Several GPS Receivers - (Updated) October 12,
We often get calls from HGIS users who ask why their maps are in
Most errors in maps are
caused by user's expectations of GPS
Misunderstood or improperly specified claims by GPS Vendors are the
primary cause of these
Improper installation by the user of the GPS is the
of these complaints.
We test GPS receivers to their
specifications and publish our reports.
We do not sell GPS Receivers,
so our test reports are not
When buying a GPS, check the
Look for important wording - For example, you may see "sub-meter
accuracy"? Over what time period? Is it accurate for 7.5
year? A large range of time periods are used when testing
receivers. For HGIS GPS mapping applications, we will be returning to
the same locations year after year. We need GPS
specifications for the long term.
How good is your confidence in the map? The word "Typical" is used to mean 50%
time; actual accuracy may be 5 times worse than "typical". Or is
it a 95% confidence level used by surveyors for mapping. Specifications
2D-RMS, or HRMS are not usually used for mapping (If you double these
particular specifications you may get an approximation of
surveyors accuracy. So if the GPS specifies "2 meter RMS", you know it
is about a "4 meter GPS".) Check for the specification
approved by surveyors: "2DRMS-95%",
also called HRMS-95% or sometimes
just 95% confidence (look for the "95%").
Is the test period specified? The GPS satellite constellation
hours. Some periods during the night or day are better
other periods. To be repeatable, a test must run for a full 24 hours.
Some testers only test a GPS receiver during the best times to make
it look better than it really is.
Is the Datum specified for submeter GPS Receivers? Submeter GPS
Receivers use different Datums, so you need to know what the GPS
reference is. For example, WAAS is referenced to a Datum called
"ITRF00" (the full name is
"WGS84 ITRF2000"). Some GPS receivers,
for example John Deere's StarFire, are referenced to the latest
ITRF(In 2007, that would be WGS84 ITRF2007).
Other GPS Receivers are referenced to NAD83. Different submeter
different Datums. Maps collected with one model submeter GPS may
not have submeter
when using a different model submeter GPS.
The Ag Leader GPS 1100:
Ag Leader started shipping the GPS 1100 in February 2006, an advance
campaign was started November 2005, and some units were recalled in
Ag Leader has had a good
reputation over the years, so we bought several of these
|Advance Publicity: Nov. 2005 to
(Catalogs, Price Sheets, and Advertising)
|Revised Claims March 2006
(Dealer only email)
|First Recall April 2006
(Letter to Customers)
"Sub meter accuracy",
"Ideal for ...
|"Correction to Listed Accuracy
Now that the GPS1100 has been released and we have had additional time
to evaluate the accuracy of the system, our tests are showing that the
unit is not providing the sub-meter accuracy performance we were
expecting. Accuracies are closer to the 1.5-meter accuracy,
pass-to-pass, that was found in last years model - GPS1000 Plus.
Therefore, we have
changed our specs on both the website and price sheets, removing the
sub-meter classification. The product catalog will be updated
when the next
batch goes to the printer. The receiver still has the ability to
4Hz NMEA , be used with DirectCommand's AutoSwath feature, and for
radar speed signals. As has always been the case with the new
this unit is not recommended for guidance. If this change in accuracy
classification affects any customers who have purchased the GPS1100,
feel free to contact us and we will work with
you and your customer to rectify the situation. We are sorry for any
this may have caused."
"closer to ... 1.5 meter
|The serial number range of this
recall is 2005860426 to 2006880165. The recall is to fix some units
that may be having difficulty tracking the WAAS differential signal
during certain periods of the day or locations in North America. (The
WAAS system has recently undergone some changes, providing more
satellites for increased coverage across North America.) This recall
will accommodate those recent changes as well as provide improvements
for better performance in general.
Due to the timing of this recall, Ag Leader will be replacing your
GPS1100 free of charge as opposed to repairing the unit. If your
GPS1100 falls in the above serial number range, please contact Ag
Leader’s support department (email@example.com) and obtain an RMA
number. Once an RMA number is obtained, the replacement GPS1100 will be
shipped and billed on your account. When the old unit is returned to Ag
Leader, a credit will be processed to remove the invoice from your
account. This offer is valid until June 1, 2006.
