RTK GPS

RTK (Real Time Kinematik) increases the GPS accuracy to a centimeter-level. PX4 supports RTK GPS using the u-blox M8P GPS device. This page describes the required setup.

You will need:

  • A pair of u-blox M8P GPS devices (see below)
  • A laptop/PC with QGroundControl (mobile devices are not yet supported)
  • A vehicle with a WiFi or Telemetry link to the laptop (for example using Pixracer or Pixhawk 3 Pro)

GPS Devices

The following are two example GPS devices that can be used with PX4.

Drotek XL RTK

Buy online : Drotek XL RTK

HEX/ProfiCNC Here+

Buy online : Here+ RTK GNSS

Setup & Usage

We successfully tested with a WiFi link (ESP8266 on a Pixracer) and a 3DR Telemetry link between the Laptop and the vehicle. When using a telemetry module, it is crucial to use mavlink version 2 and an up-to-date firmware on the telemetry device. The mavlink version can be enforced by setting the parameter MAV_PROTO_VER to 2 and the telemetry can be updated with QGC.

The base GPS device which is connected to the Laptop, needs to be in a fixed position while it is used. So make sure it is in a position where it is not moved and with a clear view to the sky, not close to a building. Preferably, it should be elevated (for example with a tripod, or on a roof). Compared to normal GPS, RTK is more sensitive and needs a careful setup.

First start QGroundControl and attach the base GPS via USB to the Laptop. The GPS is recognized automatically. Also start the vehicle and make sure it is connected to QGC. QGC will then show an additional GPS icon with the status of the base GPS:

survey-in screenshot

What you see is the status of the survey-in process. This is a startup procedure to get an accurate position estimate of the base station, by taking many measurements over time. You can see that the accuracy converges towards 0. The process ends after a minimum amount of seconds passed and a certain accuracy is reached. Both of these can be changed on the QGC configuration page.

After the process is completed (it takes several minutes), the GPS icon changes to white and QGC starts to stream RTCM data to the vehicle:

survey-in screenshot

No further configuration on the vehicle is necessary. After a while it should switch to RTK mode, which is visible in the GPS status (3D RTK GPS Lock) of the vehicle:

RTK GPS Status

There are two RTK modes: Float and Fixed. While the Float mode is easier to reach, it is a bit less accurate than Fixed mode (further explanations are for example here). The system will automatically switch to Fixed mode when the signal is good enough.

Now you can start to fly!

You may also need to tune some parameters. The default parameters are tuned assuming a GPS accuracy in the order of meters, not centimeters. For example, you can decrease EKF2_GPS_V_NOISE and EKF2_GPS_P_NOISE to 0.2.

Video Demonstration

(Video credits to Michael Ammann)

Example Vehicle Setup

This page describes an airframe setup with the Here+ RTK and a Pixhawk 3 Pro.

© PX4 Dev Team. License: CC BY 4.0           Updated: 2017-07-18 00:54:23

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