[ntp:questions] [Thumbgps-devel] good paper on timing and delay

unruh unruh at invalid.ca
Tue May 22 15:38:34 UTC 2012


On 2012-05-22, Dave Hart <hart at ntp.org> wrote:
> On Tue, May 22, 2012 at 2:07 PM, Ron Frazier (NTP)
><timekeepingntplist at techstarship.com> wrote:
>> The essential problem of time sync is to observe one or more remote time
>> servers, with variable and asymmetric propagation delays between you and
>> them, and choose what the best time to set your clock is. Obviously, not
>> simple.
>>
>> However, it occurs to me that the GPS receivers are doing essentially the
>> same thing via radio. They are observing numerous satellites at various
>> locations in orbit. Those satellites have precision clocks all calibrated to
>> within 100ns (or .1 us) of true time on Earth. The GPS receiver, cannot
>> "poll" the satellites, but it can observe their broadcasts. Each satellite
>> has a variable and at least somewhat asymmetric propagation delay.

However, the orbit of the sattelite is known exactly. Thus the propagation delay
can be calculated to nanosecond accuracy once the position of the
receiver is known, and that can be done by fitting the signals from
three sattelites. In the network situation, the delay is unkown and
uncalculable. Even if you knew exactly the path the signal took, there
are too many unknown delays (routers, switches, network cards, ...)

>
> The communication between GPS birds and receivers is one-way.  It is
> neither symmetric nor asymmetric.
>
>> That
>> propagation delay can be in the range of .25 sec or 250,000 us. Yet, the GPS
>> receiver can routinely output a PPS pulse with an accuracy of 1us or better,
>> taking the Garmin 18 as an example.
>>
>> So, the possibly simplistic question is, if our network time sync programs
>> used the same algorithm that the GPS receivers use to read their "servers",
>> ie satellites, which all have variable and perhaps somewhat asymmetric
>> propagation delays, which can be substantial, would we be able to achieve
>> much greater levels of accuracy doing synchronization via the internet?

No. 

>
> GPS birds tell the receiver the flight plans of all birds, so the
> receiver knows the dominant factor in the propagation delay, the
> distance between transmitter and receiver.  With WAAS reception, even
> more information about delay is provided in the form of atmospheric
> conditions.  NTP has a much more difficult row to hoe.
>
> Cheers,
> Dave Hart



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