What Accuracy / Repeatability Should You Expect with the LT500T?
In the United States, the LT500T
uses WAAS (Wide
Area Augmentation System) corrections to obtain differentially
corrected positions. These corrections result in excellent real-time
repeatability and when adjusted for reference-frame differences
excellent real-time accuracy.
In areas outside of the WAAS
correction footprint, completely different accuracy and
repeatability are expected.
Observations averaged over a long
period will approach IGS08 current epoch framed position
coordinates. The difference between IGS08 coordinates and NAD83
coordinates varies depending on where in the USA you are located. A
typical horizontal difference between 'IGS08 current epoch' and
'NAD83 2011 2010.0' is around 1-meter.
If you provision the LT500T with
SurvCE then you can use the localization function to build in a
horizontal and vertical offset to measurements that will be valid
for a general area.
You can read more about frame
The device repeatability is
dependant on local obstructions (including your body) and the time
that it has been tracking satellites.
We have robotically tested the
LT500T using our robotic dump machine (click the play button to
watch it move):
The robot inverts the receiver,
then returns it upright, waits 45 seconds for the receiver to
reacquire satellites, and then stores a position in a database.
After 379 dump cycles (about 6 hours
and 20 minutes) we stopped the test and end up with a lot of [
data ]. Here is a summary:
The range (which includes single
position errors) is 1.6 meters for X and Y, 6.4 meters for the
reported heights. However, a more standard measurement of error is
the horizontal error (dXY). The range of returned values is 0.94
meters, the average horizontal distance to the average position is
0.36 meters and the 1-sigma error is
0.2 meters. Remember that these results are based on
hundreds of measurements over a 6 hour period using the ubiquitous
(and free) WAAS correction service.
Here is a graph showing the horizontal
distance to the average position:
Of course, the vertical change is (you
guessed it) 3-times worse:
The LT500T is about as good as any
other receiver. It is based on the Trimble BD-910 single frequency,
fully enabled, board set. Which is about as good as you can get.
So, what if you need better accuracy
than this? Here are some ideas:
1. You can collect raw
observation data and post-process the solution.
3. If you are working in an area
with cell phone coverage and a network, you can use a network
4. You can use a
Dissecting these options:
1. You can
collect raw observation data and post-process the solution.
Collecting data to post-process is troublesome because: a.
you don't know if you have collected good data until after you get
back to the office and post-process; b. you need to wait for CORS
data to become available to have reference data to process against;
c. Stop-and-Go processing (Time-Tagged-Kinematic) is based on
continuous carrier-phase data collection. You must present the GNSS
antenna to the sky such that it always has 6 SV's available at ALL
Over the years, iGage
has specialized in selling inexpensive GPS receivers for
post-processing. We no longer sell or support these kinnematic
post-processing schemes because the price differential to full
GNSS-RTK receivers is relatively
Five years ago L1 receivers
targeted at S&G Post-Processed were $6,000 a pair and GPS RTK was
$40,000 a pair. It cost a lot of money to deploy a Base / Rover RTK
pair and the GPS only performance was not great. The price of
top-of-the-line, full multi-constellation GNSS RTK is now around
$15,600, complete with a data collector and field software. The
price for a network rover is less than $8,950!
If you need RTK, you absolutely
should purchase RTK. We (iGage) highly recommend that you no longer
risk wasting your time, collecting raw data that may or may not be
viable just to save a few bucks. GNSS RTK performance is great and
the price is now affordable.
2. You can try a global subscription
service like 'Trimble
Centerpoint RTX' (only $2,750 per
year) or 'Hemisphere
Atlas' (available 'on-sale' for
$1,280 / year). These are great services, however you
need to evaluate them VERY carefully. Both services are available by
IP (via cellular data) or delivered by L-Band satellite. Obviously
these services are best when delivered by L-Band, but that requires
keeping a constant connection (visibility) to the L-Band satellite.
This typically is not possible in canyons or under moderate canopy.
In addition, these services take an extremely long time to converge
to a 'high-accuracy' solution. It will take between 5 minutes (best
case in limited areas) and 1-hour to converge to final accuracy.
Finally, the subscription price is relatively high and the receivers
that utilize this technology are full Network Rovers.
3. If you are
working in an area with cell phone coverage and a network, you can
use a network rover. For areas of the country like Utah
where there is full state CORS Network coverage, this is a GREAT
solution. We sell complete network rovers with data collectors and
field software for $8,950.
4. You can
use a Base/Rover pair. Often this is the best
solution if you need sub-meter accuracy/repeatability. Horizontal
accuracies of 1 cm and vertical accuracies of 1.5 cm are relatively
easy to collect in real-world-field conditions. A complete RTK set
is around $15,600. Base Rover pairs provide a 100-times accuracy
improvement with a 3-times price change.
If you have questions or applications,
please call us! We have decades of experience providing mapping and