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

Tue Aug 21 12:09:57 UTC 2018

In this PDF there are examples in MATLAB for the C/A code generation,
FFT search, acquisition, carrier pull-in and tracking.
<http://www.sm.luth.se/csee/courses/sms/019/1998/navstar/navstar.pdf>
On Tue, Aug 21, 2018 at 2:31 AM Attila Kinali <attila at kinali.ch> wrote:
>
> On Mon, 20 Aug 2018 11:46:15 +0200
> Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:
>
> > On Mon, Aug 20, 2018 at 01:15:10AM +0200, Attila Kinali wrote:
> > > The online shop still works pretty well, as far as I am aware of.
> > > If you order more than 10-20 LEA/NEO modules, I recommend going
> > > through the u-blox sales. They might not resopond to you as quickly
> > > as one would like but they should still give you a decent offer.
> >
> > They do not have the UBX-M8030-KT-FT in the shop, but perhaps I should try
> > and modify a NEO module as you suggested.
>
> Ah.. you want to go for the chip. That explains why u-blox isn't selling
> to yo directly. The chips have minimum quantity of a single reel which
> is IIRC 1000 pieces.
> Keep in mind that the information needed to programm it is only available
> under NDA.
>
> > > One month? If you are not using a Cs beam standard, then having
> > > an integration time of a month is pretty pointless.
> >
> > The issue is that I only get a measurement every second, with a fairly-large
> > amount of noise, so it takes a while to get enough samples until the noise
> > averages down to below the oscillator's own noise.
> > (Hence why I want to move to carrier-phase measurements; a better receiver
> > would definitely help, though)
>
> Uhm... after saw-tooth correction the PPS jitter should be less than 10ns.
> That translates to an uncertainty of 1e-8. As PPS noise is pretty much
> uniformly distributed (whith curtains that resemble white noise), you
> can average it and get a square root improvement. So at 10s you would
> be down to 3e-10 uncertainty, which is about what a decent TCXO
> does provide at 10s (if I'm not mistaken). So, either you are using
> something different than a TCXO or your PPS noise is higher than it
> should be under good conditions.
>
>
> > I mis-spoke, though, one month was how long it took, with a particular set of
> > parameters, to get a PLL lock from a cold start (i.e. without the Kalman filter
> > being seeded with previous measurements); I guess that what I get when writing
> > late at night. :)
>
> That sounds like you got the control loop wrong :-)
> Even if you don't do anything fancy (like switching control loop modes)
> to assist faster locking, with a loop time constant of a few 10s (which
> is what you should do for a TCXO), your loop should lock in a few 100s,
> at latest in 1000-2000s.
>
>
> > > > 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. :(
> > >
> > > Hmm? What kind of problems?
> >
> > “The problem” in question was the trade-off between precision and the time it
> > takes for the PLL to lock.
>
> The trade-off between precision and locking time does not happen at the
> PLL, but at the choice of your local oscillator. The reference oscillator
> defines what your loop time constant should be, 10-20s for an XO, 10-100s
> for a TCXO, 100-3000s for an OCXO, 10ks-300ks for an Rb. Your locking time
> is then just a (small) constant factor larger than your loop time constant
> if you don't do any adaptive control (disclaimer: this is a very simplified
> view of designing a control loop).
>
> > > > - Trying to make a good GPSDO is hard enough as-is  ;)
> > >
> > > Not really. You just need to understand what the limitations of the
> > > components are and how to design a proper control loop. That's why
> > > Trimble GPSDOs or the Star-4 work so well. They were designed by
> > > people who know these things.
> >
> > I'm keenly aware; I was saying that I don't have the same expertise as
> > Trimble's engineer, so there is a bunch of learning involved.
>
> Most of this can be learned just by reading GPSDO manuals, GPSDO schematics
> and the time-nuts archives :-)
>
> What you need to know is:
> 1) How GPS works and what the uncertainties are
> 2) How your reference oscillator behaves
> 3) How to design a control loop
>
> The GPS knowledge is a bit hard to come by as there are not so many
> good web resources on it. I recommend reading books like those from
> Misra/Enge[1] and Kaplan/Hegarty[2]. Those two give in-depth knowlege
> on the system as they explain how to build a receiver, with the Kaplan
> one being easier to understand for a first read as it doesn't dwell
> as much on theory as Misra. In addition to those two I would go for
> Parkinson[3], which explains how GPS as an overall system works and
> performs.
>
> For your oscillator, I would have a look at NIST TN 1337[4] and
> NIST SP 1065[5]. Those two explain what stability of oscillators
> is and how you should talk about it. And especially how you compare it.
>
> For control loops, I usually recommend Franklin's book[6], which
> gives a good introduction, but does not cover any of the advanced
> topics which you will need for a GPSDO. But even just the basics
> will get you to a working system. It might not perform optimally
> under all conditions, but it should do quite well.
>
>
>                         Attila Kinali
>
> [1] "Global positioning system signals, measurements, and performance",
> 2nd edition, by Misra and Enge, 2012
>
> [2] "Understanding GPS : principles and applications"
> 2nd edition, by Kaplan and Hegarty, 2006
>
> [3] "Global positioning system - Theory and Application", Volume 1 & 2,
> by Parkinson, Spilker, Axelrad and Enge, 1996
>
> [4] "Characterisation of Clocks and Oscillators", NIST Technical Note 1337,
> by Sullivan, Allan, Howe, Walls, 1990
>
> [5] "Handbook of Frequency Stability Analysis", NIST Special Publication 1065
> by Riley, 2008
>
> [6] "Feedback Control of Dynamic Systems", by Franklin, Powell and Emami-Naeini
>
> --
> <JaberWorky>    The bad part of Zurich is where the degenerates
>                 throw DARK chocolate at you.
>
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