1. A method of reducing an amount of fixed pattern noise from an image signal generated by an image sensor, comprising: for each operational pixel in the image signal, applying a recursively updated offset term to generate a corrected image signal; and recursively updating the offset correction terms by: spatially filtering the corrected image signal for a current frame of the corrected image signal by applying a median filter to each pixel of the current frame and applying an anti-means filter to the output of the median filter to generate a filtered corrected frame of the corrected image signal; comparing the filtered corrected current frame of the corrected image signal with a spatially filtered corrected preceding frame of the corrected image signal, the spatially filtered corrected preceding frame being generated by applying the median filter to each operational pixel of the preceding frame of the corrected image signal and applying the anti-means filter to the output of the median filter for the corrected image signal of the preceding frame; wherein the preceding frame and the current frame are sequential image frames of the corrected image signal that have different scene registration by movement of the image sensor; updating the offset correction terms with terms generated as a function of the comparison; and wherein the median filter removes noise in the form of pixels with outlying values that could bias operation of the anti-means filter.