Robertson AP21 and AP22 Autopilots
124 20220596E
8 GLOSSARY
ASF corrections, Loran - Additional secondary phase factor is
the amount in microseconds, by which the time difference of an
actual Loran signal that has travelled over varied terrain, differs
from that of an ideal signal over an all-seaweather path. (ie.
Loran signals travel slower over ground). The ASF correction is
a value added (or subtracted) to a measured Loran position, to
attempt to restore the position accuracy.
Chart Datum - A geographic reference system based on
mathematical models of the earth's shape is the basis for
mapping and charting positions in Latitude and Longitude.
There are a number of local reference datums, each with its own
mathematical model of the earth. Therefore, the position in one
reference system does not coincide with that determined from
another reference system. Each chart lists the Datum used.
COG - Course Over Ground - The actual direction of progress
of a vessel, between two points, with respect to the surface of
the earth, The vessels heading may differ from the course over
ground due to the effects of wind, tide, currents.
GPS - Global Positioning System - This system consists of 18
satellites plus 3 spares in fixed orbits, circling the earth at an
altitude of approximately 20,200 km. The system will provide
the user with 24 hour a day all weather position coverage, with
an accuracy of 15 to 100 meters.
Loran C - A complex radio navigation network developed by
the US coast guard, to assist a navigator in determining his
precise location. The acronym, Loran C, stands for Long Range
Navigation. It is an all weather 24 hour a day electronic system
of shore based radio transmitters.
Magnetic variation - A magnetic compass points to the
magnetic north pole. The difference between this direction and
true north is the magnetic variation. The amount and direction
of this variation is dependent upon where on the earth you are
located.
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