[time-nuts] GPS 18 behavior

[Brian Alsop]

Have you considered WWVB?  Works fine within structures.
Even though the carrier today is phase modulated one can probably glean 
1 ms accuracy from it or the data transmitted.

Regards
Brian

On 7/23/2013 14:05, Magnus Danielson wrote:
> 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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