[LinuxPPS] time compensation

[Javier Herrero]

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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>

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Javier Herrero                            EMAIL: jherrero at hvsistemas.com
HV Sistemas S.L.                          PHONE:         +34 949 336 806
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