Re: Slow convergence of NTP with GPS/PPS

Unruh schrieb:
nb@xxxxxxxxxxxxxxxx (Nicola Berndt) writes:

Richard B. Gilbert schrieb:
David Woolley wrote:
Richard B. Gilbert wrote:

To turn your equipment on after months of downtime and expect it to
lock on to the correct time with millisecond accuracy within seconds
is asking for a hell of a lot.
Not really. He's starting a GPS receiver at the same time and that has
to lock to 50ns.

Doing it on a general purpose computer is more difficult, but not
particularly impossible.
Even with GPS and a full four satellite fix, ten seconds to synchronize
is extremely ambitious!! You can set the time to within whatever
precision the hardware and software support but that is only half the
problem. You also need to set the correct clock frequency. On a cold
start, the clock frequency is a moving target as the hardware warms up.

I would expect to wait at least thirty minutes for the system to
stabilize with both the correct phase (time) and frequency.

To transfer the full almanac of GPS it takes roughly 12 minutes from a
cold start. Then the receiver knows everything there is for it to know.
Some receivers (like mine) you can tell it's location, wich gets you in
the 10 s range for precise time. Then again, who claimed, it has to be
10 s? I would be very happy with these 12 mins..

For some receivers if they know their position, they can get the time
virutally instantly from "cold start". All you need is one sattelite. If the receiver has no idea where it is, it can take much longer.
Whether or not the receiver the OP has has that
capability I do not know.


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Hi,

U-Blox state on page 24 of this -
http://www.u-blox.com/customersupport/docs/GPS_Compendium(GPS-X-02007).pdf
- document a rate of 50 bits/second sent out by the satellites and a
time of 12.5 mins to transmit the full almanach. I don't know, if really
the entire almanac is needed for precise time, but I'd actually suspect
that.

Regards,
.../nico
.

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