1. A method performed by a computing device for filtering a target image frame in an associated sequence of image frames along a down-track direction, the target image frame having pixels with values, the method comprising: for target pixels of the target image frame, calculating a median filtered value for the target pixel based on the value of corresponding pixels in each of the image frames in the associated sequence; and determining whether pixels in a neighborhood of the target pixel satisfy a filtering criterion based on values of the pixels in the neighborhood and their associated median filtered values; and after determining that the filtering criterion is satisfied, replacing the value of the target pixel with the calculated median filtered value.