[time-nuts] Changes in commercial GPS clocks over the decades

[The Fiber Guru]

Thanks, it is used by Middle and high schools across the country, tech schools and many other businesses.  The goal is to get kids excited about technology but we find a lot of people just want to know more about a technology that touches their everyday life.  Glad you enjoyed it.

db

> On Oct 31, 2020, at 5:24 PM, Bryan _ <bpl521 at outlook.com> wrote:
> 
> Great tutorial read on fiber in your website.
> 
> -=Bryan=-
> 
> ________________________________
> From: time-nuts <time-nuts-bounces at lists.febo.com> on behalf of The Fiber Guru <fiber.guru at fiber.guru>
> Sent: October 31, 2020 9:29 AM
> To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] Changes in commercial GPS clocks over the decades
> 
> Yes, prior to use of GPS to discipline the Local oscillator, telecom timing was a “trickle down” topology where a cesium source in Kansas City was distributed across the continent.
> 
> The cesium was the gold standard and as the timing signals cascaded to distant geographical regions, it was obviously less pure, so if cesium was at Stratum 1 (stability), the next element in line that mediated time of downstream was at Stratum 2.  As it arrived at your local telco central office, it was at Stratum 4 (ok, but useless in today sadly networks).
> 
> Enter GPS and instantly every local central office could achieve Stratum 1 traceability, and if GPS was lost the best Rubidium's could holdover at Stratum 2e (slightly better that Stratum 2).  If the clock had OCXO, holdover was Stratum 3 or 3e depending on the quality of the oscillator.
> 
> Stratum levels are reported to most network elements by embedding the Stratum value message in the Facility Data Link of an ESF T1 signal.  The network element would read the Stratum level from the incoming timing signal to determine if it should lock to the signal, or fall back to its internal clock, usually an ocxo at Stratum 3.
> 
> If the master GPS clock suffered loss of GPS and the backup oscillator deteriorated to a low stability, the clock would generate a message to the connected elements that said “Don’t Use Me” (DUS).  This method is called, “Sync Status Messaging/SSM”, and also carry’s over to the latest packet timing designs so that subtending elements always know the pedigree of a timing/sync signals.
> 
> It is notable that, while SSM provides a label as to the purported pedigree (stableness) of the timing signal, it is no guarantee the signal is actually as stable as reported, but generally it is truth.
> 
> This has been my world for several decades so please let me know if this information is not usef