LORAN-C Frequency Measurement Capabilities

Although GPS has taken over as the primary means of precise time and frequency dissemination, another source of high-precision frequency data is the LORAN-C navigation system. LORAN can provide results almost as accurate as GPS, and has some advantages in terms of robustness; the main disadvantage is that LORAN coverage is not universal. LORAN timing receivers, like the Austron 2100F I recently acquired, are sometimes available on the surplus market for a few hundred dollars. However, it should be possible to extract frequency information from the very inexpensive LORAN navigation that are widely available both in marine stores and on eBay.

Here are some plots made from data captured over two-plus weeks from the station at Dana, Indiana, about 175 miles from my home. This data came from the phase recorder output on an Austron 2100F receiver, which shows the offset between an external reference oscillator and the Loran signal. I recently acquired an Austron 2100T receiver, which provides a PPS output in addition to the functions of the 2100F. Here is some information about the 2100T.

The first plot shows the relative phase data using a GPS-steered crystal oscillator as the reference. The GPS oscillator should be about an order of magnitude more stable than LORAN, and over time reflects the stability of the GPS satellite constellation, which is pretty darn good. So, what this data shows is the accuracy and resolution of the LORAN receiver system and the signal it receives, rather than that of the local frequency reference.

The data above shows that over the two week period, the "time" of the LORAN clock stayed within plus or minus 100 nanoseconds of GPS virtually all the time, with a few spikes that were larger, and one spike at the beginning of the run where the time excursions went to both plus and minus 450 nanoseconds. Since all the excursions (except the one at about 10.5 days) show very brief excursions of about the same amount both plus and minus, I suspect these are the result of local noise bursts that disturb the receiver. My antenna isn't what it should be, and it will be interesting to see if these bursts continue when I get a "real" antenna up this spring.

LORAN-C is often quoted as being able to provide frequency measurements to about 1 part in 10-12 over a measurement period of one day. The frequency stability charts below show that's pretty close to the case, and that resolution increases fairly linearly as the averaging time increases.

The top chart shows frequency stability using the traditional "Allan Variance" calculation, while the bottom chart uses the "Total Variance" calculation, which allows calculation at longer averaging times with reduced uncertainty. (The "tau" value listed in the chart is the averaging time; a tau of 1 means the stability of successive measurements averaged at one second intervals.)