Introduction to C94-M8P

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.

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!

there are three steps to configure the Base Station.

Set the position, by having the Base Station do a Survey-in or pre-surveyed coordinate
Set-up the port for the Radio Link Port configuration
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.

Parameters

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

DGNSS
- 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 NMEA+UBX
- Protocol out NMEA+UBX+RTCM3
RST
- Hotstart
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.

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