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C94-M8P-2 Application Board (dGPS)

  • Pay special attention to ESD protection!
  • Rover and Base antennas must use ground planes. Doing otherwise will make convergence times long. The ground plane should have a minimum diameter of 10 cm. [1]
  • Both UHF radio and the NEO-M8P operate on 3.3V. The app board provides inputs for 5V USB or 3.7-20V which we don't use.
  • This device has a single UART port. By default, it accepts RTCM (correction messages), UBX, NMEA as input and gives UBX, NMEA as output.
  • Header J8 exposes UART RX_GNSS (9), TX_GNSS (10) and GND (3). VCC (3.3V) is not available directly.
  • Follow guide [1] to setup “Base”. You need U-Center and Windows for this.
  • UBX-CFG “TMODE3” sets the Minimum Observation Time and Required Position Accuracy. If BOTH conditions are met, survey-in ends!
  • Time for base station survey-in [10cm accuracy; minimum time: 900s]: 3600 s effective

Screenshot shows test with manual displacement. Base station was surveyed-in but turned off again for some time before this test.

One of the two boards is configured as a “Base Station” and the other one is configured as a “Rover”. Some parameters will be applied to both of them in order to get the best position correction achievable.

Configure Base Station

there are three steps to configure the Base Station.

  1. Set the position, by having the Base Station do a Survey-in or pre-surveyed coordinate
  2. Set-up the port for the Radio Link Port configuration
  3. Select RTCM messages for transmission

Before you get in the configuration, go to https://www.u-blox.com/en/product/c94-m8p and download the u-center beta extension and be sure installing also the USB driver. Once the software started, click on the arrow right to the connect button (see image below) and click on the correct com port. The icon should be green after this step. The next image shows what it should looks like (After a while).

Base Station Coordinates

For most of the things, two buttons keep most of the useful things The one on the left provide information from the Base Station, i.e. for the survey-in. The one on the right allow to change Base Station parameters.


Most of the parameters are the default one. We only will talk about the changes.

    • Differential set to 3=RTK Set to fixed (see below for explanation)
  • NAV5
    • Dynamic Model Stationary
  • NMEA
    • NMEA Version set to 4.1 (As documentation recommend)
  • PRT
    • Target UART1
    • Protocol in none
    • Protocol out RTCM3
  • RST
    • Coldstart
  • TMODE3
    • Mode Survey-in
    • MOT set to 800s
    • RPA set to 0.0500m

RTK Float vs RTK fixed

The GPS system typically returns results in three different modes. In order of increasing accuracy, these are autonomous, RTK float and RTK fixed. Autonomous means the GPS rover is not receiving corrections from the base station radio, due to problems at the base station, baseline distance or topography. This mode don't concerns us at the moment of writing those lines. RTK float is similar to autonomous, in that it is a stand-alone mode. While the rover is receiving corrections from the base station in this mode, it either cannot see enough satellites to make an accurate calculation, or does not have enough satellites in common with the base station for the correction term to be valid. RTK fixed means that the GPS rover and base station can see at least five satellites in common (not sure if it is also 5 satellites for M8P) and the rover is receiving corrections from the base station. Those reason push us to set the DGNSS parameter to fixed RTK.

Rover Coordinates

To connect to the rover GPS do the same thing as for the Base Station


Also here, most of the parameters are the default one. We only will talk about changes.

    • Differential set to 3=RTK Set to fixed
  • NAV5
    • Dynamic Model Portable
  • NMEA
    • NMEA Version set to 4.1 (As documentation recommend)
  • PRT
    • Target UART1
    • Protocol in RTCM3
    • Protocol out none
  • RST
    • Coldstart

How do I know when the survey-in stop

Click on the left button described above, then navigate to UBX → NAV → SVIN On this page, you will be able to check survey-in state.

Remember that the page don't refresh by itself. To enable automatic refresh, right click on SVIN (in the left tree) and select “enable message”

  • orb/c94-m8p.1467996419.txt.gz
  • Last modified: 6 years ago
  • by sven