[time-nuts] GPS: ADEV or MDEV?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Apr 13 04:32:17 EDT 2007


Peter Schmelcher wrote:
>>>> Surely it would be much simpler (in principle at least) to just to add
>>>>  sufficient Gaussian phase noise to the GPS receiver clock so that
>>>> potential coherence problems are virtually eliminated.
>>>>         
>> The noise only really needs to be added to the PPS positioning
>> algorithm/hardware.
>>     
>
>
> My two cents. Masking the sawtooth is bad news. If you log the pps output 
> from a Z3816A you get the incorrect impression that the OCXO and/or the 
> atmosphere is unstable for time spans of tens of minutes. My Z3816A pps 
> logs look typical sd=10ns a few 50ns spikes the EFC ad=2 sd=5 etc. But use 
> the OXCO to synthesize a low noise LO for a second gps and the pps becomes 
> very stable a couple of ns. In the Z3816A the gps LO crystals 0.1 second 
> tau and the sawtooth interact and do bad things. It is best to eliminate 
> the problem and then to move on to new things. I am attenuating my HP 
> hockey puck antenna approximately 10dB to remove SV with range instability 
> from the gps pps solution. The current setup acquires SV at about 30 degree 
> and can then track them down to about 20 degree. The idea is that transit 
> time instability modulates the SV signal requiring a greater tracking 
> filter bandwidth which reduces the SNR hopefully below tracking. It is not 
> perfect - testing continues.
>
> Peter 
>
>
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>   
Peter

I only intended to indicate that not all GPS timing receivers need 
exhibit hanging bridges.
The Trimble Resolution T for example may (or may not) exhibit hanging 
bridges.
If anyone has any evidence either way it would be interesting to look at it.
If one also logs the PPS correction then you haven't lost any 
information due to the randomisation of the PPS error.

Surely its better to detect the pseudorange instability in software (if 
one has access)  rather than attenuating the signal from all SVs.
Another option is to use multiple GPS antennae operating as a phased array.

Bruce




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