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Realtime DGPS with ublox and GPSTk lib processing, relative <1m?

Posted by Noland on October 5, 2010 at 10:45am

I've read through a few discussions on this forum about poor man's DGPS and getting relative accuracy down to the ~1m range.  It seems no one's having much luck even with identical GPS units.  I'm wondering if anyone has manged this and what are your thoughts on what I'm currently building.

 

I'm laying out a PCB for a ublox LEA-6T (that's the one that outputs raw data and carrier phase) to interface to a MCU and a 433MHz radio.  That's the mobile unit.  My base station is then a single board computer with it's own LEA-6T, radio, and software running GPSTk post-processing software.  The mobile board will then radio the raw data (subframes 1 and 2 and psuedo range and carrier info) to my base station.  Base station code then chooses a set of sattellites that work well for both units and calculates position of both units using the same satallites and same ephemeris data.  (calculated position could then be radioed back to mobile unit)

 

The units are genearlly pretty close to each other (spatially) so the only error here should be that inherent to the ublox measuring psuedorange--no constellation, algorithm, iono/troposphere, ephemeris differences.

 

Has anybody tried this?  Do you think I can get a relative accuracy of ~1m?

 

 

Thanks for any thoughts guys and gals!

 

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Tags: dgps, gpstk, acurracy, post-processing, relative, ublox
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Replies

  • Jan van Hulzen October 19, 2010 at 2:41am
    Have you guys considered using any of the scientific material which is available on this subject? Using google scholar i have found a lot of publications on fusion between gps and imu.

    Best regards,

    Jan
  • Simon Wood October 18, 2010 at 8:33am
    I'll start with a statement:
    L1 RTK can give you under 10cm positioning.

    The question is whether the Ublox (and antenna) can achieve clean enough signals to achieve this and whether RTKLib can do this is real time. We (work) have been looking at the Ublox 6 with some decent (survey grade) antennas and it operated nicely, a $10 patch antenna will definately not have sufficient performance.

    There may be a problem with geting/keeping a decent link between the remote and ground station, whatever you do should be able to fly perfectly with the link down. Link latency will also be a problem, depending on what you are attempting to achieve (ie. hover at location).

    There is a planned feature for RTKlib of RTCM output (ie. raw signals converted to RTCM), whilst this is not a direct help to you as you have RTKLib on the ground, you might be able to 'steal' some code to implement a similar operation on your rover - RTCM is quite robust.

    Regarding data rates; I am informed that the Ublox 6T can output at higher rates up to 10Hz, but this relies on other CPU intensive functions being turned off as there is not enough processing power to do everything together - which is why they only advertise the slower rate of 5Hz.

    Cheers,
    Simon.
  • Tom Kent October 6, 2010 at 11:28am
    Poor man's GPS only works if you have a fix on the exact same satellites at the exact same time (and sometimes not even then).

    That said, I believe that the ublox binary protocol has the information you could use to create your own actual DGPS corrections from a base station. I've been thinking about experimenting with that, but haven't gotten around to it.
  • Tom Yochum October 5, 2010 at 3:56pm
    Well, that's the theory. If you can pull off the implementation, you should get accuracy better than 1 meter.

    I personally think the best way to sub-meter precision is to do tightly-coupled GPS/INS.
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