6Learning ObjectivesWhat 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?
7Learning Objectives (cont.) When do we have to atmospherically correct imagery, and when does it not necessarily matter?What is temporal compositing for cloud removal?
8Atmospheric EffectsEMR from the sun passes through the atmosphere TWICE before it reaches a satelliteThe atmosphere is made up of molecules that interact with EMRMany (not all) atmospheric effects are wavelength dependent!
9Atmospheric EffectsWhat are the 2 primary effects of the atmosphere on EMR?EMR can also be refracted (bent) by the atmosphere (relevant for target location applications)
11What factors affect the amount of radiance the satellite measures? Hint: What is the equation for at-satellite radiance??
12Diagram from U. of Illinois Dept. of Atmos. Sci. ScatteringAmount of atmospheric scattering is affected by:Wavelength of EMRSize of atmospheric particlesDensity of atmospheric particlesLength of travel path (optical depth)Diagram from U. of Illinois Dept. of Atmos. Sci.
13Atmospheric Scattering Lλ = (Etmr/𝜋) + LpWhere Lp is called the path radiance (atmospheric scattering TOWARDS the satellite that increases measured radiance)
14Top of Atmosphere Irradiance Path RadianceRadianceTop of Atmosphere IrradianceSurface Irradiance
15Types of Scattering Rayleigh Mie Nonselective Affects blue wavelengths most stronglyBlue sky during dayContributes to red sunsetsMieCaused by larger particles with diameter comparable to or larger than light wavelengths.Not very wavelength dependent – e.g., white cloudsNonselectiveNot wavelength dependent
16What 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.
17Atmospheric 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?)
18Atmospheric Corrections (cont.) Each band must be corrected separately!Green radiation is scattered by atmosphere 4x more than near-infraredIn general, atmospheric effects are much stronger in visible part of the spectrum than in the IR
19Correcting for Path Radiance (Scattering) Techniques includeDark pixel subtraction (a.k.a. the histogram minimum method)Regression of short wavelength band against long (unscattered) wavelength bands.
20Dark 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
22Lλ (satellite radiance) - Lp = (Etmr/𝜋 + Lp) - Lp = Ground radiance corrected for scatteringDoes not account for absorption (transmittance) by the atmosphere!
23Regression TechniqueAssumes that long-wavelength bands are not scatteredPlot 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.
25Atmospheric Absorption (or transmittance) The atmosphere absorbs some light for all wavelengthsThe atmosphere absorbs some wavelengths more than others due to specific atmospheric constituents (e.g., water vapor, CO2, ozone)
26Thermal IR – Greenhouse Effect AbsorptionOzone HoleThermal IR – Greenhouse Effect
27Correcting 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 spatiallyOften not done because it is difficult.
28Atmospheric Measurement and Modeling Requires measurement of many atmospheric characteristics at different heights above the earth at same time as satellite overpassThere are “canned” atmospheric models that work fairly well.LowtranModtranACORN
29Reasons for Atmospheric Correction Atmospheric Correction is not always necessary!Single scene studiesAtmospheric differences can be reduced by ratio based spectral indicesOften necessary when comparing multiple scenesScene matching (mosaics)Change detection studies (sometimes)Applying classification statistics to multiple scenesAlways necessary if you need to calculate ground reflectance or compare satellite radiance to ground measurements
30Clouds! Most EMR wavelengths can’t penetrate clouds Big problem in remotely sensed imagery—tropics especiallyTemporal compositing can be used to get rid of cloudsCloud shadows are a problem too
31The NE corner of the Laramie image that you’ve used in lab Clouds and cloud shadows!
32Summary – Radiometric Corrections Change the DNs of pixels from the values that the satellite measuredUsually done to remove radiance not directly from the target (e.g. path radiance)Should be considered carefully because you alter the original radiometry