3
Phase Coding
LORAN transmissions are Phase Coded to minimize the effects of
random noise, CW signal interference, and skywave interference.
Some of the pulses in the eight pulse group are transmitted with the
carrier signal inverted with respect to the rest of the pulses. That is,
some pulses are transmitted with the first cycle of the pulse starting by
going negative instead of positive. The phase code repeats in a two
GRI period called a Frame. A receiver, such as the FS700, that takes
phase coding into account will cancel any interference that occurs on an
interval longer than a frame. Also, the phase coding is arranged so that
any long delayed skywave interference will also be cancelled. The two
GRI pulse groups within a frame are called Group A and Group B and
have different phase codings. Additionally, master station frames have
a different phase coding than secondary station frames. The phase
codings for the various frames are shown in Figure 2.
Figure 2
Phase Codings of Master
And Secondary Stations
Signal Propagation
The transmitted signal from a LORAN transmitter is split into two parts:
the groundwave which travels parallel to the surface of the earth, and
the skywave, which travels upward through the atmosphere, is reflected
by the ionosphere, and returns to earth. Because the height of the
ionosphere depends on the time of day, the season, and solar activity,
the skywave propagation path and the propagation delay are very
unstable. This makes the skywave transmission less than ideal for
accurate timing. The groundwave transmission does not suffer these
problems and is a very stable source of timing information. However,
the groundwave signal is rapidly attenuated by the atmosphere and
suffers contamination from the skywave signal which arrives slightly
later in time. These considerations limit the groundwave reception
range of the LORAN signal to about 1500 miles. At this range the
skywave interference will never disturb the position of the PTR.
Blink Transmissions
Occasionally a LORAN transmitter will malfunction so that its
transmitted signal is no longer accurate. When this occurs the master
station and the affected secondary station begin blink transmissions.
The master station will blink its ninth pulse in a coded pattern indicating
which secondary is bad, while the secondary station will blink its first
two pulses in 0.25 seconds on, 3.75 seconds off pattern. The other
pulses in the transmission are unaffected. By checking for blink the
FS700 is able to ensure that it is locked to a healthy transmitting station.
Additional Information
More detailed information may be obtained from:
Group
Master
Secondary
A
++--+-+-
+
+++++--+
B
+--+++++
-
+-+-++--
