Sub-Meter Accuracy

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
WAAS?

 

Please post which manufacturers and models so that we may read the
accuracy specifications.

 

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

 

Here are a couple I seen, but there are others:

I'm looking for something with a meter or accuracy, but I don't want
to go too outragously expensive. I want something with free
differential either WAAS or Coast Guard.

AgLeader GPS 4100
http://www.agleader.com/gps4100.htm

The Satloc AgIQ
http://www.satloc.com/site_Products4_AgIQ_Features.htm

 

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.

 

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
collection.

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
corrections.

The Satloc AgIQ is a dual frequency receiver making use of WAAS signals
Ref: http://www.satloc.com/pdf/AgIQ_Brochure.pdf

 

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.

 

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

--Sam Wormley
http://edu-observatory.org/gps/gps_accuracy.html

 

Do you have a price quote for these systems?


- Carsten

 

http://www.erols.com/dlwilson AC4IU

How far were you from one of the WAAS reference locations?

 

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

http://www.csi-wireless.com/products/dgps_dgps_max.shtml

 

Seems unbelievable. What was your configuration?

Hmm, seems this report is for an older unit:

http://www.fs.fed.us/database/gps/mtdc/gps2000/geo3_3-2001.htm

- Carsten

 

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?

 

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

 

Hmm, this also seems to be an interesting approach. Sort of a 'self
contained' timeshift DGPS.

http://www.satloc.com/site_Feature_Ediff.htm

Could be a nice feature if implemented into consumer units.

- Carsten

 

Omnistar subscriptions are $800/yr in North America

 

http://trl.trimble.com/dscgi/ds.py/Get/File-128927
Trimble GeoXT literature: "Need submeter accuracy in real time?
Use corrections from a satellite-based augmentation system (SBAS)
like WAAS".

 

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
there.

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
http://trl.trimble.com/dscgi/ds.py/Get/File-128927 . 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
http://www.ngs.noaa.gov/cgi-bin/HTDP/htdp.prl?f1=4&f2=1 , 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.

 

I find that accuracy a bit hard to believe also. The William J. Hughes
Technical
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:

http://www.nstb.tc.faa.gov/reports/waaspan5.pdf (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.

 

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.