[time-nuts] GPS vagaries and binary interface

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 28 18:57:11 EDT 2006

Didier Juges wrote:
> 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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An analog loop is somewhat impractical.
With a good OCXO the optimum loop attack time will be somewhere around 
1000 sec or so.
To keep the noise down the resistor values should be relatively low 
thuss you would need to use very large low leakage capacitors in the 
loop active filter.
Suitable capacitors are virtually unobtainable.
For such time constants a digital implementation is essential.
With an EFC adjustment range of 1E-7 of the OCXO frequency the DAC needs 
to be monotonic to better than 16 bits to ensure that OCXO is the 
limiting factor.
Since high resolution Audio DACs have adequate resolution and are 
relatively inexpensive, they should be ideal.
The phase measurement resolution should be at least 3 to 10X better than 
the PPS jitter to ensure that the GPS receiver is the limiting factor in 
measuring the phase error.
The simplest method is to synchronise the input PPS signal to a high 
speed clock synchronous with the OCXO frequency and then sample the 
state of a counter clocked at this same frequency with the leading edge 
of the synchronised PPS signal.

All you need do is choose a suitable processor that can easily interface 
to the DAC and the counter.
Some PIC chips have built in counters that can be sampled by the leading 
edge of an input signal.
With some care in the design one of these may be usable although 
interfacing to a serial input audio DAC is not that straightforward.


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