[time-nuts] GPS 18 behavior

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jul 23 10:05:05 EDT 2013


On 23/07/13 05:55, Jim Lux wrote:
> On 7/21/13 6:42 PM, Chris Albertson wrote:
>> I think the way to keep the sensors in sync is to use the same method
>> they use to keep cell towers in sync. Basically each tower has a GPS
>> receiver and also a good local oscillator. The GPS disciplines the
>> oscillator and the timing is taken from that oscillator, not directly
>> from the GPS. If the GPS signal is blocked the system continues
>> normally however the oscillator may drift without the connection to
>> GPS. Then later when the GPS is available again the oscillator is
>> corrected. The system can run for a few days in holdover with no
>> GPS connection.
>>
>> I think you were talking about a system that switches to a backup 1PPS
>> signal.
>
>
> Today, I have a system with multiple modules physically connected by a
> cable that need to be sync'd to maybe 1 millisecond. I was thinking
> about using the 1pps from the GPS as the sync, if it was available all
> the time, even in GPS denied areas; that would make it always use the
> same sync from the same device, even if it's not synchronized to some
> external time scale. Since the GPS receiver doesn't put out the 1pps all
> the time, I can sync another way, and drive that sync process from the
> GPS if it's available.
>
> The long term system will have multiple modules separated by some
> distance that need to be synced and frequency disciplined, but they
> might have GPS, so that could be used to discipline a clock in the usual
> way. As a practical matter, I'm more a fan of using GPS for knowledge
> and adjust the output using a DDS rather than steer the oscillator,
> because that allows a higher Q resonator, but that's a matter of
> engineering details.
>
> There's also the situation where you're totally GPS denied, but that's
> an even more tricky problem.
>
>
> That is not the way to do it. The GPS should discipline a
>> 10MHz crystal (or whatever) and then you divide that by 10,000,000 to
>> get your 1PPS. Then when the GPS fails there is no interruption, no
>> mode change. Such a system only needs to have access to GPS now and
>> then. So if you have to go under a pile of concrete and loose access
>> to the sky there is no "hiccup". This would work for the distributed
>> system too. Your 1E-11 over 10 to 100 seconds would be met even if
>> GPS were out for a few hours.
>
> Yes, that would do. It turns out, though, that although you could
> tolerate a slow drift, assuming you can figure out what it is, it makes
> life harder if the two modules have drifted 1E-9 relative to each other
> after 10 minutes.

The movie-business have similar problems, so a sync-ones and keep drift 
low system emerged to make field recordings easier.

If it would be tolerable to have a "central" transmitter, putting a PN 
code over a voice radio system would suffice to keep the drift fairly 
well kept together for this form of system. If you choose to do it on 
the audio channel, then you can use of the shelf radios, and replace 
those or re-program those as needed. Also, they are dirt cheap nowdays.

Cheers,
Magnus


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