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Before the advent of GPS, I had two Loran-C receivers aboard my 1980
sailboat (which I still own and love).   The first cost nearly $2000
because it had the new-fangled feature of displaying one's lat/long
instead of merely one's hyperbolic coordinates.  It was typically off by
a quarter-mile because it did not have ASF correction (if I recall
correctly, "Secondary Factor" correction in Loran-C compensates for the
fact that radio waves travel more slowly through air than a vacuum,
while "Additional Secondary Factor" compensates for the fact that the
conductivity of the earth (Loran-C waves hug the earth) slows them by an
additional amount; SF can be calculated but ASF has to be based on
on-the-ground measurements).

However, the error was quite consistent, not only repeatable but
consistent over 50 to 100 miles.  Many's the time I navigated
successfully for an entire day's journey in pea-soup fog.   As I used to
explain to people, "if you break the end of a yardstick, you can still
measure distance with it, you just have to remember that it starts with
2 and not 0."   In fact, I recall repeatedly logging the "slightly-off
as shown by the Loran receiver" position of a sea buoy and wondering why
I was getting a position that varied by 0.01 minutes or so -- until I
realized that the buoy was in 100 ft of water and was simply swinging on
its chain with the tide!

After 15 years of service the first set, filled with transistors and
coils, gave up the ghost and I bought a brand new one.   This was a
tenth of the volume and displayed lat/long spot on, thanks to built-in
ASF correction tables.  I still have it somewhere, waiting for Loran to
be resurrected.

GPS is a miracle of modern technology.   But I do wish there were a
reliable electronic backup....

   

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