[time-nuts] GPS: ADEV or MDEV?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Apr 10 01:49:03 EDT 2007


Poul-Henning Kamp wrote:
> In message <004e01c77b09$d095aa60$6500a8c0 at EBS8500>, "Brooks Shera" writes:
>
>   
>> The impact of time averaging  to suppress white phase noise is illustrated
>> by a new plot TVB has created and placed on his website
>> http://www.leapsecond.com/pages/3gps/gps-adev.gif.
>>
>> These revealing plots show that just a few hundred-second time average has 
>> largely removed the performance
>> difference between the aged VP and the newer M12+ and has almost entirely
>> removed the benefit of sawtooth correction (CNS-CNS II).  These important
>> results are not evident in the usual ADEV plot.
>>     
>
> This is simply not true.  No averaging period can ever guarantee you
> will not get a constant offset from the sawtooth.  You may get rid
> of the noise, but not the bias.
>
>   
Poul-Henning

Not to mention the case of the GPS receiver with a pathologically stable 
local oscillator frequency that generates lots of "hanging bridges" and 
little sawtooth.
Since software sawtooth correction is virtually free why not do it, when 
such data is made available by the GPS receiver?

Hoping that the local oscillator frequency is sufficiently unstable 
isn't good engineering, one should design to accommodate the worst case 
possibilities.

Bruce





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