[LinuxPPS] time compensation

[clemens at dwf.com]

> 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>

-- 
                                        Reg.Clemens
                                        reg at dwf.com