[time-nuts] GPSDO TC

[Lux, James P]

> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Poul-Henning Kamp
> Sent: Thursday, January 08, 2009 3:27 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPSDO TC
>
>
>
> And timekeeping lends itself well to experiment with this,
> not the least because getting it wrong doesn't smash anything
> valuable :-)

But you might show up early or late for dinner, which could have dire consequences.

>
> For instance, one thing to think about in the context of
> GPSDO's is that in addition to the PPS signal, we also have
> all the other information.
>
> For intance it would make sense to loosen the PLL a bit when
> satellites enter and leave the solution, because that often
> moves the GPS signal a few nanoseconds abrubtly, which is
> enough to throw most PLLs into thinking you had a phase jump.


This brings up an interesting question.  A lot of the discussion here is about taking an off the shelf GPS receiver of one sort or another, and then putting something around it to "improve" the system.  A goodly part of what's in the "around it" is essentially deconvolving (conceptually) the peculiarities of the receiver.

These days, it's not that hard to build the RF section of a GPS receiver, and one can do the processing in an FPGA and attached CPU.  Is there an "open source" signal processing chain (i.e. to acquire and track the PN codes, and generate the raw observables, and then to do the timing/nav solution)?  If such a thing exists, or can be created, then you can do a fancier nav solution that explicitly accounts for all the satellites and weights them differently as they appear and disappear.

Navsys sells a product that generates GPS signals by simulation and then you load them into a USRP and play them with Gnuradio. They also sell the receiver software.
Here's a review of Matlab toolboxes:
http://www.constell.org/Downloads/gpsmatlab.article.pdf

Xilinx has an article:
http://www.xilinx.com/publications/magazines/dsp_01/xc_pdf/p50-53_dsp-gps.pdf
That describes a GPS receiver implemented using SystemGenerator, etc., but I suspect that they're not distributing the source code.

There's this paper, too:
http://old.gps.aau.dk/downloads/IONGNSS2005_BorreAkos_paper.pdf

Here's someone who did it as a project in school:
http://cegt201.bradley.edu/projects/proj2008/gps/
He tried to convert the Matlab from Akos, et al., to C++


>
> There is also the complex 12h signal in most GPS receivers
> PPS, should that be notched out of the PLL so that it will
> not react to offsets that have a 12h period ?  (obviously
> only in stationary
> applications.)
>
> So many things to try, so little time...

Indeed...


But hey.. Why not start hooking up a USRP or Xilinx Eval board..

Jim