[time-nuts] Can eloran Backup GPS?

Bob Martin aphid1 at comcast.net
Sat Sep 8 14:58:27 EDT 2018


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 <aphid1 at comcast.net> 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 <hmurray at megapathdsl.net> wrote:
>>>>
>>>>
>>>> kb8tq at n1k.org 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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