[LinuxPPS] time compensation

[Paul]

Thanks for that. I see the difference in the specs from the modules
which are designed primarily for navigation.

On Sun, 2010-06-06 at 20:49 +0200, Javier Herrero wrote:
> Oncore M12M is a GPS timing receiver capable of providing +/-25ns 
> accuracy on its PPS signal, before sawtooth correction (that would 
> increase the accuracy to +/-2ns), and it is priced below $100 if I'm not 
> wrong. It is manufactured by i-Lotus, and it is a derivative of the 
> Motorola M12T (i_Lotus purchased the GPS branch from Motorola).
> 
> And you also can populate a 1U 19" drawer with several tens of it :) 
> http://www.rabel.org/archives/Motorola_Oncore/m12m%20users%20guide%201.0.pdf
> 
> Best regards,
> 
> Javier
> 
> 
> 
> 
> Paul escribió:
> > Can this accuracy be achieved with 'consumer' GPS modules? Or are such
> > accuracies only available with 'professional' units (you know the things
> > I mean 19" rack and costing tens of thousands dollars, pound euros?)
> >
> > On Wed, 2010-06-02 at 17:27 -0600, clemens at dwf.com wrote:
> >>> On Wednesday 02 June 2010 12:05:47 am Paul wrote:
> >>>> Hmm, delay in the attenna cable? Your GPS unit will just think it is in
> >>>> a different point in space. 
> >>> This is true for non-timing specific GPS units.  But timing specific units like 
> >>> the Oncore VP, UT, UT+, M12T... are setup to have the antenna position set 
> >>> based on a site survey.   This is typically done by averaging the positions 
> >>> gathered by the GPS over a several hour period (typically about 10,000 
> >>> positions).  This averaged position is perhaps with in 0.5 meters of the 
> >>> actual position of the antenna if reception is good at the time of the survey.  
> >>> Users that require the most accurate possible timing information (IE. 
> >>> astronomers) will typically pay to have a survey crew measure the antenna's 
> >>> position.  This surveyed position is then coded into the ntp configuration file 
> >>> for the receiver and this position is fed to the receiver by the driver.  By 
> >>> also giving the driver an antenna to receiver cable delay, which is also fed 
> >>> to the receiver by the driver, the receiver can compensate for the delay.  
> >>>
> >>> One other benefit of having a surveyed antenna position is that the receiver 
> >>> can give accurate timing with only one satellite being tracked where as a non-
> >>> timing specific GPS needs at least 4 tracked satellites to give accurate time 
> >>> data since it also needs to calculate it's position in order to calculate time 
> >>> information.
> >>>
> >>> Keep in mind that these timing specific receivers can, if properly setup, put 
> >>> out raw PPS signals that are accurate to with in 12 to 50 nanoseconds (newer 
> >>> models are more accurate) and the majority of this error is what is known as 
> >>> saw tooth error (this is related to the granularity of the oscillator on the 
> >>> receiver) for which the receiver can calculate a correction factor with the 
> >>> high degree of accuracy. in addition these receivers also feed saw tooth 
> >>> correction data to the ntp driver and I think the driver uses this to correct 
> >>> for the saw tooth error.  Reg is this correct?
> >> Yes.
> >>
> >>>  Also do we lose this capability using the kernel consumer?  
> >> Probably not.
> >> It would depend on how the kernel consumer is coded.
> >> But instead of just USING the saw tooth correction in the user-code 
> >> calculation,
> >> the ONCORE driver passes this offset to the kernel and lets it do the 
> >> arithmetic.
> >> As such it has the corrected timestamp for its own use.
> >>
> >> <snip>
> >>
> >
> >
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