Sub-Meter Accuracy

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

  1. deecee

    deecee Guest

    I see some vendors advertising submeter accuracy with a receiver with
    only WAAS differential. Other vendors advertize 3-5 meter accuracy.
    What is the difference and can you really get sub-meter with only
    deecee, Sep 18, 2003
  2. deecee

    Sam Wormley Guest

    Please post which manufacturers and models so that we may read the
    accuracy specifications.
    Sam Wormley, Sep 19, 2003
  3. deecee

    Angus Og Guest

    The receivers that advertise sub-meter accuracies with WAAS generally tend
    to be dual frequency (L1, L2) receivers (expensive). Most L1 receivers can
    get you in the 3-5 meter range, but it will generally take a dual frequency
    receiver to get you sub-meter with WAAS. Having the second frequency
    available generally allows the receiver to correct for the ionosphere and
    multipath which tend to be the next largest error contributing factors.
    Also to effectively perform carrier smoothing on the L1 frequency an
    effective method for cycle slip detection is needed an this is provided by
    the availability of the L2 band. The best I have seen a dual frequency
    receiver perform with just the WAAS corrections is 80cm. On average you
    will see 1 - 2 meters, but this all depends on the environmental conditions.

    Angus Og, Sep 19, 2003
  4. deecee

    deecee Guest

    deecee, Sep 20, 2003
  5. deecee

    matt weber Guest

    WAAS reduces (but does not eliminate) the largest single source of
    error, changes in the propogation characteristics. because it is wide
    area, it only gives an approximation for any given area, as opposed to
    a precise correction for the immediate vicinity. So if you want sub
    meter accuracy there are only two ways to get it.

    1). You can use a differential beacon and a lot of patience with
    carrier phase measurement

    2) you can use two units and do it in post processing with carrier
    phase and doppler shift. The latter is barely sub meter, the former
    can get you to about 10cm without much trouble. Magellan pioneered the
    doppler shift method for hand helds.
    matt weber, Sep 20, 2003
  6. deecee

    Sam Wormley Guest

    Looking at the information for AgLeader GPS 4100 ...

    The GPS 4100 DGPS smart antenna combines high-performance GPS and
    L-band satellite differential in one package. Plus it's compatible
    with the Wide Area Augmentation System (WAAS) service. It's compact
    and light weight, making it ideal for crop scouting and data

    The GPS 4100 will connect to any Ag Leader PF Series monitor to
    record your vehicle's path. The PFadvantage display provides the
    ideal way to configure and interface to the GPS 4100 for grain yield
    mapping, cotton yield mapping, and variable rate controlling. For
    accurate field guidance without foam markers, the GPS 4100 supports
    the Light Bar. The GPS 4100 also supports the GPS functions of the PF
    monitor including site verification, soil sampling, field mapping,
    and navigation.

    o Combined GPS/DGPS and L-band satellite differential receiver and
    antenna in a single, rugged compact housing

    o Worldwide operation with both Omnistar and Landstar subscriptions

    o Control from either the PFadvantage, PF3000, PF3000 Pro, or
    included Windows-based AgRemote software

    o Submeter accurate, real-time DGPS operation

    o WAAS compatible

    The high performance submeter performace comes drom a combination of
    L-band satellite differential from either Omnistar and Landstar
    subscriptions and probably very good multipath rejection antenna design
    and signal processing. WAAS is an alternative differential source but
    most likely falls short of submeter performance.

    Down the road I will post position accuracy data with a mapping
    receiver from Trimble comparing WAAS with L-band satellite differential

    The Satloc AgIQ is a dual frequency receiver making use of WAAS signals
    Sam Wormley, Sep 20, 2003
  7. deecee

    Ron Wilson Guest

    The Trimble GeoXT advertises submeter accuracy with only WAAS. In my
    testing, I achieved 0.61 meter 2DRMS (95% accuracy) for open canopy
    measurements over NGS benchmarks. I also post-processed the data using code
    only and achieved less accuracy than with the WAAS corrections.
    Ron Wilson, Sep 20, 2003
  8. deecee

    Sam Wormley Guest

    Thanks Ron--And one of the critical factors is the Everest Multipath
    rejection available on the Trimble GeoXT.

