L1A Processing: Initial Radiometric Interpretation & Correction
To create calibrated hyperspectral products, Pixxel L1B, L1C, and L2A, suitable for use as Analytics Ready Data ARD, an initial L1A preprocessing step is conducted to eliminate predictable image artifacts. Radiometric correction coefficients (NUC) are calculated prelaunch, and their accuracy is validated onorbit using data collected over established vicarious sites such as RadCalNet and FLARE sites.
All Pixxel satellite images are collected at a bit depth of 10 bits per pixel. Once on the ground, the images are converted to 16bit format. Onground image processing involves applying radiometric correction coefficients to convert the pixel Digital Numbers (DNs) from the sensor to absolute atsensor radiance units.
The pixel values of Pixxel’s L1A product represent absolute calibrated radiance units for the image.
L1B Processing: Geometric Correction
Further radiometric processing of L1C and L2A requires accurate georeferencing. L1B processing orthorectifies all bands using a calibrated camera model and a terrain model. Geolocation is computed photogrammetrically, and good bandtoband registration is obtained.
The initial attitude data received from the satellite has poor accuracy due to hardware limitations. This creates a noticeable geolocation error that must be addressed before L1B processing. To rectify this, Pixxel bands are registered against Sentinel2 reference images to recover an “adjusted pointing” that would lead to the same geolocation upon projection. A proprietary, multichannel registration technique is employed where four Pixxel bands in a collection are simultaneously registered to the corresponding four 10meter bands [blue, green, red, NIR] of a contemporaneous Sentinel2A or 2B truth ortho. The registration creates tie points for those four bands, which are then fed into a leastsquares fit that deduces the adjusted pointing. This "corrected quaternion list" is then stored for all subsequent product generations. This process is as accurate as the truth orthos, which themselves typically have an absolute geolocation accuracy of around 12 meters CE90.
Also relevant is the geometric accuracy of the terrain model DEM used during orthorectification. Freely available, 30meter Copernicus DEMs have a horizontal accuracy of 6 meters, or 1/5th of a posting. Vertical accuracy for the DEM is about 4 meters or 1/7th of a posting.
The output of L1B processing is a multiband geoTIFF, complete with geospatial headers that specify the projection and extent. The map projection can be anything, but it is defaulted to geodetic EPSG4326. The hyperspectral stack is trimmed to the common overlap region of all bands. A configurable value is used to fill areas outside the projection. The bit depth is made 16bit just prior to saving the file.
L1C Processing: RadiancetoReflectance Conversion
To generate the L1C scaled product from Pixxel’s L1B radiance values, parameters such as EarthSun distance, extraterrestrial solar irradiance, and solar zenith angles are considered for calculation using the following formula:
The above formula assumes that the targets are Lambertian in nature and that the BRDF effects of the target are not taken into consideration.
The delivered Pixxel L1C product is then scaled between 050000 values, where 0 is for 0 reflectance and 50000 represents 100% reflectance (i.e., 1. Thus, to convert the pixel values of the L1C product to TOA reflectance, the pixel values are to be multiplied by a reflectance scaling factor of 2e5, as provided in the corresponding metadata XML file, as follows:
L2A Processing: Atmospheric Correction
To generate a bottomoftheatmosphere BOA L2A product, Pixxel L1C products are atmospherically corrected using Pixxel’s copyrighted atmospheric correction software  pHSICOR. The software accounts for aerosols and water vapour content in the atmosphere and their variations with altitude. Based on libRadtran lookuptables, popular approaches such as DarkDense Vegetation and novel methods such as the Water Absorption Strength Ratio were used in combination to estimate the BOA products. Some limitations of the model are:

The current version of the atmospheric correction model does not correct for oxygen absorption bands around 760 nm.

The surface BRDF effects have not been accounted for.
Comments
0 comments
Please sign in to leave a comment.