[time-nuts] Can eloran Backup GPS?

Bob Martin aphid1 at comcast.net
Sat Sep 8 17:16:25 EDT 2018


Bob,

That seems pretty conclusive to me but wait there's more..

By adding a letter to the name they are attempting to address the 
very issue you've raised.

https://rntfnd.org/wp-content/uploads/eDLoran-Reelektronica-Paper.pdf

  I'm sure after a few more prefix letters are added to Loran it 
will work for everyone!

Time for a new house to flip or dead horse to flog,

Bob Martin





On 9/8/2018 2:44 PM, Bob kb8tq wrote:
> Hi
> 
> The differential approach to eLoran involves running two local receivers. You look at the time of arrival on one
> and use it to “calibrate" the time of arrival on the other. Put another way - you look at the difference between the
> two arrival times. They can both “wander” over a 250 ns range, as long as they stay within 50 ns of each other
> they meet the “differential spec”.
> 
> For disciplining a local reference, you really need an absolute number. The fact that both are wandering over a
> pretty big range *does* matter if you are looking at a stable local source (and trying to make it more stable).  What
> would / does work is having a very accurate standard at one of the locations and using the difference measure
> to “distribute” that source. That gets into bandwidth.
> 
> Since the difference information is *very* local, there really isn’t a practical way to distribute it on the eLoran signal.
> As you pile on more correction stations, your data bandwidth goes up. There are a very limited number of bits
> available on the eLoran signal.
> 
> Another way to look at it: If you have a standard sitting in your basement, and don’t have a buddy in town with a
> better standard. Does a difference measure to his house do you any good?
> 
> Bob
> 
>> On Sep 8, 2018, at 2:58 PM, Bob Martin <aphid1 at comcast.net> wrote:
>>
>> 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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