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

[Bob kb8tq]

Hi

> On Aug 19, 2018, at 5:26 PM, Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:
> 
> On Sat, Aug 18, 2018 at 08:25:11PM +0200, Attila Kinali wrote:
>> 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 :-(
> 
> Ah, that's a pity.  :(
> 
> FWIW, I'm going to try going through a ublox reseller that says they have
> that timing GNSS IC available, ask whether I can purchase in small quantities
> and whether I could have the datasheet.
> 
> 
>> 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. 
> 
> I've considered that, and it ends up being mostly equivalent to what I'm
> currently doing. Part of the issue is that I don't want to wait ~1 month
> for a PLL lock, but I also need/want an integration time about that long,
> as that's about where the GPS becomes more stable than my local XO.

If you *assume* 2 ns on the GPS ( could be better … might be worse) then at 
100,000 seconds ( = about a day) you are at 2x10^-14. That’s a pretty good LO.
A month gets you to 6.7x10^-16. A LO that is in that range is also in the “very 
expensive” range. 

Going to an L1 /L2 approach will drop the GPS errors by an order of magnitude or 
more. Given that you already are in the “very expensive” range, the cost of the receiver
should be trivial.

Bob


> 
> I was able to work around the problem in part, by dynamically adjusting the
> constants of my IIR (and so the integration time), and it works pretty OK
> despite being highly non-linear, but there is only so much one can do when
> fixing hardware deficiencies in software. :(
> 
> 
>> 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.
> 
> That seems pretty much equivalent to using a “naked” GPS IC, as the part
> I care about is clocking it with my XO and getting phase measurements
> (wrt. the time-code and the carrier) out.
> 
> OTOH, it might be much easier than getting a datasheet out of u-blox, so
> I will keep that in mind, in case I cannot do it the way I wanted.
> Thanks a lot for the suggestion.
> 
>> 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 :-)
> 
> Thanks a lot for the pointers  :)
> 
> I indeed low-key considered rolling my own GNSS receiver, as there are now
> some RFSoCs that would make it not too bad, but I decided against it as:
> 
> - Trying to make a good GPSDO is hard enough as-is  ;)
> - I would need anyway to be able to validate that the PLL works correctly
>  and gives the expected accuracy, with a known-good GNSS receiver.
> 
> 
> Best,
> 
>  nicoo
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