It's almost certain that there will be some sort of checksum on the data uploaded to the nav computer. It's very likly that this will be written at the end of one of the files. The best way of finding this would be to try and work out the checksum on the origional unaltered file in various ways and see if you can find a match for that value. Once you know how to calculate the check sum and where it is, the rest should be easy!