UM982 Dual-Antenna RTK

The FlyCore flight controller has an onboard UM982 GNSS/RTK module, which is connected to GPS1 in ArduPilot.

Warning:

Reversing the primary and secondary antenna connections, using an incorrect baseline direction, or having an unstable GPS yaw can cause heading anomalies. Do not take off until the heading has been physically verified.

Official reference: ArduPilot GPS for Yaw. The official webpage may be updated. When using it, verify the parameter definitions for ArduPilot Copter 4.8.0.

For the pinout of the antenna and RTK-related interfaces, see Appendix: FlyCore-FC Interface and Pin Definitions.

Prerequisites

• Both the ANT1 primary antenna and ANT2 secondary antenna are securely mounted and will not move during flight.
• The antennas are in an open outdoor location, with obstructions, metal structures, and multipath environments avoided as much as possible.
• The parameters have been backed up, and the GPS and EKF status can be viewed in Mission Planner.

1. Understand the Test Configuration

2. Install the Primary and Secondary Antennas

1. Connect the primary antenna to ANT1 and the secondary antenna to ANT2.
2. Securely mount both antennas to prevent changes in their position or angle during flight.
3. Maintain sufficient baseline distance.
4. Record the position of the primary antenna relative to the secondary antenna in the body coordinate frame.

The body coordinate frame is defined as follows: X points forward toward the nose, Y points to the right side of the aircraft, and Z points downward. The unit is meters.

3. Set the Baseline Offset

GPS1_MB_OFS_X/Y/Z represents the position of the primary antenna relative to the secondary antenna:

1. Measure the X, Y, and Z offsets of the ANT1 primary antenna relative to the ANT2 secondary antenna.
2. Enter the measurements in meters into GPS1_MB_OFS_X/Y/Z, respectively.
3. Save the parameters and restart the flight controller.
4. Wait for the GPS yaw to stabilize in an open outdoor environment.
5. Point the nose in a known direction and verify the heading displayed in Mission Planner.

For example, when the primary antenna is 0.3 m to the right of the secondary antenna:

GPS1_MB_OFS_X = 0
GPS1_MB_OFS_Y = 0.3
GPS1_MB_OFS_Z = 0

4. Set the GPS1 Position Offset

GPS1_POS_X/Y/Z represents the position offset of the GPS1 antenna relative to the aircraft reference point. The positive coordinate directions are likewise forward, right, and down.

1. Determine the aircraft reference point.
2. Measure the X, Y, and Z offsets of the GPS1 antenna relative to this reference point.
3. For installations with a large airframe, a long baseline, or an antenna far from the reference point, enter the actual measured values.
4. Save the parameters and restart the flight controller.

5. Inject RTCM Differential Data

• External injection: After receiving RTCM data, the video and data transmission module forwards it to the positioning module through a serial port or network link.
• Onboard system injection: The onboard system receives and forwards RTCM data to the positioning module.

After correction data is received, the GPS status will change to RTK Float or RTK Fixed.

6. Verify Status and Heading

1. Confirm in Mission Planner that the GPS has been detected.
2. Check the satellite count, HDOP, or accuracy indicators.
3. Observe whether the status changes to 3D Fix, RTK Float, or RTK Fixed, and confirm that it remains stable.
4. Point the nose in a clearly known direction and verify the heading angle.
5. Slowly rotate the aircraft horizontally and confirm that the heading changes in the same direction as the actual rotation, without sudden jumps.

Common Issues

If the heading or positioning status is not stable, stop takeoff preparations and retain the logs for troubleshooting.