheric Transmittance Must correct separately for each wavelength (band) Must either measure or make assumptions about optical depth, atmospheric density of various constituents, etc. Transmittance can vary spatially Often not done because it is difficult.

Atmospheric Measurement and Modeling Requires measurement of many atmospheric characteristics at different heights above the earth at same time as satellite overpass There are “canned” atmospheric models that work fairly well. Lowtran Modtran ACORN

Reasons for Atmospheric Correction Atmospheric Correction is not always necessary! Single scene studies Atmospheric differences can be reduced by ratio based spectral indices Often necessary when comparing multiple scenes Scene matching (mosaics) Change detection studies (sometimes) Applying classification statistics to multiple scenes Always necessary if you need to calculate ground reflectance or compare satellite radiance to ground measurements

Clouds! Most EMR wavelengths can’t penetrate clouds Big problem in remotely sensed imagery—tropics especially Temporal compositing can be used to get rid of clouds Cloud shadows are a problem too

The NE corner of the Laramie image that you’ve used in lab Clouds and cloud shadows!

Summary – Radiometric Corrections Change the DNs of pixels from the values that the satellite measured Usually done to remove radiance not directly from the target (e.g. path radiance) Should be considered carefully because you alter the original radiometry