Bob,
I believe that information is transmitted with the eloran signal.
Way back when, I remember there was an added pulse called the LDC
pulse. I had to modify that pulse with each transmission based on
an input to the transmit timing unit from the computer.
I found the following on it:
http://www-personal.umich.edu/~tmikulsk/loran/ref/eloran_ldc.pdf
Also, the article referenced previously on The Great Britain
system mentions that the differential corrections are sent on the
LDC pulse.
To be honest, I don't know if this addresses your "gotcha".
Best,
Bob Martin
On 9/8/2018 12:38 PM, Bob kb8tq wrote:
Hi
The gotcha is the differential corrections. That’s not the way these systems
are set up to work. They
function with no external input other than the timing signal its self.
Providing bandwidth to do correction
signaling just isn’t part of the overall system design. If you wanted to use
bandwidth, you would go
with 1588. Then you have a backup and no fiddling with anything else.
Indeed with an area wide 1588, you can do it all without even a GPS primary.
Simply agree on a
“something” as the master source. The man with one watch *always* knows what
time it is ….
The 250 ns "without correction" is the number that directly compares to the ~10
ns number for GPS.
Stretch out the distances to “USA” sort of stuff and it does not improve things
at all.
Bob
On Sep 8, 2018, at 1:40 PM, Bob Martin <[email protected]> wrote:
Bob,
I agree that eloran needs to be analyzed with regard to it's
usefulness for each potential application. You are also 100% correct that
timing requirements get tighter and tighter as technology advances. In some
ways the question isn't whether eloran can
match GPS but rather would it suffice in a pinch were GPS to go down?
I think the 50ns accuracy is actually "as received" not "as transmitted".
The link below is an analysis of eloran in Great Britain. The
receiver/transmitter distance was 300 miles.
http://www.ursanav.com/wp-content/uploads/On-the-Uses-of-High-Accuracy-eLoran-Time-Frequency-and-Phase-2015.pdf
I've attached a screen capture of one of the pages that compares
eloran with GPS in case anyone is interested. This is where it
appears that the 50ns is received as opposed to at the transmitter.
Best,
Bob Martin
On 9/8/2018 9:35 AM, Bob kb8tq wrote:
Hi
I believe the 50 ns is the “as transmitted” signal from the tower. The “as
received” signal after going
through all the various gyrations is not that good on a ~1 second basis.
====
One of the gotchas here is that we lump “systems” into one giant bag. That’s
not a good way
to analyze things. One system may be quite happy with 10 ms timing another may
be happy
with 10 us and yet another may die completely at 1 us and only run right at 100
ns. All of that
is on a 2 second basis for CDMA (they time every other second).
By far the biggest / baddest / most venerable system out the that uses GPS
timing is the
cell tower system. They started out back in the 80’s with a 10us max timing / 1
us running
spec on CDMA. AFIK they were the first major system to adopt GPS time as their
reference
(rather than UTC).
This worked out fine for a few decades while companies got a lot of towers
built. People started
using those systems and they became congested. Others started streaming video
over them
and they ran out of bandwidth. Upgrades followed. There have been a lot of
them. Much of what
we TimeNuts buy on the surplus market comes to us as a result of older systems
being scrapped
out.
The latest set of upgrades does / will / is getting them into the sub 1 us
range at the end of holdover.
In normal operation they are spec’d at 100 ns worst case. To do that, you need
a timing source in
the roughly 10 ns range. No you don’t see those GPSDO’s on the surplus market.
You will see
them someday ….
Again, they went this way a decade ago. Rolling that all back …. not at all
easy.
Are there other systems that have issues with sync? Of course there are. There
also are a lot
of instances where miss-configuration ( or junk implementation) is a much
bigger issue. Sorting
that all out requires a deep dive into the timing of each individual system /
implementation. No
two systems do things quite the same way. Unless you want to deal with the
numbers and the
implementation details, simply moaning and groaning isn’t going anywhere.
Bob
On Sep 8, 2018, at 3:23 AM, Hal Murray <[email protected]> wrote:
[email protected] said:
You are not trying to run a cell system when checking your local oscillator
against LORAN.
The eLoran committee said 50 ns. Is that good enough for cell towers?
Too bad it isn't up so we could collect some data.
--
These are my opinions. I hate spam.
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