# RTK GPS Heading with Dual u-blox F9P
Two u-blox F9P RTK GPS modules mounted on a vehicle can be used to accurately compute a heading angle (i.e. an alternative to compass-based heading estimation). The two GPS devices in this scenario are referred to as the Moving Base and Rover.
# Supported Devices
This feature works on F9P devices that expose the GPS UART2 port (access to the port is required for setup).
The following devices are supported for this use case:
- SparkFun GPS-RTK2 Board - ZED-F9P (opens new window) (www.sparkfun.com)
- SIRIUS RTK GNSS ROVER (F9P) (opens new window) (store-drotek.com)
- mRo u-blox ZED-F9 RTK L1/L2 GPS (opens new window) (store.mrobotics.io)
Ideally the two antennas should be identical, on the same level/horizontal plane and oriented the same way, and on an identical ground plane size and shape (Application note (opens new window), section System Level Considerations).
- The application note does not state the minimal required separation between modules (50cm has been used in test vehicles running PX4).
- The antennas can be positioned as needed, but the GPS_YAW_OFFSET must be configured: GPS > Configuration > GPS as Yaw/Heading Source.
- The UART2 of the GPS devices need to be connected together (TXD2 of the "Moving Base" to RXD2 of the "Rover")
- Connect UART1 on each of the GPS to (separate) unused UART's on the autopilot, and configure both of them as GPS with baudrate set to
Auto. The mapping is as follows:
- Main GPS = Rover
- Secondary GPS = Moving Base
- Set GPS_UBX_MODE to
- EKF2_AID_MASK parameter bit 7 must be set (see GPS > Configuration > GPS as Yaw/Heading Source).
- GPS_YAW_OFFSET may need to be set (see GPS > Configuration > GPS as Yaw/Heading Source).
- Reboot and wait until both devices have GPS reception.
gps statusshould then show the Main GPS going into RTK mode, which means the heading angle is available.
If using RTK with a fixed base station the secondary GPS will show the RTK state w.r.t. the base station.
# Further Informaiton
- ZED-F9P Moving base applications (Application note) (opens new window) - General setup/instructions.
- GPS > Configuration > GPS as Yaw/Heading Source