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

[bg at lysator.liu.se]

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