A-6
Magnetic variation offset
The location of the magnetic north pole is dif-
ferent from the geographical north pole. This
causes a difference between the true and mag-
netic north direction. This difference is called
magnetic variation, and varies with respect to
the observation point on the earth. This varia-
tion may be entered automatically or manu-
ally.
Navigation calculation
The GPS receiver calculates the range, bear-
ing and cross- track error to next waypoint
when you select a destination. The calcula-
tion of that data is called navigation calcula-
tion.
NMEA 0183
The National Marine Electronics Association's
signal format which enables connection of
electronic equipment of different marine elec-
tronics manufacturers.
Plotting interval
The plotting interval determines both how the
track will be reconstructed on the display and
track storage time. The shorter the interval the
more accurate the reconstruction of track line,
however total storage time is reduced. The
plotting interval can be selected to time or
distance. Plotting by distance offers the ad-
vantage that the track is not stored when the
vessel is anchored.
S/A
GPS was developed by the US Department
of Defense mainly for use by its marine ves-
sels and aircraft. For civil users the accuracy
of the system is purposely downgraded for
national security reasons. This intentional ac-
curacy reduction is called S/A. Because GPS
position error may be greater than 100 meters
any GPS position should be double checked
against other sources to confirm position.
Time-to-go (TTG)
The amount of time necessary to get to a des-
tination, maintaining current speed and course.
Time differences
Time differences (or TDs) are the position
information generated by the Loran C and
Decca position-fixing systems. TDs are the
time in microseconds between the transmis-
sion of pulsed signals in the Loran C and
Decca systems.
Total distance
This is the number of miles from starting point
to end point in a route.
Trip distance
The distance run from a position.
Trip elapsed time
The amount of time passed since departing a
starting point.
True bearing
Bearing relative to North; compass bearing
corrected for magnetic deviation. The GPS
receiver can display true or magnetic bearing.
Velocity to destination
The amount of speed in the direction (course)
of the desired destination.
Waypoint
A waypoint is a particular location on a voy-
age whether it be a starting, intermediate or
destination waypoint.