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

[Bob Camp]

Hi

On Sep 16, 2013, at 9:48 AM, Jim Lux <jimlux at earthlink.net> wrote:

> On 9/16/13 6:03 AM, David J Taylor wrote:
> 
>> 
>> 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.
>> 
> 
> 
> It's trying to get nice flat group delay in the filter that causes all the issues with Light Squared.  *small*, *inexpensive* brickwall bandpass filters tend not to have nice delay properties, or at least ones that are temperature stable.  Spectrum regulators know this, of course, and assign adjacent services accordingly.
> 
> 
> If you're only worrying at the few nanosecond level, you probably don't have to take into account continental drift (periodic resurveys of location to account for several cm/year?) and solid earth tides (on the order of 30-50 cm).  And, really, for a lot of applications, you're interested in relative timing, so the solid earth tide shift of 1-2 ns every day isn't a big deal.

Well maybe it is….

If 

1) Your antenna is ~ 2 ns out of position
2) you have a small number of sats
3) Everything else is doing very well

Then as you take sat's in and out of the mix, you will could get 2 ns more "pop" than you would have otherwise. 

Bob

> 
> 
> 
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