"We fixed WAAS"
The GPS 1100 specification should say
operating pass-to-pass with 7.5 minutes between passes, the
Cross-Track-Error(XTE) of the GPS 1100 will be greater than 4.9 feet
(1.5 meters) 50% of the time."
For mapping applications, the specification of the GPS 1100 should read
"14 feet (4.4 meters) 2DRMS-95%".
The GPS 1100 is less than "Ideal ...
mapping." At the current time, AgLeader is still making
Your repeatability when returning
at a later time is expected to be about 18 feet (95% confidence).
Our tests show that the reason for the first recall is that the early
1100s contained an inferior patch antenna incapable of tracking WAAS.
AgLeader is now using a patch antenna that they consider to be an
improvement for WAAS. (We tested the improved WAAS Antenna in the field
In our tests below, we tested the GPS 1100 at premium
locations where we could track WAAS (we also know how to test a GPS for
The first recall does not affect any of our test results, as we
tracked WAAS 100% of the time in our static and dynamic tests.
Our Actual Tests:
|WAAS STATIC TESTS
Low Multipath Environment
100% WAAS Differential
A submeter GPS would be
within the red circle 95% of the time.
|WAAS DYNAMIC TESTS
100% WAAS Differential
One Hour Test = 1.9 meters
from best average 95% of the time.
Actual Field Test with the WAAS
improved GPS 1100.
There should be NO white space between swaths. Note that for a short
time in the lower right two swaths are adjacent. White space means that
the GPS was in error as we know the swaths didn't overlap.
Often the GPS 1100 error was an entire swath (20 feet). This matches
our other test results.
Static (Stationary) Tests
Carefully run static tests are the best indication of the performance
of many GPS Receivers. We run our static tests in a low
interference environment. If the environment is not
ideal; interference at the test site can affect the test results that
would not affect the GPS in the field. One common type of
is called "Multi-path Interference". Some GPS Receivers can
also have special features (such as "Pinning" or "Psuedo-range
Tracking") which can affect
testing. We carefully look at all these factors, and compare the
static results with the dynamic results. Statically, the GPS 1100 was
horizontally within 4.44 meters of WAAS (ITRF00) 95% of the time. Also
statically, the GPS 1100 was horizontally within 3.72 meters of its own
best 24 hour average 95% of the time.
Dynamic (Moving) Tests
We ran a one-hour dynamic test to verify the "vintage" of the GPS
1100 electronics and test features that did not show up in static
compared the GPS 1100 using a Trimble AgGPS 132 as the reference GPS,
and measured the error of the GPS 1100. Relative to the reference
receiver, the GPS 1100 was within 2.4 meters of WAAS (ITRF00) 95% of
the time. Relative to itself, the GPS 1100 was within 1.9 meters
95% of the time. We normally see smaller errors in the dynamic
test because it is a one hour test compared to the 24 hour static
test. GPS errors are localized over time periods shorter than 24
hours. We found that the GPS 1100 static tests were
representative of its absolute dynamic accuracy. If we ran a 24
hour dynamic test, we would expect to see errors in the 3.7 - 4.4
meter range, as we did in our static tests.
WAAS Sensitivity (Before the April
The WAAS Sensitivity of the original GPS 1100 is very poor compared to
current generation GPS Receivers we have tested. WAAS sensitivity of
the GPS 1100 is made worse by using the antenna's magnet mount.
where you cannot track WAAS, your repeatability will be about 21 feet
(95% confidence). If the GPS 1100 can track WAAS in your fields, you
can expect the 18 foot (95% confidence) repeatability that it is
capable of with WAAS.
WAAS Sensitivity (After the April 2006
The field plot above was with the WAAS improved GPS 1100.
Using the Magnet Mount should still be a problem with the newer
GPS 1100 receivers; if you raise the unit 12-16 inches above metal
surfaces, it will improve its tracking of WAAS and the overall accuracy
of the GPS 1100 to about 15 feet.
Run Your Own Tests
The demo version of HGIS GPS
Mapping software (available at http://www.starpal.com/v800/HGIS_GPS_User.php
) can be used to run a one hour test on a GPS. The demo will run
for one hour, but can be repeated as many times as you want. Start the
demo with a "PRO All Features" profile; select Start GPS (Do not check
the Save Files checkbox, as using this feature will cause pauses in the
demo.). Open the GPS Statistics Screen (only available on the PRO
profile) under menu GPS Misc Info..., and watch the accuracy statistics
collect for one hour (it starts reporting after 10 minutes). If you
check "Show GPS Trail", you can watch the GPS drift around on the
screen while you are stationary (you need to zoom in with submeter GPS
receivers). You can also try the other features of HGIS during the
hour. With the GPS 1100, you will see a pattern similar to the dynamic
test shown above right.
