[time-nuts] eLoran test

John Ponsonby jebponsonby at gmail.com
Fri Feb 3 12:58:39 EST 2017


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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