Atmospheric compensation in satellite imagery
View Patent ↗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; and
converting the image from radiance to surface reflectance by using the determined Aerosol Optical Depth, including correcting for light that has been scattered at least a first time by the atmosphere onto one of the non-target regions and then reflected by the one of the non-target regions and then scattered at least a second time by the atmosphere onto the path to the sensor (C 2 );
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;
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;
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.
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 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.
4. A method as defined in claim 1 , wherein the radiance received for the background region is derived by taking an average radiance for the region surrounding the target.
5. A method as defined in claim 1 , wherein the entire method is performed automatically, without human intervention.
6. A method as defined in claim 1 , wherein the image data used to determine the Aerosol Optical Depth is purely panchromatic image data.
7. A method as defined in claim 1 , wherein the image data used to determine the Aerosol Optical Depth includes multispectral image data.
8. A method as defined in claim 1 , wherein the at least a portion of the image includes a plurality of non-contiguous pixels.
9. A method as defined in claim 1 , wherein the convolution filter includes different types of scattering effects, including forward-scattered light that then impinges upon the target and is received as part of the image data and light that reflects from the target that is then forward scattered in a direction to be received as part of the image data.
10. A method as defined in claim 1 , wherein the image data is very high spatial resolution image data.
11. 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 ).
12. 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; and
converting the image from radiance to surface reflectance by using the determined Aerosol Optical Depth, including correcting for light that has been scattered at least a first time by the atmosphere onto one of the non-target regions and then reflected by the one of the non-target regions and then scattered at least a second time by the atmosphere onto the path to the sensor (C 2 );
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;
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;
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.
13. A method as defined in claim 12 , wherein the image data used to determine the Aerosol Optical Depth is purely panchromatic image data.
14. A method as defined in claim 12 , wherein the at least a portion of the image includes a plurality of non-contiguous pixels.
15. A method as defined in claim 12 , 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 ).
16. A method for determining an Aerosol Optical Depth in the atmosphere relative to a satellite or aerial image of the Earth, comprising:
receiving a pixelated image having radiance values for each pixel in the image, the pixelated image having been captured by a sensor having a plurality of pixelated sensor elements in spaced apart relationship from each other;
retrieving values for certain atmospheric parameters, including an initial estimate of an Aerosol Optical Depth atmospheric parameter;
determining a portion of the image for which an estimate of the surface reflectance can be made, the portion of the image being a plurality of pixels in the image, at least some of which pixels are spatially non-contiguous with each other;
estimating the surface reflectance of the portion of the image;
based on the estimated surface reflectance of the portion of the image and the certain atmospheric parameters, calculating the expected radiance for the portion of the image;
comparing the radiance from the received image for the portion of the image to the expected radiance for the portion of the image; and
adjusting the Aerosol Optical Depth atmospheric parameter if the comparison reveals a difference greater than a threshold value;
wherein the calculating, comparing, and adjusting operations are repeated as necessary until the comparison reveals a difference within the threshold value.
17. A method as defined in claim 16 , wherein the portion of the image for which an estimate of surface reflectance can be made is a dark object.
18. A method as defined in claim 17 , wherein the estimate of surface reflectance for the dark object is based on the radiance value for the dark object, the radiance value for other portions of the image, and the certain atmospheric parameters.
19. A method as defined in claim 16 , wherein the portion of the image for which an estimate of surface reflectance can be made is an object that varies in a known fashion throughout the year.
20. A method as defined in claim 19 , wherein a determination is made as to which of a plurality of object classes the object belongs to.
21. A method as defined in claim 20 , wherein the plurality of object classes include forests, crops, and man-made surfaces.
22. A method as defined in claim 20 , wherein typical surface reflectance values throughout the year are known for each of the object classes.
23. A method as defined in claim 16 , wherein the Aerosol Optical Depth and the retrieved atmospheric parameters are used to convert the image from an array of radiance values to an array of surface reflectance values.
24. A method as defined in claim 16 , wherein the image data used for determining the Aerosol Optical Depth is purely panchromatic image data.
25. A method as defined in claim 16 , wherein the image data used for determining the Aerosol Optical Depth includes multispectral image data.
26. A method as defined in claim 16 , wherein the image includes a plurality of dark objects and the Aerosol Optical Depth is determined independently for each dark object, with kriging used for determining the Aerosol Optical Depth between the dark objects.
27. A method as defined in claim 16 , wherein the image data is very high spatial resolution image data.
28. A method for determining an Aerosol Optical Depth in the atmosphere relative to a satellite or aerial image of the Earth, comprising:
receiving a pixelated image having radiance values for each pixel in the image;
retrieving values for certain atmospheric parameters, including an initial estimate of an Aerosol Optical Depth atmospheric parameter;
determining a portion of the image for which a non-zero estimate of the surface reflectance can be made;
estimating the surface reflectance of the portion of the image, using the Aerosol Optical Depth initial estimate, using the radiance values, and using a transfer function that includes a plurality of different types of scattering effects;
based on the non-zero estimated surface reflectance of the portion of the image and the certain atmospheric parameters, calculating the expected radiance for the portion of the image;
comparing the radiance from the received image for the portion of the image to the expected radiance for the portion of the image; and
adjusting the Aerosol Optical Depth atmospheric parameter if the comparison reveals a difference greater than a threshold value;
wherein the calculating, comparing, and adjusting operations are repeated as necessary until the comparison reveals a difference within the threshold value.