DGPS

Working draft

The principle

The Differential GPS present here is based on the Ublox chip M8P. There are two types of Ublox M8P:

• NEO-M8P-0: acting as the rover (mobile)
• NEO-M8P-2: acting as the rover (mobile) or as the ground station

The NEO-M8P-2 acting as the ground station must know its position. This can be done by locating the ground antenna to a know position and initializing the ublox to this position, or programming the ublox to send its estimated position after a given converging time and precision. Furthermore the ublox acquire observation data and output them with its position to the NEO-M8P rover (mobile).

The ground station output three messages types of the RTCM3 standard to the rover (mobile):

• RTCM3.2 1005 : Ground station position
• RTCM3.2 1077 : GPS observation data
• RTCM3.2 1087 : Beidou observation data

The NEO-M8P acting as a rover (mobile) computes the station position with the data received from the ground. By comparing the computed position with the position sent, the rover integrate this error while computing its owns position from its own observation data.

Technically

To not duplicate radio links the ground and board ublox use the paparazzi telemetry.

• From the ground side, the NEO-M8OP-2 is connected to the laptop using the USB connector (should be /dev/ttyAVCMX).
• From the rover (mobile) side, the NEO-M8P is connected to the autopilot (Apogee) using the UART. The ublox output UBX (position) messages to the autopilot, and input RTCM3 message coming from the ground.

Implementation

We use gps boards from DROTEK. The boards are very small, and equip ed with USB and UART. The antenna are active ones from Tallisman.

Ublox Configuration

The ublox chip firmware update and configuration are done with the tool U-CENTER V8.24. This is a Windows application provided by ublox that needs to be run within a Windows virtual machine. Previous version (ex.V8.16) that could be run with Linux/Wine are not compatible with the NEO-M8P. It is recommended to configure also the on board ublox with U-CENTER tools, as the modules/gps/gps_ubx_ucenter is not working for the time beeing

Paparazzi Configuration

   <module name="gps" type="ublox">
     <configure name="GPS_BAUD" value="B115200"/>
   </module>
   <define name="USE_GPS_UBX_RTCM" value="TRUE"/>