Sub-Meter Accuracy

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

  1. deecee

    Carsten Kurz Guest

    Don't get me wrong here. It would be too good if reliable submeter is
    possible using only WAAS. But with an internal, rather simple antenna,
    and L1 only?

    I just don't think that the spacial resolution of the WAAS is capable of
    doing that. I would expect submeter as a very optimistic figure for very
    short baseline DGPS.

    But we'll see. Making and applying WAAS corrections is complicated and
    may still be improved significantly as the people involved get more

    This report looks very promising:

    - Carsten
    Carsten Kurz, Sep 21, 2003
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  2. deecee

    Sam Wormley Guest

    Statistical data is the final arbiter!
    Sam Wormley, Sep 21, 2003
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  3. deecee

    Sam Wormley Guest

    Although this has nothing to do with the submeter accuracy attained with
    some GPS receivers using WAAS as a source of differential corrections, it
    is interesting to see some statistical data of some of the same receivers
    collecting data in working conditions that include obstruction (such as
    forested areas).
    Trimble Geo XT
    Trimble Pocket GPS
    Garmin GPS MAP 76
    Garmin eTrex
    Garmin GPS III with CSI Differential Correction (real time)
    Magellan Meridian Platinum

    And older data
    Garmin and Magellan Recreational GPS receivers
    Trimble GeoExplorer 3
    Trimble Pathfinder Pro XR
    Rockwell PLGR 96

    It should be noted that this is not a comparison of all of
    these GPS receivers under identical conditions. Technology
    changes, the GPS as a whole improves, and test conditions are
    not uniform!
    Sam Wormley, Sep 21, 2003
  4. deecee

    Ron Wilson Guest


    It appears that the report you referenced above may be almost 3 years old
    now, so I would imagine that many improvements to WAAS have been made since
    then. Even so, the document states that WAAS is "at least as accurate" as
    the signal supplied by other commercial providers of differential
    corrections. I found this to be the case in my testing with the GeoXT as
    well. I post-processed (code only) many of the points using the nearest
    CORS station and found the errors to be significantly larger than the real
    time WAAS corrections. In theory, post-processing with a given differential
    source should be more accurate than real time corrections, yet the CORS
    corrections were much less accurate. I soon quit going to the trouble of
    post-processing and have used WAAS only since then.

    The GeoXT's internal antenna has a ground plane set up plus the unit uses
    the Everest Multipath rejection software, which is not cheap or "simple".
    As I said in an earlier post in this thread, Trimble actually only claims a
    submeter RMS figure for this unit. Choosing a worst case scenario of 0.99
    meters as an RMS value, the 2DRMS value would balloon up to 1.71 meters and
    still be within their specs for the unit. I have not observed this much
    error, however, at least under open canopy conditions. You can pretty well
    bet that Trimble would not advertise submeter performance for this unit,
    unless it was capable of delivering it. They wouldn't stay in business long
    by doing things like that! Why don't you try to borrow or rent a GeoXT and
    do some testing for yourself? I think you will be amazed with its accuracy,
    as I was. I was a doubter too, before trying it.
    Ron Wilson, Sep 21, 2003
  5. deecee

    Sam Wormley Guest

    I want to further point out that there a least two knowledgeable posters
    from this newsgroup who find the USFS testing "lacking" and I have to
    agree, but on the other hand, the tests to give some indication of
    expected performance in the field, where the variables are many.

    Sam Wormley, Sep 21, 2003
  6. deecee

    Carsten Kurz Guest

    I, too, find it strange that two GEO XT reports show very different
    results. And some of the other reports, though marked as 'preliminary',
    should better not be published online before they have been

    - Carsten
    Carsten Kurz, Sep 21, 2003
  7. deecee

    Frank Looper Guest

    Frank Looper, Sep 21, 2003
  8. deecee

    Jack Yeazel Guest

    Well, if you are willing to spend 30 minituts of your time, the new Magellan
    SproTrak Color can do it:
    Jack Yeazel, Sep 22, 2003
  9. deecee

    John Bonde Guest

    Are others who are using professional grade WAAS receivers getting
    horizontal accuracies in the 0.6 meter (95%) range without averaging?

    Since SA was eliminated more than 3 years ago, I haven't paid much
    attention to code differential as the ~4.5 meter (95%) accuracy of
    autonomous GPS is good enough for some of my work and when I
    need higher accuracy I just go straight to cm-level carrier phase
    differential. The 0.6 meter figure is just a couple of decimeters
    better than any other I've seen published so it surprised me a bit.
    If others could share some numbers, be it 0.5 meters or 1.5 meters,
    along with an approximate location within the WAAS service area
    I'm sure readers would appreciate it.

