10
The entire search process takes between 15 and 40 minutes depending
on the signal-to-noise ratio of the station selected.
It is important to understand that at large distances (>1000 miles) from
the LORAN transmitter, the LORAN skywave signal can be many times
larger than the groundwave signal. The FS700 can detect and correctly
handle this condition until the skywave is about 5 times the groundwave
amplitude. At this point, the FS700 will most likely lock to the skywave.
This results in poor long term stability because of the diurnal shifts in
the skywave timing. If this occurs, a different station should be chosen.
Locking to LORAN-C
Once in LOCK the FS700 keeps the frequency difference between the
internal oscillator and the LORAN transmission at a minimum. This is
accomplished by using a software frequency-locked loop (FLL). The
frequency-locked loop adjusts the internal oscillator's frequency so that
the frequency difference at all times is zero. This is different than a
phase-locked loop (PLL). A PLL attempts to always keep the phase
difference at zero, and may introduce a large instantaneous frequency
offset to change the phase. Thus, a FLL will have better phase noise
and short-term stability than a PLL. The time constant of the FS700's
FLL is set according to the signal-to-noise ratio of the signal, and is
generally about 2000- 5000 GRI. The FS700 also has an auxiliary PLL
that keeps the phase difference between the FS700 and the LORAN
signal small. The time constant for the PLL is much longer than the FLL
time constant, and has no effect on short-term stability (the frequency
offset due to the PLL averages to zero and is rarely instantaneously
larger than 1-2 x 10
-11
).
Signal Errors
During locking, the FS700 monitors the received signal and checks for
error conditions. If an error is detected, the FS700 will halt its FLL (to
prevent erroneous frequency adjustments) and wait. If the error
condition goes away within 20 minutes, the FLL will restart and will
continue as before. If the error does not go away, one of two things
may happen. If the search mode is set to manual, the FS700 will
terminate its lock. If the search mode is set to auto, the FS700 will
attempt to reacquire a station. If the station selection is set to a
specific station (not auto station selection), the FS700 will continue to
try to acquire the station selected until the station returns to health.