    --Sam Wormley
    Sam Wormley, Sep 20, 2003
  9. deecee

    Carsten Kurz Guest

    Carsten Kurz, Sep 20, 2003
  10. AC4IU
    How far were you from one of the WAAS reference locations?
    David L. Wilson, Sep 20, 2003
  11. deecee

    deecee Guest

    No, but I do have a list price on the CSI Wireless DGPS MAX for
    $2995...they also have a Mini-Max that does sub-meter for $2195.

    It looks like SatLoc that I mentioned before is owned by CSI-Wireless
    so I presume the technology is similar
    deecee, Sep 20, 2003
  12. deecee

    Carsten Kurz Guest

    Seems unbelievable. What was your configuration?

    Hmm, seems this report is for an older unit:

    - Carsten
    Carsten Kurz, Sep 20, 2003
  13. deecee

    Ron Wilson Guest

    Hi David,

    I'm having trouble finding a website that has the exact locations of the
    WAAS reference stations, but it seems the nearest station to me is the one
    that appears to be in or near Washington DC. I'm located roughly 110 miles
    from that position, as the crow flies. If I had a lat/lon for the reference
    station, I could give you a much more accurate figure. Does that baseline
    distance seem too long for the accuracy I reported?
    Ron Wilson, Sep 20, 2003
  14. deecee

    Carsten Kurz Guest

    Navtechgps quotes $3350. Then you need the antenna and OmniStar
    subscription. I hear one year Omnistar is about 1000-1400US$.

    I really doubt that WAAS will give submeter in a reliable fashion. It
    may depend on your location.

    But it seems that using two DGPS-Max + wireless link will make a decent
    L1 RTK system at comparably low cost. I would trust that configuration
    more than a WAAS-only solution.

    - Carsten
    Carsten Kurz, Sep 20, 2003
  15. deecee

    Carsten Kurz Guest

    Carsten Kurz, Sep 20, 2003
  16. deecee

    Sam Wormley Guest

    Omnistar subscriptions are $800/yr in North America
    Sam Wormley, Sep 20, 2003
  17. deecee

    Sam Wormley Guest
    Trimble GeoXT literature: "Need submeter accuracy in real time?
    Use corrections from a satellite-based augmentation system (SBAS)
    like WAAS".
    Sam Wormley, Sep 20, 2003
  18. deecee

    Ron Wilson Guest

    I'm not sure why you find the submeter results of the GeoXT "unbelievable".
    First of all, the study you referenced in the link above is not even for the
    GeoXT. It dealt with the GeoExplorer 3. The date of that study was June of
    2000. The GeoXT didn't come out to the fall of 2002. It represents the
    next generation of technology from the GeoExplorer 3 unit that was tested

    The GeoXT is a professional grade mapping receiver and goes for about $4300
    as a standalone unit. You will need at least $500 worth of additional
    software to make it do anything useful. Trimble advertises it as a submeter
    unit with real time WAAS corrections and says so in the specs for it. See . In my experience,
    Trimble's specs tend to be conservative and they are not known for rash,
    undeliverable claims of performance. Just the same, I doubted that it could
    deliver this level of accuracy, so I spent the first few months after buying
    one doing a lot of testing. For that kind of money, if it had not delivered
    submeter performance as advertised, you can be assured that I would have
    returned it. I made that clear when I bought it.

    My testing methods were fairly straightforward. I recorded over 200 points
    by single point capture (no averaging) over 3 separate, open canopy NGS
    benchmarks that were separated by a distance of over 20 miles. Two of the
    benchmarks had Horizontal Orders of B, while the third had a Horizontal
    Order of 1st. All were established by GPS observations. The GeoXT was
    placed on a tripod and the internal antenna was used. The data points were
    recorded on the GeoXT in WGS84 lat/lon. The coordinates given on the data
    sheets for the benchmarks were in NAD83 lat/lon. Those coordinates were
    corrected to WGS 84 lat/lon by using the HTDP site from NGS at , before doing the
    error analysis calculations. The points were collected on 5 separate days
    spanning a period of nearly 3 months.

