High-Level PX4 v1.9 Release Notes

Highlights

ARM M7 silicon errata fix
Ethernet networking support
Orbit mode (multicopter)
Multicopter airmode
Obstacle avoidance in missions (with full 3D navigation, not just basic points)
Collision prevention in position mode (with full 3D navigation)
New flight tasks system enabled by default
Smoother takeoff for multicopters
Smooth trajectory generator for multicopter auto mode
GPS heading support for use in magnetically challenging environments
Forgiving default filter settings for larger multicopters with high vibration levels
Proven failsafe between different IMUs (Pixhawk 2.1 hardware errata of HEX products successfully detected and handled)
Operating system: NuttX upgrade from 7.22 to 7.28+
Option to build single-vehicle-type binaries for flash-constrained microcontrollers
Simulation improvements
Better VTOL models
Simplified multi-UAV simulation with JMAVSim
Run simulation faster/slower than realtime (lockstep simulation)

With over 3000 commits a lot of the changes are not listed here. These changes represent the main improvements in the system.

Improved support for tiltrotors
Rate controller architecture change (run fixed wing & multicopter rate controller together to have dedicated rate controller for one set of actuators)
Active weathervane based on direction of desired thrust vector
Migration guide (vtol roll direction parameter - V19_VT_ROLLDIR)

Reduced latency from receipt of IMU data to output of vehicle Pose data
Multi GPS support (GPS blending and failover)
Support for Fusion of GPS heading
Miscellaneous optical flow data use improvements
Miscellaneous VIO data use improvements
Improve stability of attitude alignment and yaw reset at large tilt angles (eg tailsitters)

Full bidirectional VIO uORB messaging, with linear/angular velocity and covariance propagation, besides pose data;
Fast-RTPS bridge client and IDL definitions code generation improvements for a better integration with px4_ros_com (PX4 bridge with ROS2).

Collision Prevention in Position Control
Obstacle Avoidance in Mission Mode
Pre-flight checks and failsafe mechanisms for Obstacle Avoidance in Mission mode

SITL lockstep (jMAVSim + Gazebo support)
Faster or slower than real-time simulation, support to pause or step through code, enables SITL on older or slower hardware
Simulation in Hardware module
Avoidance in Gazebo
Serial port configuration improvements