I have a three channel LORAN-C receiver of my own design which I made over
thirty years ago. I built it for navigating my small yacht. I think the eLoran
signals are compatible though of course my receiver doesn't have the means to
extract the data channel.
Here follows a brief tutorial.
All LORAN-C stations transmit on 100kHz precisely. The signals come in groups
of eight pulses spaced precisely 1ms apart. Master stations transmit a ninth
identifying pulse spaced 2ms after the last of the 8 so they look like:
{●●●●●●●● ●} The phase of the RF is switched between 0° and 180° according to
a fixed pseudo-random pattern from pulse to pulse. There are two phase
patterns, the A pattern and the B pattern, and these alternate from one 8 pulse
group to the next. The Master patterns are different from the Secondary
patterns. Each LORAN-C chain has its own GRI (Group Repetition Interval) and
this is the number of ten's of μs from the start of one 8 pulse group to the
start of the next.
For precise navigation one needs to lock onto the ground-waves. If the receiver
is far from a transmitter the skywave can arrive very shortly after the
ground-wave, so it is essential to only use the start of each pulse. Officially
one is supposed to lock onto the third positive going zero crossing point on
the rising edge of each pulse. Close to a transmitter the skywave can arrive up
to 1ms after the ground wave so that it interferes with the start of the next
pulse. The phase coding is designed to avoid this causing trouble. The skywave
delay varies with time of day as the ionosphere goes up and down.
Because the wavelength is so long (3km), the near-field of the transmitter
extends quite a long way. That has the effect of changing the phase of the
received signal from what one might first think it would be assuming a fixed
velocity of propagation. The velocity of propagation is less than the
free-space velocity of light and is different between over-sea and over-land
paths.
My receiver just gives two relative time delays in μs and I used a TI59
calculator to convert to latitude and longitude. The program only just fits
within the capacity of the calculator.
I don't know anything very specifically about eLoran but I think the main
modification is to add extra phase coding on the pulses to form the data
channel.
John Ponsonby
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