[time-nuts] Time in a cave

Oz-in-DFW lists at ozindfw.net
Wed May 13 09:19:54 EDT 2015


On 5/12/2015 6:00 PM, Tucek, Joseph wrote:
> I'm looking for information on non-GPS time sources.
>
> For background, I need to provide PTP to a cluster where we don't have line of sight to the sky, and are unlikely to get roof-rights without a fight.  There are CDMA solutions that would work (e.g. Endrun Technologies), but I was wondering if there were any other options.  I either need an indoor capable PTP, or an indoor capable PPS.  Microsemi claims to have an indoor capable "GNSS" system, but I've yet to find a sales rep to talk about it; if anyone has a link to one who can, I'd love to find out the problems^W^W^W^W talk to them about it.
>
> For an example of something that almost but doesn't quite work, Beagle Software has a CDMA NTP server, but they do neither PTP nor PPS in the CDMA version.  Similarly, Meinberg will sell a PTP unit that freeruns (if you override the config), but they have no solution to discipline via CDMA.
>
> I'm also curious if anyone has any idea about non-GPS time sync after CDMA gets turned off (can I get time from 4G?).
>
> My endgame worst case is to just do PPS from a stratum 2 NTP (or even a freerunning oscillator) and lie to my PTP server; hard sync to UTC is a secondary concern so long as the cluster agrees with itself.  Endrun is looking pretty good, but I'd really like to have a second option to compare against.
>
> -Joe
LTE and WCDMA both provide time, but it's a function of how carefully
the operators actually maintain it.  That's less of a problem today, but
there are no absolute /guarantees/, even with IS-95 CDMA.  The only
guarantee is that the basestations in the system will track within a few
microseconds of each other.

What you don't say is how much accuracy you need.  Is a hundred
milliseconds enough, or do you need sub-microsecond absolute accuracy?
How much holdover accuracy do you need? These are considerably different
probelms.  You indicate that you need to provide PTP to a 'cluster.'  Is
relative accuracy within the cluster all that's important, or do you
need to coordinate with the outside?  If so, there are a host of other
considerations.

Too many applications grossly over specify some requirements in the name
of safety and miss critical performance needs.  More information might
let the group a more complete answer. 

-- 
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Oz
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