M2: NMEA (GGA) message content does not match web UI information

Reach Module / M+ / M2 Hardware integration
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Hardware: Emlid Reach M2 reciever
Software: Ubuntu 20.04, connection via USB and TCP NMEA Position Streaming, NTRIP real-time Correction via WiFi Hotspot to use in ROS Application
Reach Firmware version: 31.7

Hi everyone,

I successfully connected the M2 device via TCP position streaming to my ROS environment using the following community-provided ROS node: GitHub - rpng/reach_ros_node: ROS driver for the Reach RTK GNSS module by Emlid.

Now, I was trying to get a bit more information on the recieved signal quality from the device. I compared the different NMEA messages and decided to get the following criteria from different messages:

  • number of satellites in view from GGA
  • pdop, hdop, vdop from GSA

While looking at the GGA sentence in more detail, I noticed that the values don’t entirely match with the Status overview provided by the M2 Web interface.
I have attached a screenshot with the received NMEA sentences as terminal output with the GGA sentence roughly at the center of the terminal window with the content parsed to variables according to the official NMEA definition as output below the original sentence for better readability.

If we compare the coordinates with the Status overview you can see a slight offset in latitude and longitude and altitude, but also a completely different number of satellites in view. First, I thought the no. of satellites in view only displayed the amount of GLONASS satellites, but the number kept changing to different values. Also the “GN” in “GNGGA” indicates that it should display a solution for combined coordinate systems.
For the dilution of precision parameters it turned out that the GSA message are separate for different coordinate systems so instead of recieving overall dop values I get them seperately and I can’t really calculate average dop-values without the number of viewed satellites for each system, or am I thinking falsely?

Screenshot from 2023-12-07 15-10-02
Screenshot from 2023-12-07 15-10-021920×1200 372 KB

So, I’m wondering now, what the GGA output actually shows compared to the Status overview and if there is any other way to recieve the position, number of satellites and dilution of precision parameters that match the values represented on the web UI as these seem to include the recieved NTRIP corrections.

Thanks a lot in advance for any thoughts put into this and looking forward to your replies
Best,
Noel

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Hi @blunderTUHH,

Welcome to the forum! Sorry, it was so quiet in here.

There might be a slight delay before you receive the data streamed over TCP to your computer. In your case, it looks like the reason for the different coordinates in the web interface and in the ROS environment.

but also a completely different number of satellites in view. First, I thought the no. of satellites in view only displayed the amount of GLONASS satellites, but the number kept changing to different values.

GGA shows only the number of satellites used to calculate the solution and not all available satellites.

For the dilution of precision parameters it turned out that the GSA message are separate for different coordinate systems so instead of recieving overall dop values I get them seperately and I can’t really calculate average dop-values without the number of viewed satellites for each system, or am I thinking falsely?

Do you mean you get DOP values separated by different satellites? You can use GN Talker ID. As you mentioned, it’s a combination of multiple satellite systems.

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Hi @olesia.riman,

thanks a lot for the reply!

  1. Thanks for pointing out the delay as possible reason for the difference, that makes a lot of sense!

  2. Do you know, how the satellites are chosen for the calculation of the solution? Is there a kind of threshold value in terms of signal strength or quality?

  3. Thanks for pointing out the Talker ID. I thought, I printed all recieved NMEA messages to the terminal in the screenshot above and there were only GP GL GA GB IDs for the GSA messages, as you see above, but I will double, if I somehow filtered the GN IDs out.

Thanks again for your help and
best regards from Hamburg
Noel

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We can’t provide a specific threshold as it depends on different factors. As you mentioned, it could be the quality and strength of the signal, and which satellites were available at that time. The desired “quality” of the signals can be chosen in the settings in the Elevation mask angle and SNR mask.

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