Trimble VRS accuracy calculation question

That is the way I understand it.

We utilize the very system and while I do not have any specfic expected accuracies as you've identified. It is RTK afterall and the farther you are from the base the poorer the accuracty. With that said we have tested it on NGS monuments more than 25 miles from the base and checked in under 0.20', which for certain types of work like GIS data acquisition work just fine. In town, within 1-4 miles of the base our values come out great almost everytime.

> The specs state +/- 10 mm + 1ppm RMS. It is about 20 KM away which would be 2 cm added to the 10 mm stated in the specs, so 3 cm or +/- 0.10’. Am I understanding this correctly?

Taking the spec at face value, that's what I'd think is the expected standard error of a position claimed, i.e. about 68% of positions should be in error by less than 0.10 ft. and 95% in error by less than 0.20 ft.

Disclaimer: I'm not an RTK user, but I'd think that the 1ppm component of the error would be reduced by repeat occupations at much different times.

> The specs state +/- 10 mm + 1ppm RMS. Is the 1 ppm calculated by using the distance from my project site to the CORS station listed in the DC file? It is about 20 KM away which would be 2 cm added to the 10 mm stated in the specs, so 3 cm or +/- 0.10’. Am I understanding this correctly?

That would be how you would do it if you where actually measuring a vector between an actual base at said location and your rover. But there is no base at the location. The VRS is more like having a base right close by, wherever you may be, as long as you are within the confines of the network. So you ignore the ppm part of the equation. But add in the uncertainty in the VRS network itself.

The Virtual Reference System computes the atmospheric distortion correction for where you actually are performing your observations based on the CORS RTK NETWORK that presumably surrounds your observation site. Because of that, there is NO parts per million correction error based on distance from your base since it is computed AT YOUR OBSERVATIONS SITE.

When you are outside of the Real Time Network for your Virtual Reference System, the ppm error is then applicable as if you were using a single base with a radio for RTK differential corrections.

The advantage of using a VRS RTN is zero ppm error accumulation.

I am not an expert, but I think that the fact that you are using a VRS (real time network) changes that. The power of a RTN or VRS is that it "creates" a vertual reference station close to where you are working and therefore takes the ppm error out of the equation. I think you would just be looking at 1cm horizontal. But again I am not an expert.

> The Virtual Reference System computes the atmospheric distortion correction for where you actually are performing your observations based on the CORS RTK NETWORK that presumably surrounds your observation site. Because of that, there is NO parts per million correction error based on distance from your base since it is computed AT YOUR OBSERVATIONS SITE.

That would depend mainly upon the number and spacing of the CORS sites used in computing the atmospheric models wouldn't it?

Edit: I wonder if the spec Liz posted was just for RTK performance, not necessarily Trimble's VRS network in her area.

If this and if that is best left to the Trimble Tech folks, but in general ... yes.

For Kent's post above:

I thought about that but the spec sheet is specifically for the "Trimble R8 GNSS VRS Rover". I didn't see any footnote explaining how or why the ppm value was included...maybe for instances when you use the rover with a conventional base.

I called the Trimble Tech folks at the number on the bottom of the spec sheet but the automated recording wanted my Premium Support Account number or otherwise call my dealer...

I called my dealer and he said that someone at Trimble told him one time that you should divide the 1 ppm to the nearest CORS station by two. He did not know if this was for a 68% or 95% confidence level but personal experience tells me it would be closer to 95%.

...maybe for instances when you use the rover with a conventional base.

I think that is right, it's for use of the rover with a conventional base.

Network RTK accuracy calculation question

All measurements have error!

The baseline from the actual reference station to the VRS point contains error. That error would increase by the distance you are away from the actual reference station. So, the baseline from the actual reference station antenna to the non repeatable VRS point would contain the standard error for RTK plus 1 ppm for the distance of this baseline. Each RTK measurement you take with a VRS system contains this error. The error can be approximates as about 0.005 per mile of baseline length. For example if you have a 20 mile baseline from the reference station to the non-repeatable VRS point you will have +/- 0.11 feet in error. This cannot be avoided no matter what brand or flavor of GPS you are using. The claim with a VRS system is that it will compute a non-repeatable reference point that is 5 feet away from you position that you initialized at and then will claim that the have 0 ppm errors when in fact that have just excluded the errors encountered for the baseline to the site. The VRS points will be recalculated over and over again during the day when you get to far away or reinitialize.

As an industry we should all be using the Master Auxiliary Concept (MAC)that is built into RTCM 3.1 and higher. This was created by all manufacturers and GNSS organizations as a group. That way users would not be lulled into thinking their measurements contain no errors.

> I called the Trimble Tech folks...
>
> I called my dealer ...

Try Scott Harris at FDOT.

Liz,

You understand this correctly. You have the error of the baseline to the site plus any error from the non-repeatable VRS point. The error involved from the reference station to the non repeatable VRS point contains the specified error plus the ppm's in the distance of this baseline. The distance from the non-repeatable VRS point and the rover is kept very short so any ppm value for that baseline is just noise in the data.

> That would depend mainly upon the number and spacing of the CORS sites used in computing the atmospheric models wouldn't it?
Sure it does. That is the bad side of VRS systems. You never quite know, and have little control over, the exact quality of the positions you get. No residuals to examine. But my experience with the Oregon VRS was that the positions you got with a minute of VRS-RTK data were right there with what you got out of hours of OPUS-S observations.

Of course site selection, satellite geometry, and redundancy is still key to a proper control survey all the same.

gschrock,

I was not posting about your first post. I selected the wrong link to post a general reply. I do appreciate all of your information. It is interesting to hear about all of the comparisons of different systems.

I've always wondered if our geographic location on the west coast of Florida should actually affect those statistics.

PS. Missed you at the conference, you must have been busy GPSing! 🙂

Two questions to answer.

What is the network (Ref Sta) acuuracy?

What is the local (rover) accuracy.

Local is easy.
95% confidence of your observation set results. What? you only took 1 observation? You can't calc accuracy with one observation.

Here in fly over land we are about 1 cm locally accurate relative with NAD83 (2011) published coordinates FBN or CBN.

Network RTK accuracy calculation question

Gavin,

I've read that when using RTK as a network rover with multiple base stations, that the receiver can model the distance biases, and just about eliminate it. I've always assumed that means the PPM correction is a non factor in a multi-base network RTK system. Glad to see you post that because it clears it up for me.