RTK-GPS VTG-track issues with SONTEK rs5

[Daniel Wennerberg]

 

Hi all,

Any suggestions why the RS5 would report the Emlid RTK VTG data to be like the above orange/brown line?

Compass calibration was passed and looked good. I have had this quite consistently across a couple of sites now. No overhead conditions, RTK approx 30 cm from the water surface. Same phenomenon seen both with RQ-pod powered transects and cableway transects. Reasonable boat/water speed.

/Daniel

• This topic was modified 1 year, 5 months ago by Daniel Wennerberg.

Topic

[Kristoffer Florvaag-Dybvik]

An offset that is supposed to be 5.0 degrees that has been entered as 50 degrees by accident?

Kristoffer

 

[Daniel Wennerberg]

Ah, perhaps there is an offset here. But I don’t think so, the angle between GGA and VTG varies by quite a bit. Perhaps I’m not moving fast enough to make the VTG-positioning work?

[Kristoffer Florvaag-Dybvik]

I did not read your post well, it seems. Nor did I recognize the colors. Shame on me! I have no idea why VTG runs away from GGA and BT-track. Have you ever seen OK data from this GPS unit?

[Daniel Wennerberg]

I’m not sure I have seen ok VTG-data to be honest… We have four Emlids, and Mikael Lennermark has also seen this VTG inconsistency, however I can not say for sure whether he used this exact RTK or one of the others.

We have done a lot of normal RTK-work with them though without any apparent issues, but I think for taking point measurements they use mostly GGA.

[Kristoffer Florvaag-Dybvik]

To state the obvious: It must be something wrong in the Emlid VTG data. Either the output or the readings. Or in the RS5 VTG reading.

Is it possible to connect another gps (another brand) to the rs5 to see if you have the same issue? If so, the issue is how the RS5 reads/interprets the VTG data. Another suggestion: If it is possible to connect the Emlid to a PC/PDA/phone and read VTG data, try walking north at 1 m/s and check if VTG data agrees. (Or: Return GPS and rs5 to manufacturer/dealer and tell them that something is wrong)

[dave@genesishydrotech.com]

VTG having the correct speed but the wrong direction is something I have seen in the past. It is the biggest concern when using VTG without GGA for comparison. When we evaluated the issue we couldn’t correlate when this happened with any of the GPS quality indicators. When using VTG it is recommended that the boat speed be greater than 0.24 m/s (C.R. Wagner, D.S. Mueller / Journal of Hydrology 401 (2011) 250–258) but I don’t think that guarantees this won’t happen.

[Daniel Wennerberg]

Kristoffer Florvaag-Dybvik wrote:

To state the obvious: It must be something wrong in the Emlid VTG data. Either the output or the readings. Or in the RS5 VTG reading. Is it possible to connect another gps (another brand) to the rs5 to see if you have the same issue? If so, the issue is how the RS5 reads/interprets the VTG data. Another suggestion: If it is possible to connect the Emlid to a PC/PDA/phone and read VTG data, try walking north at 1 m/s and check if VTG data agrees. (Or: Return GPS and rs5 to manufacturer/dealer and tell them that something is wrong)

I’m in touch with Sontek about some other issues, I’ll add this to the pile..

Dave: Thank you for confirming my hypothesis, I will try to do a fast transect and see whether it starts to align better. I must say with the RS5s slight issues with bottom tracking, GPS has been very useful. What is a bit concerning is that of course the discharge with a bit of lost bottom track tend to be biased slightly low.

Another question, which may deserve it’s own topic is that the RS5 reports altitude based on a “non-published, proprietary geoid” instead of pulling height above the ellipsoid from the GGA string. The geoid seems to be quite close to MSL, but since it is “secret” altitude mapping with the RS5 is, at least from my limited geodetic abilities, impossible?

I know the raw gps data is exported into the Qrev matlab files. Can we also pull altitude above ellipsoid from Qrev Dave?

/Daniel

[Mikael Lennermark]

Yes, we have got ok VTG data from these Emlid RTK-GPS.

This is from a distance made good test I did two months ago:

• Two of the 370m transects plus a transect where I tested for GPS lag. VTG follows well.
• I used a Emlid GPS and the RiverPro in the transects below.
• The StreamPro was there to compare StreamPro bottom track with the GPS GGA track on the RiverPro.

In the examples below, speed and heading was steady prior to recording.

[Daniel Wennerberg]

Mikael, at how many Hz did you set the VTG string to be sent to the ADCP?

/Daniel

[Mikael Lennermark]

5Hz for both GGA and VTG and the boat speed was 1m/s

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