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

[Dave Hart <hart@ntp.org>]

On Tue, May 22, 2012 at 2:07 PM, Ron Frazier (NTP)
<timekeepingntplist@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.

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?

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