GPS card for laptops synchronisation

[davide]

I'm planning a lightning measurement campaign for next summer.
I will measure the electromagnetic field radiated by the same lightning strike at two different locations A and B, so I need to synchronize the laptops installed in A and B with the GPS time.
Since each lightning flash is usually composed by several subsequent strokes (each one with a typical duration of a hundred microseconds) which are several milliseconds spaced, I need a sub-millisecond time resolution from the GPS card.
Looking at the specs of GPS units, resolution is frequently expressed in meters. Does anyone know how to convert that into time resolution?
Does anyone have experience on that to suggest me which CF-type GPS card could be suitable for my application?
Thanks a lot.
Davide

[lbendlin]

the spatial resolution has nothing to do with your problem. All you need is the time reference.

There are two issues that come to mind.

a) the GPS time is pretty accurate but it is only coming out of the GPS receiver every second! So your best precision is 1 Hz.

b) the time from different GPS receivers will be different by definition. Hell, that's the whole idea of GPS!

What you can do is ignore this delta and synchronize your laptop clocks with the GPS clock. Then you can use the sub-millisecond accuracy of your operating system (there are some low level API calls available for that). Be aware though that any and all programmatic approaches will again skew the results (other runnung processes, disk access, NMIs etc). Ideally you would want to run a real time OS on your laptops...
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Lutz

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[davide]

Thanks for your suggestions, Lutz.
What I meant actually was that I need a time reference, as you have well expressed, that's why I'm looking for GPS cards.
The idea is that when the digitizer triggers on a lightning signal, the laptop labels those data with the current GPS time. I will see how to do that in the LabVIEW routine that I'm using to control the digitizer.
Do you have any suggestions regarding which PCM-CIA card should I purchase? Let me add that I do not need powerful navigation tools, but rather a strong reliability, since my system is supposed to operate non stop for the entire summer, in order to catch the highest number of lightning strikes. I read something about an internal battery of the GPS cards, is it something that I should worry about if I'm planning a three months operation time?
Thanks again.
Davide

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