[time-nuts] Lost GPS lock or 1PPS recently?

Bob kb8tq kb8tq at n1k.org
Sat Sep 8 11:35:25 EDT 2018


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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