Loran-C (LOng RAnge Navigation) is an electronic aid to navigation
consisting of shore-based radio transmitters. The Loran system enables
users to determine their position quickly and accurately day or night in
any weather. Your position is determined by locating the crossing point
of two lines of position on a Loran-C chart. Most units today will give
you a direct readout in latitude and longitude, which will allow you to
plot your position even if you do not have a Loran overprinted chart.
Loran-C is generally accurate to 1/4 nautical mile. The Loran-C system
allows you to determine your position by means of radio signals
broadcast by stations of known position. A fix is determined by Loran
through the intersection of lines of position obtained by reference to
shore stations whose locations are known.
In Loran-C, you locate a LOP by determining the difference in time of
arrival of signals sent out by each of a pair of broadcast stations. This
interval is constant when the ship is located anywhere along a previously
determined Loran-C LOP. To say it another way: When the time
interval is a specific amount, the ship must be somewhere on a
predetermined Loran-C LOP that is a focus of all points where the
interval between arrival of signals is the same.
Most of the Loran-C receivers in use today provide the user with two
Difference Lines displays of fix information. The most commonly used is a readout of
the latitude and longitude of the vessels position. The other is a
readout of the time delay of each LOP, which can then be plotted on a
Loran-C overprinted chart. The steps for plotting the time difference
LOPS are described briefly below:
Examine the Loran-C chart for your area. LOP lines are
marked with time difference numbers; chain and secondary
identification also appear every few lines.
Read the time difference in the TDA display.
On the chart, locate the line that most closely fits that reading
for the secondary selected.
Examine adjacent lines and interpolate to determine where the
line that corresponds to your reading is to be plotted
Repeat steps 2 and 3 for the TDB display
Locate the point where the two lines cross and label the fix
with a small triangle along with the time affixed close by.