[time-nuts] backup to GPS after jan 2010, was: OT - GPS and North

[Bill Janssen]

Hal Murray wrote:
> stanley_reynolds at yahoo.com said:
>   
>> Further if at least one cell site has a accurate clock how far could
>> it be repeated before it lost it's useful accuracy using the CDMA
>> signal as in : 
>>     
>
> I think there are two cases.
>
> The first is if the GPS receiver on a single site breaks, I think it would be 
> reasonable to have that site lock on to it's neighbors.
>
> The second case is when GPS dies so that all CMDA sites need to coordinate 
> time without help from GPS.  Chains of PLLs are tricky.  If they didn't need 
> GPS they probably wouldn't have used it to begin with.
>
> I think you could do it if each site had a rubidium or something that was 
> stable enough.  I don't know what "enough" means.  I think the key idea is 
> something like the time constants for distributing the information have to be 
> much faster than the time constant for tweaking the local clock.  That is all 
> the sites have to agree on what to do and understand where they fit into the 
> plan.
>
> Early SONET ran into troubles with chains of PLLs.  I forget the details if I ever knew them.  I think they got the time constants wrong and ended up amplifying noise in a certain band rather than filtering it out.
>
>
> If you told me the stability of the local clock and the round trip times to adjacent cells and how much bandwidth I could use and such, I could probably come up with a stable algorithm and tell you how far and/or how many hops it would work over.
>
>
>   
I think that the site to site communications are via digital systems. 
All digital systems I know of
use buffers on the transmitting end and on the receiving end. So the 
time delay between sites can
vary depending on the buffer fill. The average frequency is controlled 
to tight limits but the time
delay can vary. So time transfer is not feasible without doing a delay 
measurement which takes
cooperation between the sites.

Bill K7NOM