The way I see VRS working with Trimble equipment is that it creates a virtual base near the rover position
What I've always wondered is how much more accuracy is obtained by using the virtual base over a single-base position to the nearest actual base.?ÿ I know there's some additional iono (and maybe tropo) modeling involved, and perhaps some orbit refinement, I just don't know how much that adds to positional accuracy at, say, 20 km from a real base.?ÿ (This sounds like a question for Gavin Schrock!)
A friend off line tells me;
Many of our users use the RTN to set up control, and break out a base if they need a little more vertical.
In response to your initial post, even using a base/rover combo your grades are still not going to be tight enough for curb layout.?ÿ Some people lay out horizontal with GPS and then hit their hubs with a level for grade, but no GPS manufacturer's equipment will get you the vertical accuracy you need for good curb layout.?ÿ Still need the robot or a level for that.
When I say highway work I mean aerial mapping for design.?ÿ
That sounds like how the Leica approach to NRTK is supposed to work vs Trimble's VRS which calculates a pseudo base position in the area your working and if you move too far it recalcs a new pseudo base position. Both use all the CORS in network to model atmospheric errors just have different methods of reporting "baseline" vectors
NC's RTN network just started giving corrections for Beidou and Galileo in addition to Glonass and GPS.?ÿ I'm hoping it helps with vertical. I haven't had a chance to give it a real test, but I definitely noticed the increase in available satellites. Twenty-two seems to be common in the open now.?ÿ?ÿ
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Reply to Murphy
Seems to help on vertical, hit a Ncgs horizontal and vertical published monument the other day. Before the Bediou and Galileo vertical results was always a little high to published vertical. Right on the published vertical that day. Going to hit a couple more just for fun, to compare. Horizontal has always been good. No base just RTN.
As usual, it depends.?ÿ In my case TDOT's RTN is only dual constellation, so at typical baseline lengths and common sat availability, I see higher rms values for both h & v.?ÿ Double checks confirm that the RTN points are sloppier.?ÿ
Today I was working on a large tract where the only open area was on a small hill under the low point of a 500 kV transmission line.?ÿ I set the base and connected to the RTN for a VRS position.?ÿ It floated between 12-14 sats and would reset every 20-30 seconds due to high rms values. After a few tries, I switched to the single base option of the RTN and got through two 3-minute occupations without any issues.?ÿ Not sure if the VRS issues were due to the transmission line, weather, or an issue with the VRS itself. At any rate, the rms values were 0.04' h and 0.10' v for both occupations and sure enough, the positions differed by about that much.
This portion of the boundary was about 800' long and straddled a hill.?ÿ On the north end both corners were in the open and on the south end both corners were 50'-60' into the woods. After switching to base/rover I performed an integrated resection to locate my S7 on the south end of the boundary.?ÿ I picked a spot where we had line of sight to both corners in the woods but were far enough from the wood line to be able to set stakes along the line for several hundred feet without having to move.?ÿ I located the north corners with RTK, then got the south corners with the robot. I staked points along the boundary line in the woods with the robot.?ÿ After storing each point I'd toggle over to RTK to check them. In every case the RTK position agreed with the robot within 0.02'.?ÿ We broke out of the woods on the north side of the hill just beyond where the robot lost line of sight and continued staking the remainder of the line with RTK.
I could not have completed this job with the RTN that I have access to. There would have been too much error and the time penalty would have been too high.?ÿ In hindsight, Base/Rover alone would have gotten it done, but with my concerns about interference from the transmission line, incoming weather, and the thickness of the woods it wasn't a chance I wanted to take.?ÿ Switching to integrated surveying (Trimble's term) and utilizing base/rover RTK with my robot allowed us to toggle between the two technologies as needed for each shot.
FWIW, I have my base set to log data during RTK so that I can post process or grab an OPUS position later if I need to tighten things up a bit.
@murphy TDOT announced at the spring TAPS conference that they are working on updating all of their stations to 4 constellations but I think it was still in the RFP process. Seems like it was going to be a couple of years before the upgrade was complete. That's one of the reasons I went with two R10-2's rather than using a cheaper base unit. I figured I'd enjoy the benefits of base/rover for now and have the future prospect of being able to send out two very capable Net Rovers once the upgrade is complete. Good to hear that you're getting good rover results in NC.
@norman-oklahoma as @lukens describes, Leica holds a master CORS, the one you are seeing in your vector, but the network correction is based on it and multiple auxiliary CORS stations surrounding your location. In the Leica software, and maybe others, you can view the auxiliary CORS as well.
@frozennorth years ago a manufacturer, was marketing a laser level / GPS hybrid that would utilize GPS for horizontal and the laser for vertical. It sounded interesting but must not have been that effective. At the time I was doing a lot of airport construction verification and it sounded like just the ticket. Sadly, I could not even get a demonstration.
@bill93 Except with your own base, you'll get to your desired precision sooner if your VRS CORS is much, much farther away.
Relative position accuracy is dependent on how similar satellite propagation is to the rover versus to the base. Thus having the base near the rover has to generally help with iono/tropo propagation concerns. That would be especially true at times when conditions are changing.
Distance does little with respect to canopy and other multipath that affect only the rover.
Let me restate that.?ÿ I didn't meant that distance from the base/CORS was not at all important when the rover is under canopy.?ÿ A local base would be better then, but ...
Errors fall into two categories. Those due to the iono/tropo propagation will be smaller if the base is near the rover.?ÿ Errors added on top of that, due to canopy and other local multipath conditions at the rover, are pretty much independent of where the base is. If the multipath predominates on short measurement times, you may see little improvement with a close-by base relative to a more distant one. If you can average out the multipath with a longer session, then you may get down close to the iono/tropo errors so that the local base is indeed noticeably better.
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@lukenz Nothing is ideal all the time or even most of the time. There's time when I have very good sky visibility, but PDOP is low due to lack of satellites and their geometry. Sometimes just 10 with GPS and GLONASS. I'll also take Galileo and Beidou anytime! And who is only using GNSS for control?
@john-hamilton It only create that fake base station depending on the broadcast format you select in the the survey style. If you choose the correct one, it will show your base as the actual CORS it is using.
