  The checksum keywords described here provide an integrity check on the
information contained in FITS HDUs. (Header and Data Units are the basic
components of FITS files, consisting of header keyword records followed by
optional associated data records). The CHECKSUM keyword is defined to have a
value that forces the 32-bit 1's complement checksum accumulated over all the
2880-byte FITS logical records in the HDU to equal negative 0. (Note that 1's
complement arithmetic has both positive and negative zero elements). Verifying
that the accumulated checksum is still equal to -0 provides a fast and fairly
reliable way to determine that the HDU has not been modified by subsequent data
processing operations or corrupted while copying or storing the file on
physical media.