    The greatest horizontal error observed for any single measurement was 0.78
    meters and the RMS value for the set of data was 0.354 meters. The 2DRMS
    error (95% confidence) was 0.61 meters. If I were publishing these results,
    I would have taken many more data points, but it became fairly obvious to
    me, that the results were converging to a value around 0.6 meters. It was
    clearly submeter, so I was satisfied.
    Ron Wilson, Sep 20, 2003
  19. deecee

    John Bonde Guest

    I find that accuracy a bit hard to believe also. The William J. Hughes
    Center - part of the FAA - produces performance reports on WAAS. They use
    fixed reference stations and analyze data on a quarterly basis. In the last
    report: (3376.8K)

    The best 95% horizontal accuracy was from Columbus OH at 0.734 meters.
    The worst was at Elko, NV at 2.132 meters. A year earlier the best horizontal
    accuracy was at Chicago at 0.884 meters (95%). What you are claiming -
    and I don't totally dismiss it - is better accuracy with your ~$4000 GeoExplorer

    than any of the more expensive reference stations in the above referenced
    report. It does border on "unbelievable".

    One way I could see this type of accuracy being achieved is if the receiver
    "auto-averaged" the position - perhaps without telling the user. Did the
    receiver sit on the tripod for a while before you recorded a position?

    If this receiver can provide 0.61 meter (95%) accuracy while moving -
    in open terrain - then I'm certainly surprised and would like to know more.
    It could be checked by recording carrier phase info while wandering around
    an open field and then comparing the WAAS corrected realtime output
    to the carrier phase post-processed position info.
    John Bonde, Sep 21, 2003
  20. deecee

    Ron Wilson Guest

    I spent about 20 minutes or so wading thru the acronynms of this report,
    trying to make some sense out their experimental method for actually
    collecting the data they analyzed. In referring to the data collection
    method, the report said "This was accomplished by utilizing the GPS/WAAS
    position solution tool to compute a MOPS-weighted least squares user
    navigation solution ... ". It almost sounds like it was a simulated result,
    rather than an actual collection of data by a GPS receiver? If the data
    were collected at the WAAS reference stations, where the correction
    calculations are performed, shouldn't the error be zero there? I'm afraid I
    never made any sense out of that, so maybe you could enlighten me. I would
    also throw out that the specs that DOD has put out over the years for GPS
    accuracy have always been extremely conservative, so maybe some of that is
    going on here? Perhaps a CYA approach, if you will. (Even us civilians can
    use acronymns sometimes.)

    I noticed that the Elko, NV 95% vertical accuracy was also the worst of all
    stations, so it makes you wonder if something else was going on there. Once
    again, I didn't collect a tremendously large amount of data, since the
    object of my testing was only to determine whether or not the unit was
    capable of submeter performance with WAAS corrections. Even so, after
    looking at about 200 data points and seeing that the greatest error recorded
    was only 0.78 meters, it seems safe to say that the unit passed the test.

    Actually, Trimble only claims a submeter RMS value in their specs, not
    2DRMS, as my results indicate. Looking at a worse case scenario of 0.99
    meters as an RMS value, then the corresponding 2DRMS value would be 1.71
    meters. This would still satisfy their claims for the unit, but I found it
    to be much more accurate than that. We have a saying when doing wetland
    delineations, "It is what it is!" There's nothing quite like going out and
    doing an experiment for yourself, in the environment where you will be using
    the GPS. This is what I observed with the methods I used. Maybe if I catch
    up on my work load, I will go out to some different NGS benchmarks and do
    some more testing.

    The receiver was not set to average the position and does not "auto-average"
    without telling you. These were all single point captures.

    I never attempted any tests while moving, so I can't comment on that. The
    unit does have the ability to do post-processing with carrier phase, with
    the proper software. According to the specs, the GeoXT is capable of
    achieving a 30 cm RMS accuracy (52 cm 2DRMS) by doing that. I don't have
    the software to attempt this, so I have never tested this figure.
    Ron Wilson, Sep 21, 2003
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