[time-nuts] lightening protection of a GPSDO system / optical isolated distribution amp

Bob Camp kb8tq at n1k.org
Sun Dec 7 14:49:49 EST 2014


The 12 hour periodicity of GPS is (in general) less obvious in an ADEV plot than 24 and 48 hour effects. Part of this is due to the lower “floor” at longer tau. Another part of it is due to things like the  ionosphere being at different places at the 12 hour points. 


> On Dec 7, 2014, at 2:27 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
>> Bob,
>> can you explain more about the effect of antenna performance on a GPSDO
>> system?
>> Now you have told me it is important,  I would like to know more! My lab
>> has an East facing window.
> Physically what is happening here is that the error bars on the fix your
> GPS gets will cycle over a 12  hour period because of the sats all have 12
> hour orbital periods.   If you can only see part of the sky the amplitude
> of this is greater.
> As an example thing of the worst possible case where you antenna can only
> see a few degrees of the sky.  Every 12 hours one GPS satellite comes into
> view and your GPS gets a decent "fix" but then for 8 hours the GPS sees
> nothing and drifts off.  Now think about moving the antenna to a marginally
> better place where it always sees at last ONE GPS stiletto but for 8 hours
> there are two and for 2hours there are three satellites.   The quality if
> the "fix" would still vary but would be better.   In the best case your 12
> channel GPS receiver ALWAYS is able to select the 12 BEST places GPS
> satellites that are in view.
> This is not a great effect as long as there is enough sky that there are
> always some in view.
> Don
> t worry about listening strike on your antenna.  If is FAR MORE likely that
> lighting will strike some utility pole within 1/4 mile of your house and
> the surge will come in through the AC mains power.  So if you want to fix a
> problem fix that one first, then worry about less and less likely things.
> This is not to say not to take normal precautions and ground the iron pipe
> with a $7 aluminum ground wire just like you would do to an old fashioned
> TV antenna.
> -- 
> Chris Albertson
> Redondo Beach, California
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