Sub-Meter Accuracy

Discussion in 'General GPS Discussion' started by deecee, Sep 18, 2003.

  1. deecee

    Andy Guest

    The "rather simple" antenna's are all tuned individually, if you look
    closely at the patch you can see the tuning stubs in the centre of each
    side. I'd recommend just taking my word for it rather than doing that,
    you have to take a lot apart to get to the antenna. Plus it does have a
    very large ground plane (and large antenna for that matter) for a
    handheld internal antenna.

    Also the internal antenna is a dual feed design which gives it better
    multipath performance to start with. That is partly why in some
    situations the internal antenna can be better than the standard geo
    external antenna.


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    Andy, Sep 26, 2003
    #61
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  2. deecee

    Andy Guest

    Thanks for the vote of confidence, we try our best :)
     
    Andy, Sep 26, 2003
    #62
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  3. deecee

    Sam Wormley Guest

    Hi Andy--could you elaborate on the dual feed design antenna a bit?
    Thanks,
    -Sam
     
    Sam Wormley, Sep 26, 2003
    #63
  4. deecee

    John Bonde Guest

    There's nothing like running a real test to find out. If Ron Wilson,
    or someone else, wants to collect a little data I'll process it. With
    a single test it's always possible to run into some problem that
    screws things up so I'd suggest recording ~30 minutes of static
    data at some control point within 10 km of a CORS station.
    That should be enough data to obtain a cm-level position with
    a high degree of confidence assuming the receiver is capable.
    Just to make it interesting the receiver could be setup, say, one
    decimeter due south of the control point or whatever and
    if cm-level accuracy is possible I should be able to determine
    the correct offset - without knowing it beforehand of course.
     
    John Bonde, Sep 26, 2003
    #64
  5. deecee

    Ron Wilson Guest

     
    Ron Wilson, Sep 27, 2003
    #65
  6. deecee

    Ron Wilson Guest


    John,

    I would love to. I tried to email you, but it bounced. Send me an email
    and we can set up the details of the data collection.
     
    Ron Wilson, Sep 27, 2003
    #66
  7. deecee

    Andy Guest

    Most standard patch antennas are single feed, this means that there is a
    single connection from the patch to the amplifier. The GeoXT/XM use a
    dual feed patch, two connections from the antenna to a differential
    amplifier. Since ground bounce and most other multipath signals have the
    opposite polarisation they can be filtered out by the diferential amplifier.
    I know that's not really a huge amount more, RF isn't exactly my strong
    point. Filtering and shielding I can cope with, antenna and amplifier
    design I leave to the experts, trimble has plenty of them.


    ____________________________________________
    reply address : newspost at . org
    talon . uk
    ____________________________________________
    I'm not paranoid about spam.
    Well ok, maybe a little bit.
     
    Andy, Sep 27, 2003
    #67
  8. deecee

    Sam Wormley Guest

    Thanks Andy
     
    Sam Wormley, Sep 29, 2003
    #68
  9. deecee

    John Bonde Guest

    In working with Ron Wilson to try and process some carrier phase
    info from his GeoXT I came across the following:

    http://trl.trimble.com/dscgi/ds.py/Get/File-29670/SprtNote_PfOffice_CarrierPhaseProcessing.pdf

    So it does seem that the carrier phase output on the GeoExplorers
    is crippled and they can't do cm-level work. About a year ago I
    had received some RINEX files, which did process to cm-level,
    that I thought were from a GeoExplorer IIIc. It seems that they
    were most likely from a Pro XRS and that I was wrong about
    the potential capability of the GeoXT.

    It would be interesting to hear why Trimble did this. The
    only reason I can think of is to sell more expensive receivers.
     
    John Bonde, Sep 29, 2003
    #69
  10. Sam Storm van Leeuwen, Sep 29, 2003
    #70
  11. deecee

    Andy Guest

    The performance of the GeoXT is not intentionaly capped or limited in
    any way. It is the best we could make it given the size, battery life
    and unit price restrictions on the design.
    Do you really think we would not take advantage of the marketing benefit
    of having a cm level handheld if we could do it with just a software
    change? Trust me, if we could do it we would. We'd probably end up
    selling it for much the same price as the top end ProXR, that way there
    would be no risk of cutting into other products from other business
    units. If a higher accuracy GeoCE is ever released it's only going to be
    after a large amount of development work.

    Andy
    ____________________________________________
    reply address : newspost at . org
    talon . uk
    ____________________________________________
    I'm not paranoid about spam.
    Well ok, maybe a little bit.
     
    Andy, Sep 30, 2003
    #71
  12. deecee

    John Bonde Guest

    Oh, come on Andy. It is intentionally limited to keep it from hurting
    the Pro XR market. As long as your company believes it can make
    more money by running two entry line products it will. Nothing
    really wrong with that as you are in the business to make money,
    but don't pretend it's a technical problem.

    Motorola had a receiver, the LGT1000, which was remarkably
    similar to the GeoXT in size, weight, and functionality. Even the
    costs were fairly close. It was marketed for GIS data collection,
    like the GeoXT. Its interface was a bit more primitive, being
    based on DOS instead of WinCE, but that was to be expected
    since it was released in 1992. Oh yeah, and it provided carrier
    info with enough resolution for cm-level work. So you are
    going to need a large amount of development work just to
    match 1992 Motorola technology?
     
    John Bonde, Sep 30, 2003
    #72
  13. deecee

    Andy Guest

    I'm not going to pretend to be an expert on the inner workings of our
    GPS recievers. But I do know what is discussed in the GIS engineering
    group and I do know what the GeoXT reciever can do, one of the plus
    sides to designing a product is knowing what it can and can't do.
    Trust me, cutting into the ProXR market isn't an issue, the Geo has
    already done that, ProXR sales have dropped a huge amount. And if we
    could sell a cm level Geo at close to the price of a ProXR we would do
    it, the profit margins would be huge.
    Obviously all the evidence of this is confidential so you'll either have
    to take my word for it or continue to think we cap it. For that matter
    you only have my word that I even know what an XT looks like let alone
    know what it can do.

    As for keeping up with 1992 technology, the GPS portion of the XT is
    only a fraction of it's size, the bulk of the space is taken up with the
    Windows CE front end and the battery. If you take one apart you will
    find that the GPS reciever is all on one board with a diameter of around
    3 inches.

    Andy
    ____________________________________________
    reply address : newspost at . org
    talon . uk
    ____________________________________________
    I'm not paranoid about spam.
    Well ok, maybe a little bit.
     
    Andy, Oct 1, 2003
    #73
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