These are common ASCII text files containing the corresponding image's 3-by-4 projection matrix.
In order to use 3x4 projection matrices as input for SURE, a folder containing one .txt file for each image is required.
Make sure, that the folder containing the .txt files contains only this kind of .txt files with the exactly same count as your imagery.
The correspondence will be established in alphabetical sequence.
Preferably, the .txt files have the same name as the image file, so the ascending sorting ensures the same sequence.
Structure of a Projection Matrix:
Each ASCII .txt file contains one (arbitrary) word at the beginning, followed by the 12 elements of the matrix.
The elements should be separated, e.g. by white spaces, tab stops, or line breaks.