Atmospheric compensation in satellite imagery
Techniques for atmospheric compensation in satellite imagery that include converting an image including an array of radiance values to an array of surface reflectance values. The conversion is performed in an automated fashion by identifying one or more portions of the image for which the surface reflectance can be estimated and determining the Aerosol Optical Depth (AOD) by iteratively comparing the radiance value captured by the image sensor to a calculated radiance value (based on the known surface reflectance, historical values for other atmospheric parameters, and the AOD) and adjusting the AOD until the calculated radiance value is substantially the same as the captured radiance value.
1. A method of compensating for atmospheric events in satellite images, comprising:
receiving image data representing an image of at least a portion of the Earth based on the radiance received by a sensor on the satellite, wherein the image data is pixelated and each pixel represents light directed along a path from a target region on the Earth to the sensor, with other portions on the Earth being non-target regions;
receiving Earth coordinate data related to the image data for the image;
receiving temporal data related to the image data for the image;
retrieving one or more atmospheric characteristics from a database, the retrieving being based on the earth coordinate data and the temporal data;
processing the image data for the image to determine Aerosol Optical Depth for the image, without using image data from another image, wherein the image data used to determine the Aerosol Optical Depth is purely panchromatic image data; and
converting the image from radiance to surface reflectance by using the determined Aerosol Optical Depth, including correcting for light that satisfies each of the following: (1) has been scattered at least a first time by the atmosphere onto one of the non-target regions, (2) has then been reflected by the one of the non-target regions, and (3) has then been scattered at least a second time by the atmosphere onto the path to the sensor.
2. A method as defined in claim 1 , wherein the one or more atmospheric characteristics include water vapor, temperature, elevation, and aerosol type.
3. A method as defined in claim 1 , wherein the processing operation includes:
assuming an Aerosol Optical Depth for at least a portion of the image;
deriving a surface reflectance of the portion of the image from the Aerosol Optical Depth, the retrieved atmospheric characteristics, and the radiance received;
calculating an expected radiance for at least the portion of the image based on the derived surface reflectance, the assumed Aerosol Optical Depth, and the retrieved atmospheric characteristics;
comparing the expected radiance to the actual radiance received for at least the portion of the received image; and
adjusting the Aerosol Optical Depth in a direction to bring the expected radiance closer to the actual radiance received;
wherein the calculating, comparing, and adjusting operations are performed iteratively until the comparison shows the expected radiance to be within a predetermined threshold of the actual radiance received.
4. A method as defined in claim 3 , wherein the deriving the surface reflectance includes deriving the surface reflectance for each of a target portion of the portion and a background portion of the portion.
5. A method as defined in claim 3 , wherein the image is used to derive both a radiance received for a target region of the image and a radiance received for a background region of the image.
6. A method as defined in claim 5 , wherein the radiance received for the background region is derived by taking an average radiance for the region surrounding the target.
7. A method as defined in claim 5 , wherein the radiance received for the background region is derived by using a convolution filter for the region of the image proximate to the target region, in order to account for adjacency effects including light scattered by a plurality of different types of scattering pathways.
8. A method as defined in claim 5 , wherein the radiance received for the background region is derived by using a gamma-based filter based on the retrieved atmospheric characteristics and the wavelength of the radiance received.
9. A method as defined in claim 1 , wherein the entire method is performed automatically, without human intervention.
10. A method as defined in claim 1 , wherein the image data used to determine the Aerosol Optical Depth is purely panchromatic image data.
11. A method as defined in claim 1 , wherein the entire method is performed automatically, without human intervention.
12. A method as defined in claim 1 , wherein the image data used to determine the Aerosol Optical Depth is purely panchromatic image data.
13. A method as defined in claim 1 , wherein the image data used to determine the Aerosol Optical Depth includes multispectral image data.
14. A method as defined in claim 1 , wherein the image data is very high spatial resolution image data.
15. A method as defined in claim 1 , wherein the converting further includes correcting for light that has been scattered by the atmosphere onto the path to the sensor without ever having reached the Earth (A), light that is scattered by the atmosphere onto the target region and reflected onto the path to the sensor (B 2 ), and light that has been reflected by one of the non-target regions and then scattered by the atmosphere onto the path to the sensor (C 1 ).