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.
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