[time-nuts] GPS vagaries and binary interface

Didier Juges didier at cox.net
Sat Oct 28 18:15:30 EDT 2006

Dr Bruce Griffiths wrote:


> Good timing antennas have built in ceramic or equivalent bandpass 
> filters to minimise the effect of interference.
> A patch antenna is not as satisfactory as a quadrifilar helix or a choke 
> ring ground plane antenna for accurate timing purposes.
> If GPSDO did some statistical filtering instead of just blindly 
> accepting all PPS signals as valid and usable such dropouts would cease 
> to be much of a problem.
> There's no substitute for a a correctly engineered design with an 
> appropriate tracking loop bandwidth and statistical filtering of outliers.
> A good crystal will drift very little over half an hour or so when the 
> GPS derived PPS signal may be unreliable.
> Bruce
> _______________________________________________
That's the impression I am getting. I do not know if any of the GPSDO 
that I have seen described in recent literature take care of this properly.
It seems when the GPS goes nuts, the 1 PPS goes quite a bit out of 
normal range, so it should not take too much processing power to 
determine if it's in range or not.
Of course, an analog solution would require many more parts to do that 
determination, filtering and switching, so it seems the most *practical* 
way to implement a GPSDO is with a uC of some sort.
The uC could even monitor what's coming out of the GPS receiver's serial 
port and open the loop if there are not enough satellites in range.

Now, about the Jupiter receiver, it seems the only way to set a mask 
angle is through the binary interface that is not well documented at all 
in the Navman documentation. They do not even say if the mask is saved 
to flash or EEPROM, or if it has to be reloaded each time the GPS is 
powered up. That would also require a uC.

I would be really grateful if someone had done that research and could 
help me.

In the mean time, I may just put the spectrum analyzer at the output of 
the patch antenna (with proper biasing of the LNA of course) and see if 
I am getting junk other than the GPS signal. I know the GPS signal will 
be too low for the spectrum analyzer, but if I see anything else between 
1 and 2 GHz, I know that would be a problem. I live about 5 miles from 
the largest US air base (Eglin AFB) and I am sure they have L-band 
radars running there, and probably a bunch of other ether pollutants. 
Assuming the radar signal is not swamping the LNA, a good filter could 
not hurt.

Didier KO4BB

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