Now the GPS file header contains a 64 byte hash of the data, to prevent attacks
like truncating the file. If you truncate in the right places, the receiver
of the file would not be able to tell that some of the file is missing. Now
the entire plaintext plus user data content must match.

And the hash is placed securely in the file header, rather than appended to
the file in plaintext, just to be extra careful.