Raytheon STD 21 GPS Operation Manual page 3

Please note that all ships of 500 gross tonnage and upwards
according to SOLAS and other international regulations must
be equipped with a gyro compass. The gyro compass must be
operational. For this reason it is not allowed to have a
switched- -off gyro compass during voyages.
For the application "GPS Compass in combination with Gyro
compass STD 22" see also respective manuals:
Manual no.: 3646 for Gyro compass STD 22
Manual no.: 3648 for Operating Unit
After an alarm message occurs and the reason for this alarm
is eliminated either automatically or by repair, the displayed heading
has to be checked. In case of an implausible heading value,
the GPS Compass should be switched OFF and ON again.
Edition: Feb. 05, 2007
Satellite Compass
STD 21 GPS
I
STD 21 GPS
3717/110--850.DOC020002