    In one of the references which Sam Wormley pointed out the
    unaveraged WAAS accuracy of the GeoXT was in the 6 meter (95%)
    range (in the open). I gotta think this guy did something wrong though.

    As for getting significantly worse accuracy when postprocessing with
    CORS data as compared to WAAS corrections, that shouldn't (in
    theory) be the case unless the base station is hundreds of km distance.
    My guess is that the CORS station you used had an Ashtech
    receiver. In that case you will get worse results as the code
    performance of the Ashtechs is worse than basically all other
    higher end receivers. Try post processing with data from an AOA
    or Trimble CORS station, even if it is a little farther away. You will
    see better results by a factor of 2 or 3 (as compared to using an
    Ashtech CORS, not necessarily WAAS).

    The specs for the GeoExplorer may say 30cm RMS with carrier
    phase post-processing, but any receiver designed to output good
    carrier phase info is capable of ~ 1 cm rms accuracy. A little
    worse when moving, a little better with redundant static measurements.
    You just need the appropriate software, good receiving conditions,
    and good data collection techniques. The software is expensive and
    there are more limitations when working with carrier phase data, but
    the accuracy you can get makes 0.6 meters (95%) seem quite
    John Bonde, Sep 22, 2003
  10. deecee

    Jack Yeazel Guest

    Actually the result was 0.8m for 95% and 0.6m for the "mean"... But I'd think
    that for hand-helds, it would require averaging... In the case of Magellan,
    that graphic is an average of an average...
    Please don't get the idea I know anything about professional receivers! -One
    thing I can say is that Magellan's spec for WAAS-corrected accuracy of 3m 95% of
    the time is easily attained...
    Jack Yeazel, Sep 22, 2003
  11. deecee

    Ron Wilson Guest

    I believe you are referring to my experience with code post-processing here.
    Actually the CORS station I used was closer than the nearest WAAS station.
    I used the Trimble Pathfinder Express service to do the post-processing.
    This service does not give you a choice of which CORS station you want to
    use. It only uses the closest available station. I received corrections
    from several different stations, however, during my testing, as some were
    not available at various times. It didn't seem to make any difference.
    They were all significantly less accurate.

    I realize that post-processing should be more accurate, because of the
    better coordination of time, but you seem to have the notion that WAAS is
    inherently inferior to other DGPS sources. If so, why do you feel that this
    is the case? I'm not sure it is.
    The 30 cm RMS figure is just the spec that Trimble is quoting. As I said
    before, I have heard that their specs are on the conservative side. The
    GeoXT may be capable of 1 cm RMS accuracy with carrier phase, but I doubt
    that Trimble wanted to advertise that, since their cm units go for much more
    Ron Wilson, Sep 23, 2003
  12. deecee

    stretch Guest


    stretch, Sep 23, 2003
  13. deecee

    John Bonde Guest

    Ah, black-box post-processing by a third party. I thought you were
    doing it yourself with your own software. There are always so many
    variables. I just thought I'd point out one common problem that might
    be the cause. Sounds like there is a problem with their service.
    I have no idea where you came up with this. Coordination of time
    isn't really a factor since SA was discontinued. WAAS should be
    inherently superior to other DGPS sources in the most general sense.
    Of course there are many factors involved. If I happened to be
    300km from the nearest WAAS base station and 300km from the
    nearest single-point DGPS base station the corrections from
    WAAS should be superior. If there was a single-point DGPS
    base station at 50km distance I would choose it over the WAAS
    corrections. These differences aren't going to be huge. This is
    assuming everything else is the same - usually not the case.
    That's exactly it. It's all about marketing and maximizing profit.
    They get you to spend 4 grand for a good "submeter" receiver. Then
    after a while you decide a bit more accuracy would be nice. They
    tell you that you can use your existing receiver and just buy some
    carrier phase processing software for ~$2000 (I have no idea what
    the current price would be). You try it out and your happy because
    you are usually getting better than 30cm, but because you aren't an
    expert yet there are times when the accuracy is at the 5 cm level and
    others when it's up around 30 cm. Now you want to reliably
    obtain a couple cm accuracy. Well the specs for your receiver
    don't claim 1 cm accuracy so it's time to upgrade both hardware
    and software.

