GPS Heading/Yaw (aka Moving Baseline)

PX4 Guide for RTK

PX4 Firmware GPS Heading with Dual u-blox F9P is supported in the UART version, but not the DroneCAN version at the moment.

RTK GPS Heading with Dual u-blox F9P | PX4 User Guide

RTK GPS | PX4 User Guide

Ardupilot Wiki for RTK

Ardupilot GPS Heading with Dual u-blox F9P is supported in both the UART and DroneCAN version.

GPS for Yaw (aka Moving Baseline) — Copter documentation

GPS/Compass (landing page) — Copter documentation