DGPS
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. Added to the common usage to output UBX (position) messages from the ublox, the UART is also used in the other way to inject 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
Both NEO-M8 Both NEO-M8P are uploaded with the last firmware. There is a dedicated firmware for ground station or rover (mobile). The ublox chip firmware update and configuration are done with the tools U-CENTER V8.24