Phase center error with antenna height = 0 m

I just used the Flow app to measure some points with the RS2. I didn’t use a pole, so my antenna height was 0m (I placed the RS2 on the ground). I figured the height was a bit off so I checked it on a control point.
If I input AH=0, there is no correction for the 0.134m L1 offset anymore (this worked fine in RV3!). The exported csv does contain the 0.134m antenna height, but my measured point is 0.134m too high.
When I use a pole (2m), it DOES correct for L1 offset. The exported csv mentions 2.134m.

See screenshot below, the actual height of the point is 6.530m, which is exactly 0.134m lower than what POSCHECK says.

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Have you done several measurements with the RS2 on the ground ? Analysing such repeated data would help to determine if it is an app error as you suggest or a RTK engine inaccuracy.

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Hi Sean,

It’s a curious case, so I’ve tried to reproduce it. However, it can’t be reproduced. I’ve measured the same point three times with different antenna heights (2.0, 1.0, 0.0 m) and the decimal part of elevation stays almost the same in all cases. You can see it in the screenshot below:

Is it repeatable behavior? Have you tested it on other control points, if you have them?

Please share with me the full CSV file, I’d like to check it. You can do it to support@emlid.com or via PM if there’s sensitive data.

UPD: fixed some typos

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Hi Kirill,

Thank you for the investigation. I also tried to replicate it this morning.
I’ll share the csv file with the measured points privately, but the antenna height doesn’t seem to be the issue indeed.

I used two NTRIP providers of which one makes a VRS near the rover based on a few nearby reference stations. When this NTRIP profile is selected when the Emlid is booted, I think the very first GGA string is sent over to the NTRIP provider to be used as a position for VRS. That resulted in a reference station 200 meters below me. I’m not sure if this is an Emlid-thing or should be discussed with the NTRIP provider.

interesting information

Hi Sean,

Indeed, it looks like something from the NTRIP side. VRS usually creates a base station a few meters nearby the rover, not 200 meters below.

What about the second NTRIP provider? Was there any elevation discrepancy when you changed the antenna height?

Yesterday I sent the data to support@emlid.com. I think the height difference with different antenna heights was just a coincidence (also a coincidence that the difference was in the order of the PCO of 13.4cm).
With a non-VRS reference, the positions are correct, but with the VRS there are some strange outliers. Errors of >5cm occur in horizonal and vertical direction (however mostly vertical). With a measurement period of 30seconds I think these are pretty high.

Please have a look at the data I sent you. There I measured the same benchmark with different antenna heights, two different RS2 systems and the two different NTRIP providers (VRS and non-VRS).

Sean,

Oh, I see that you sent the data yesterday! I’ll examine it and write you back.