Altitude (GPS) injection to SITL


I’m lifting a plane using an octocopter - the plane will release itself at a preset altitude and switch to Auto mode. I’d like to simulate the dronekit script that the plane’s companion computer will be running but I can’t see a way to increase altitude without actually starting a flight.

Does anyone know if it’s possible inject GPS altitude changes into the simulation and have an example please?

Dronekit script here:

Thanks, Tim