[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 14 18:55:02 EST 2006


bg at lysator.liu.se wrote:
> On Thu, December 14, 2006 23:07, Dr Bruce Griffiths said:
>
>   
>> Thus devising inexpensive phase detectors/TICs with subnanosecond
>> performance allows one to take advantage of improvements in GPS timing
>> receiver performance when they occur.
>>
>> The possibility of utilising GPS carrier phase tracking techniques in a
>> timing receiver offers a potential timing resolution and jitter in the
>> picosecond range which would allow enhanced  GPSDO performance.
>> Alternatively one could then achieve much better performance with less
>> expensive oscillators. Currently dual frequency GPS geodetic receivers
>> achieve subnanosecond resolution and stability when the data is
>> processed, albeit not in real time.
>>
>> Bruce
>>     
>
> I have had an idea for some time, even have the hardware pieces since a
> year or more. Wish there were more time to spend on realizing projects...
> :-( Its not very original, but I have not seen it explored in any GPSDOs.
> Why not do the phase detection/frequency measurement inside the GPS
> receiver?
>
> Find a GPS that can be driven by your VCOCXO. (Zarlink's GP4020 accept
> 10MHz. CMCs Allstar and Superstar receivers are still available.) Control
> the oscillator softly enough that the tracking loops will not unlock.
>
> PLL augmented code measurement noise is in the low dm region for a good
> receiver design. Using a known surveyed position this would give one sub
> ns measurement for each satellite tracked. And then there is the phase
> measurements that should be usable in some sense. Other information that
> is available for a static receiver with internet connection is the
> ultra-rapid ephemeris and clock-parameters that are available for
> surveying use. These are much better than the broadcasted ephemeris. It
> appears as if this concept would open performance enhancement
> opportunities that are not used by the current OEM GPS timing receivers.
>
> This structure would not need to generate the 1PPS from the GPS, and there
> is no need for an external phase/TIC. It does instead ad an adaptable
> amount of software complexity to solve the GPS time error outside the GPS.
>
> What is the catch, that leads every(?) GPSDO designer along a different path?
>
> --
>
>    Björn
>
>
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>   

Björn

You will need a dual frequency receiver to more accurately correct for the ionospheric delay.
Currently non approved users do not have access to the codes required to use the L2 frequency signals in an optimum way.
Various kludges are required to extract some info from the L1 frequency carrier phase if one doesn't have the despreading codes.
However this should change as the GPS system is enhanced to provide 2 or more frequencies to civilian users.

Multipath effects become even more critical as one attempts subnanosecond timing performance.
One may have to use either a choke ring ground plane antenna (as used in geodetic receivers) or resort to phased array techniques.

There is at least one commercially available GPS disciplined OCXO system 
that uses carrier phase measurement techniques to discipline a crystal 
oscillator.
A fractional standard deviation of 1E-11 for a measurement time of 
around 1 second is claimed. When using carrier phase measurement 
techniques it is advantageous to use the oscillator being disciplined to 
generate all the receiver local oscillator frequencies as well as the 
correlator clock frequencies.

Bruce




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