Furuno FEA-2107-BB Operator's Manual page 59

Verifying position sensors (Posn page)
On the Posn page of the Sensors dialog
box, the data field of a position sensor
contains a label(s) (in the figure at right it
is DGPS, GPS+ZDA) which indicates the
type of the sensor. Primary-Secondary-
OFF indicates sensor status and priority.
Position in local datum, speed and course
are also shown. A DGPS position sensor
also shows the text "Diff" if the
differential signal is in use.
Position sensors have priority, which is
indicated by Primary and Secondary.
Only one sensor can be primary while the
others can be secondary or off position.
After a sensor is turned off, its status is
changed to Secondary state. When a
position sensor state is changed to
primary and another sensor was primary,
the sensor formerly primary becomes
secondary.
Choose the "Primary" navigation sensor as the sensor that is considered to be most accurate and reliable. Set all
other navigation sensors as "Secondary".
Verifying Kalman filter (Posn page)
The Kalman filter is an automatic process that will calculate the most probable position based on all position
sensors. The filter uses in its process all sensors that are not chosen to OFF. For more information about the Kalman
Filter, see section 19.7 Filter Operation.
Verifying alignment (Posn page)
The alignment is a correction given by the chart align function, and it is used to move the ship to the "correct"
position. Normally turn this feature off in the start-up procedure. For more information about position alignment, see
the section 19.8 Position Alignment.
6. Setup Before Departure
6-9