inertial navigation system that measures the ship’s
motion in several planes and integrates the results
with a high degree of accuracy.
Although the
methods of dead reckoning may vary, they all share
the following characteristics: (1) the accuracy of the
estimated position never exceeds the navigation
method used to obtain the last fix, and (2) the
accuracy of the estimated position deteriorates over
time.
ELECTRONIC NAVIGATION
Simply put, electronic navigation is a form of
piloting. Piloting is the branch of navigation in which
a ship’s position is determined by referring to
landmarks with known positions on the earth. These
reference points may be bearing and distance to a
single object, cross bearings on two or more objects,
or two bearings on the same object with a time
interval in between.
Position in electronic navigation is determined in
practically the same way as piloting, though there is
one important difference—the landmarks from which
the ship’s position is determined do not have to be
visible from the ship.
Instead, their bearings and
ranges are obtained by electronic means.
The advantages of electronic navigation are
obvious. A ship’s position maybe fixed electronically
in fog or heavy weather that makes it impossible to
take visual fixes. Also, an electronic fix can be based
on stations far beyond the range of any local bad
weather.
Since electronic navigation is the primary form of
navigation in today’s Navy, the rest of this chapter
will deal with electronic navigation and the roles
played by the following systems:
1. Long Range Aid to Navigation (LORAN)
2. VLF Radio Navigation (OMEGA)
3. Ship’s Inertial Navigation System (SINS)
4. Navy Navigation Satellite System (NNSS)
5. NAVSTAR Global Positioning System (GPS)
We will also briefly discuss navigation radar,
surface search radar, and fathometers.
We will cover TACAN in chapter 2.
LORAN/OMEGA—TRANSITION AND
BASIC OPERATION
LORAN and OMEGA have been the “workhorse”
systems for many years.
However, they are being
phased out. Based on the DOD policy statement
reprinted below and because you may see a civilian
version aboard your ship from time to time, we will
simply give you an overview of the two systems. In
accordance with the 1992 Federal Radio navigation
Plan (FRP), NAVSTAR will become the primary
reference navigation system for surface ships,
submarines, and aircraft. The DOD requirement for
LORAN-C and OMEGA will end 31 December 1994
and TRANSIT will be terminated in DECEMBER
1996. Land-based TACAN and VOR/DME are to be
phased out by the year 2000.
LORAN BASICS
LORAN is a long-distance radio navigation
system used by ships at sea to obtain a position fix,
The system is based on the difference in the transit
time required for pulsed radio signals to arrive at the
LORAN receiver from multiple, synchronized,
omnidirectional ashore transmitters.
LORAN also
takes advantage of the constant velocity of radio
signals to use the time lapse between the arrival of
two signals to measure the differences in distance
from the transmitting stations to the point of
reception. The receiving set provides a direct reading,
in microseconds, of the time difference in the arrival
of the signals. (Some sets automatically convert the
readings into latitude and longitude.) When the time
difference is measured between signals received from
any two LORAN transmitter stations, a ship’s line-of-
position (LOP) can be determined.
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