    I haven't processed any data from a GeoXT so I don't know
    for sure, but I have from a GeoExplorer III and 1 cm rms is

    Back to the original question though. There must be people
    reading this newsgroup who have done tests with "professional
    grade" WAAS receivers. Any results on non-averaged data
    would be appreciated.
    John Bonde, Sep 23, 2003
  14. See below
    I have both WAAS (SporTrack Pro) and a DGPS/GPS (Garmin 12xl w Garmin DGPS
    receiver) hardware. I am about 10 miles from a NDGPS beacon in Ft Edward
    NY. I have not run definitive tests, and since many variables can influence
    the perception of accuracy, I won't hazard a guess anymore which setup
    produces the most accurate fixes. I think they are close. When WAAS first
    became a viable system, I had thought that since it was a far more regional
    system, it must inherently be more generalized and that the provided
    corrections would not improve position fixes as much as a local DGPS beacon
    signal. However, I have been surprised to find the accuracy specs for WAAS
    versus DGPS suggest that WAAS is, in fact, more accurate.
    Pieter Litchfield, Sep 23, 2003
  15. deecee

    Frank Looper Guest

    Stupid question time here. Isn't the former L1 only and the latter L1/L2?
    That makes all the difference in the world that I can't afford (yet, he
    Frank Looper, Sep 23, 2003
  16. deecee

    Sam Wormley Guest

    Both are L1 only.
    Sam Wormley, Sep 23, 2003
  17. deecee

    Frank Looper Guest

    That is great news. I'm finally making some money mapping a few State
    Parks trail systems, and though I've grown quite fond of my GPS16/PocketPC
    combo, the best I can get from it kinematically, even in post processing, is
    around 2 meters. Plus YOU Sam Wormley have posted so many interesting
    Trimble articles that I have pored over that I have learned lust, though
    thank God, not covetousness. (weird Christian joke there, I guess). I'm
    guessing that the GeoExplorer III is quite a bit more than the GeoXT?
    I now need to go back to Dennis Milbert's triple difference carrier phase
    software, find a close order horizontal control marker nearby and find out
    if I've been accidentally lying to people about how close (actually not
    close) an L1 receiver can get. I realize there's worlds between a handheld,
    a low/mid grade mapping/avionics receiver like mine (GPS 16. It just runs
    circles around my GPS 12. You'd be shocked.), and a higher grade mapping
    receiver like the Trimble, but things just keep on changing. Now if OPUS
    would go ahead and open up to L1, it'd just save me lot's of trouble. Well,
    all but the fact that OPUS also doesn't do kinematic. :)


    P.S. Go to bed. You've got school tomorrow.
    Frank Looper, Sep 23, 2003
  18. deecee

    John Bonde Guest

    The data is online at
    for you to check out yourself. Use the maps and station descriptions
    to find various combinations of receiver brands/models and baseline
    lengths and do some tests. As I said in a previous post I haven't
    bothered much with code DGPS for the past three years, but I did
    a lot of testing before that. With my own equipment and CORS

    Just to refresh my memory the other day I downloaded a day
    of data (Sept. 20) from MIL1, CALU, and LCDT. LCDT is
    about 75km from both MIL1 and CALU. MIL1 is an Ashtech
    receiver and the other two are Trimble. Making the LCDT
    data kinematic and processing the CA code using MIL1 as
    a basestation and CALU as a basestation shows much lower
    CA code residuals for the Trimble/Trimble baseline then
    the Ashtech/Trimble baseline.

    I've done this type of test dozens of times in the past with
    similar results. The CORS data is a great resource for doing
    all kinds of testing be it short baselines, long baselines, CA
    code only, L1 only, or L1 and L2.

    You do have to be a little careful though as various
    processing packages can behave differently. If you are
    getting comparable CA code performance from Ashtech
    CORS and, say, Trimble CORS I'd like to hear about it.
    John Bonde, Sep 23, 2003
  19. My Garmin Extrex Vista goes from 6 meters without WAAS down to 2 meters
    with WAAS enable and running. This is Switzerland, two WAAS birds (maybe
    three, cannot remember when the third was to become operational).

    Roberto Divia, Sep 23, 2003
  20. deecee

    Frank Looper Guest

    It's a slightly different thing. The sub-meter real-time WAAS accuracy was
    achieved using software inside the receiver that reduced multipath error, a
    prime cause of inaccuracy in the longer bandwidth L1 receivers.

    Frank Looper, Sep 23, 2003
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