Looking for ArduPilot in one place: Mission Planner, firmware and the vehicle wikis. [url=https://ardupilot.club/ardupilot/index.html]ArduPilot Documentation[/url] is the page I send people when they ask where to start.
GPS has to sit still before the first flight or Loiter is a mess. I used [url=https://ardupilot.club/copter/docs/common-positioning-landing-page.html]GPS/Compass[/url] for the modules and the Mission Planner checks. After a 3D fix the rest of the setup is normal.
Need RTL and battery failsafe in writing, not a Discord guess. [url=https://ardupilot.club/copter/docs/failsafe-landing-page.html]Failsafe[/url] is the page I bookmarked. Set that before you leave the field with GPS.
For Mission Planner on Windows this is the install path I used. The short guide is here: [url=https://ardupilot.club/planner/docs/mission-planner-installation.html]Installing Mission Planner[/url]. After that, Connect over USB and load Copter or Plane firmware.
Which boards actually run this firmware? The list I used is [url=https://ardupilot.club/copter/docs/common-autopilots.html]Choosing an Autopilot[/url]. Pixhawk, Cube Orange, Holybro, Matek: same ArduPilot, different hardware.
