[ntp:questions] Indirect GPS time source options

William Unruh unruh at invalid.ca
Thu Mar 13 19:06:44 UTC 2014

On 2014-03-13, Steve Kostecke <kostecke at ntp.org> wrote:
> On 2014-03-13, William Unruh <unruh at invalid.ca> wrote:
>> On 2014-03-13, Olivier Drouin <ol.drouin at gmail.com> wrote:
>>> Also, from what I understand and please correct me if I'm wrong but
>>> cellular networks are indeed designed to be accurately timed because
>>> it's needed for the normal operations of the cell network.
>> Yes, but they are not designed to deliver accurate time to the rest of
>> the world AFAIK.
> http://www.cdg.org/technology/cdma_technology/a_ross/systemtime.asp
> CDMA requires accurate time synchronization among all base stations and
> mobile stations. The accuracy must be within a few microseconds among
> base stations because the pilot code phase is used to distinguish them.
> When a mobile station is communicating with a base station they must be
> synchronized to within a fraction of a chip (814 ns). And the "clocks"
> (the PN generators) that must be synchronized have a period of 37
> centuries.
> http://www.endruntechnologies.com/gps-cdma3.htm
> GPS Absolute timing accuracy of unit is under 30 nanoseconds.
> CDMA Absolute timing accuracy of unit is typically under 10
> microseconds.
> Network timing accuracy 1/2 - 2 milliseconds, typical for both GPS and
> CDMA NTP products.
> Also see: http://www.endruntechnologies.com/cdma.htm

Thanks. By the way, network timing I have found is much better than
1/2ms. If I get the time from a gps and a stratum 1 source 2000 miles
away, the timing accuracy of that source is in the 50usec range. (the
delay is clearly much larger than that ). From a local stratum 1 source
(ie in the same building) it is more like 15usec.

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