GPS 1100 Summary
The GPS 1100 looks to be very shock resistant. The cable is
already fitted with connectors and has a cable shield. You can
just plug it in. By default, it reports position once per second,
but this can be changed to 4 times per second (4 Hertz) with supplied
It has a simulated Radar output which we did not test.
users discover the GPS 1100 does not give advertised performance
when they return to a mapped field.
Unfortunately, the published AgLeader recall will expire before some
users find out the actual capabilities of the GPS 1100.
If you purchased a GPS 1100 for
use with StarPal HGIS GPS Mapping Software; Ag Leader
is privately offering the following "No Questions Asked" (no time limit
specified) refund because this GPS still does
not meet its specifications.
We will keep you posted as Ag
Leader further addresses this problem.
"[W]e will be contacting all the specific dealers, and customers if we have
that info, who we have sold GPS1100's to to inform them of the situation
and offer them the chance to return their units, no questions asked, if
they had intended to use the unit for operations requiring sub-meter
accuracy or guidance." - AgLeader
comparing several GPS Receivers:
Column in Bold is most often used to compare
different models of GPS
* Expected Repeatability combines the GPS error of the initial
measurement plus the GPS error when you return to that same location at
a later date.
|Your Expected 95% Repeatability*
|Ag Leader GPS 1100 (Self Average)
|Ag Leader GPS 1100 (Compared to
|Garmin GPS 18-5 Hertz (Self
|Trimble Ag GPS 106 (Compared to
|Trimble Ag GPS 132 (WAAS)
Alternatives For Mapping
We have two recommendations that will beat the GPS 1100 for less
money. At $795, the Trimble AgGPS 106 is the GPS that the GPS 1100
claims to be. The Ag GPS 106 has a better antenna element than the GPS
1100 which will improve WAAS reception and reduce multipath
intereference. It reports position at 5 Hertz. The downside is that it
does not have a radar output (the accurary of the GPS 1100 radar output
will be poor as the GPS jumps around as demonstrated in the field
At $199, the Garmin GPS 18 5 Hertz
will also beat the GPS 1100. The Garmin GPS 18 5 Hertz is
available at http://www.garmin.com
It is waterproof to one meter,
has a rugged polycarbonate case, magnet mount, and reports position
five times per second. Garmin calls the 2DRMS-95%
specification "DGPS (WAAS)
accuracy: Position: < 3 meters, 95% typical" and
accurate according to our tests. The GPS 18 5 Hertz is more
accurate than the GPS 1100 in all our tests. The GPS 18
needs a 5 volt automotive power adaptor available at any store with an
automotive/electronics department such as Target($10), Radio Shack($17)
or Wal-Mart, K-Mart,
Best Buy, Circuit City as well. You must
attach a Serial Connector to the cable end (available at Radio Shack)
to match your computer. The
meets industry standards for GPS cables. Your total cost of all parts
including shipping should be less than US$250. The GPS 18 does not have
the radar output as does the GPS 1100.
For submeter GPS alternatives, please review our report on submeter GPS
Receivers at http://www.starpal.com/Reports/Submeter_GPS_Report_e.html
GPS 1100 internals:
The GPS 1100 contains a GPS engine rated by the GPS manufacturer
at 2.0 meters "typical " (CEP).
As can be seen above, 2.0 meters CEP is in-line with our test results
of 1.7-2.3 meters "typical" (CEP).
We inquired of Ag Leader about their specification being better than the GPS
manufacturer's own specification,
this was Ag Leader's response:
"Historically we have been very
conservative when listing accuracies of products so that
customers did not get the
wrong impression about out products capabilities.
Over the last 3-4 years our
competitors have done the opposite, often quoting accuracies that
could not be achieved but all
the same made our offerings look inferior because we would not
claim such specific values."
- Ag Leader
Perhaps being conservative is why Ag Leader has had such a good
We hope that this information aids in your choice of GPS Receivers.