[time-nuts] Req: Decent GPS AntennaActive/Passive Recommendation

[David J Taylor]

From: Bob Camp

Hi

Once the sat's all arrive at the antenna, a fixed delay (in general) does 
not impact things a lot. The gotcha with the filter is that it's delay is 
unequal across the GPS band. Doppler puts the sat's all over the place. That 
gives some more delay than others. Having unequal delay on sats is indeed an 
issue. As the filter changes delay with temperature things will move a bit 
more.  Since it's a "frequency high on approach / low on departure" sort of 
thing it will average out on each pass. The main impact would be an increase 
in wander.

Bob
====================================

Bob,

I am thinking about exact time measurement - getting your PPS edge exactly 
on the nanosecond.  People can add in the length of the cable as an offset, 
so they must also need to enter any delay through any filters, mustn't they?

Agreed that for position alone it doesn't matter as much.  It's the 
antenna's approximate position which will be measured.

Your points about dispersion in the filter, and temperature coefficient of 
delay are good ones.

Cheers,
David
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Email: david-taylor at blueyonder.co.uk