v20180710
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.

Accuracy

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 translation [ here ].

Repeatability

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.

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).

3. If you are working in an area with cell phone coverage and a network, you can use a network rover.

4. You can use a Base/Rover pair.

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 TIMES.

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 small.

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 survey GPS/GNSS!

 

iGage Mapping Corporation
1545 South 1100 East #1;  Salt Lake City UT 84105 USA
Voice:
+1 801 412-0011 Fax: +1 801 412-0022
email
orders@igage.com   General iGage Information