DroneKit

DroneKit-Python allows developers to create apps that run on an onboard companion computer and communicate with the ArduPilot flight controller using a low-latency link.

The API communicates with vehicles over MAVLink. It provides programmatic access to a connected vehicle’s telemetry, state and parameter information, and enables both mission management and direct control over vehicle movement and operations.

Drone-kit is a python API corresponding to the functionality of mavproxy. This means you can arm, change flight modes and send waypoints. The constraint being that all of this happens at a low frequency.

If you want to implement an an autonomous vehicle with your very own AI you would choose the GUIDED mode. There are two ways to control the vehicle

Position. In this mode positions in relative/global frame are sent to vehicle (over mavlink) in the form of goto commands.
Velocity The function send_ned_velocity() below generates a SET_POSITION_TARGET_LOCAL_NED MAVLink message which is used to directly specify the speed components of the vehicle in the MAV_FRAME_LOCAL_NED frame (relative to home location). The message is re-sent every second for the specified duration.

RC overid

teleoperation_erle_rover

PX4 pro

PX4 Pro

Build an Agent on your offboard compute which communicate with the PX4 autopilot client. The client and agent both run as daemon process.

PX4-user-guide PX4-dev-guide

PX4-Firmware This repository holds the PX4 Pro flight control solution for drones, with the main applications located in the src/modules directory. It also contains the PX4 Drone Middleware Platform, which provides drivers and middleware to run drones.
PX4-flight stack
PX4-middleware

multiple applications known as Apps are runnig on the Pixhawk2:

~/src/Firmware/Tools$ ./mavlink_shell.py /dev/ttyACM0
Connecting to MAVLINK...

NuttShell (NSH)
nsh> help
...
Builtin Apps:
  adc
  tone_alarm
  fmu
  px4io
  rgbled
  mpu6000
  mpu9250
  lsm303d
  l3gd20
  hmc5883
  ms5611
  sf0x
  ll40ls
  trone
  gps
  pwm_out_sim
  ets_airspeed
  ms4525_airspeed
  ms5525_airspeed
  sdp3x_airspeed
  frsky_telemetry
  sensors
  px4flow
  vmount
  pwm_input
  camera_trigger
  bst
  lis3mdl
  ulanding_radar
  bl_update
  config
  hardfault_log
  mixer
  mtd
  nshterm
  param
  perf
  pwm
  reboot
  top
  ver
  commander
  send_event
  load_mon
  navigator
  mavlink
  gpio_led
  land_detector
  camera_feedback
  ekf2
  fw_att_control
  fw_pos_control_l1
  gnd_att_control
  gnd_pos_control
  mc_att_control
  mc_pos_control
  vtol_att_control
  logger
  uorb
  dataman
  px4_simple_app
  serdis
  sercon

Individual message channels between Applications (Apps) are called topics in PX4.

modules