[LEAPSECS] Executive Order on Strengthening National Resilience through Responsible Use of Positioning, Navigation, and Timing Services

[Tom Van Baak]

 >> 2 (or more) Loran-C stations gave you more accurate time. But if you
 >> know where you are, and since the stations don't move, you can
 >> manually adjust for signal propagation delay to some extent. This is
 >> not unlike how one obtains time from WWV or WWVB.
 >
 > One of the big issues with LORAN C, I thought, was that those
 > variations were measured in microseconds...

Right, the variations are huge compared to GPS. Microseconds (Loran), 
even milliseconds (WWV*). So that's why when people talk about a "source 
of UTC" the accuracy should be specified. Otherwise a even wristwatch or 
sundial could be considered a source of UTC.

 > But I guess those average out over time so if you have a stable local
 > oscillator you can recover time fairly well.

Before Loran-C was canceled and before the WWVB format was "enhanced" 
there were a number of disciplined time & frequency products that did 
just that. Stuff from Austron, Spectracom, and SRS. Some of us time nuts 
have this gear.

 > Fun fact, LORAN has no leap seconds (since it is just a bunch of
 > different rates). So to calculate time of coincidences with UTC you
 > had to basically use TAI - 10s since the LORAN signal is paced with
 > SI seconds. When I was working on the replacement timing system for
 > the LORAN chains, this was a big deal since to start LORAN you needed
 > to know both the UTC time and a recent / near future table of leap
 > seconds... This leads to a lot of edge cases, especially when you
 > needed to cope with the cold spare GPS reciever case. :(. It's also
 > where all the love I feel for leap seconds left my body...

Yes, exactly. BTW, Loran TOC (for the 9940 chain) is available here:

http://leapsecond.com/java/gpsclock.htm

USNO used to publish Loran-C TOC tables for all the chains on their web 
site. Last year most of their user-facing pages went away.

/tvb