[time-nuts] Choosing a GPS IC for carrier phase measurements

Attila Kinali attila at kinali.ch
Sat Aug 18 14:25:11 EDT 2018

On Thu, 16 Aug 2018 17:45:30 +0200
Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:

> The main issue I'm running into is that most timing GPS modules will
> happily give you carrier phase measurements... for their internal
> oscillator, and the few ICs I can find that would possibly do the job, like
> ublox's UBX-M8030-KT-FT, do not have publicly-available datasheets & docs.
> (I tried contacting ublox to ask, and never got an answer...)

Yeah. u-blox isn't as nice as they used to be to small customers :-(
There are two ways to proceede: 

1) use the timing of the PPS to deduce what the phase relation between
your clock and the internal oscillator of the LEA is.
In principle, this is possible, but I have not worked out the math,
so I cannot say for sure. 

2) replace the internal oscillator with one phase locked to your OCXO.
The internal clock of the LEA is derived from a single TCXO. You can
easily unsolder it and feed your own signal in. It should be relatively
easy to reverse engineer what you need. If you don't want to reverse
engineer it and if you get hold of a u-blox representative, you can get
the required information from them. I got the data from them a couple
of years ago. Unfortunately, I was explicitly asked not to share this
information :-(

> So, what are people around here using for that purpose?  Bonus points if the
> chip can work with a differential-signal clock.  :)

The alternative is to build your own GPS receiver. If you only want
GPS L1 C/A, then you can use the design of The Witch Navigator[1]
with one of the VHDL/Verilog projects out there (e.g. cu-hw-gps [2])

If you want to go for L2C, L5 or Galileo, you have to do your own coding :-)

			Attila Kinali

[1] http://www.witchnav.cz 
The website seems to be down at the moment so use archive.org for now:

[2] https://github.com/perfaram/cu-hw-gps

<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.

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