How are atmospheric parameters sourced for the correction to surface reflectance?

As the Dove sensor only records visual and near-infrared spectral bands, it is difficult to measure atmospheric parameters directly from the Dove imagery. As a result, key atmospheric parameters are currently sourced from MODIS near-real time (NRT) global datasets for use in the correction to surface reflectance. Aerosol optical depth and water vapor parameters are sourced from MOD09CMA dataset and ozone is sourced from the MOD09CMG dataset. The parameters are retrieved from the MODIS data for the same day and within the footprint of the image being processed.  While there is some uncertainty introduced to the correction to Surface Reflectance using atmospheric data with an acquisition delta of approximately 1 hour, the largest uncertainty is the varying quality of the MODIS NRT inputs caused by clouds, ice, and bright deserts.

In some instances there is no MODIS data overlapping a Planet scene or the area nearby. In those cases, Aerosol Optical Depth (AOD) is set to a value of 0.226 which corresponds to a “clear sky” visibility of 23km, the aot_quality is set to the MODIS “no data” value of 127, and aot_status is set to ‘Missing Data - Using Default AOT’. If there is no overlapping water vapor or ozone data, the correction falls back to a predefined 6SV internal model.

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