[time-nuts] GPS: ADEV or MDEV?

[Dr Bruce Griffiths]

Hal Murray 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. 
>>     
>
> It doesn't look reasonable to me, but I'm not good at this sort of math.
>
> The clock is, say, 40 MHz.  That's 25 ns, so the output jitter will be 25 ns 
> p-p.
>
> If you are at the top of a hanging bridge, you have to add enough noise to 
> hide a 12.5 ns offset.  Will a typical GPS receiver still work with that much 
> trash on its clock signal?  What sort of noise spectrum would you use?
>   
Hal

The noise only really needs to be added to the PPS positioning 
algorithm/hardware.
In principle (if you can get at the firmware) it would be possible to 
implement noise shaping using a sigma delta modulator (possibly with 
added noise dithering to break up idle patterns - like hanging bridges) 
so that the short term PPS positioning noise increases but the average 
position is more stable. This may be useful when a long time constant 
PLL is used to discipline an oscillator with the PPS output of the receiver.

Bruce