Gazebo

=Gazebo=
The nice sim framework Gazebo from here http://gazebosim.org/ can now be use from within Paparazzi. Are you doing work on UAS in combination with e.g. Vision based navigation check it out. I might make testing your new work so much simpler.
Be warned, using it can be highly addictive, and might tempt you into buying a new computer with high specifications. To be able to use Gazebo a very good Video card is needed, consider upgrading you hardware if everything runs slowly.
Note that from stable version 5.14 onwards, only version 8 and 9 of Gazebo work in Paparazzi on Ubuntu 16.04 or higher.
Gazebo back-end for NPS NPS simulator
{TODO: Animated GIF here}}
=Why=
What is the use of this all.
Well... to be able to test your routines before you get cold, or to warm flying your aircraft for real outside and save some travelling time in the same run.
Of course there should be a much better text here.. It's wiki, so please add it, and remove what you see fit.
=Installation=
We assume you already have Paparazzi installed via the One liner. If Not install Paparazzi first please. If that all worked, come back to this page.
==Gazebo Installation==
Steps to install Gazebo
1. Make sure Gazebo is installed, version 9 if you are on Ubuntu 18 and higher
 $ sudo apt install gazebo9 libgazebo9-dev
If for some reason this doesn't work, you can find some additional instructions here: http://gazebosim.org/tutorials?cat=install&tut=install_ubuntu&ver=9.0
===Model===
Prepare your Gazebo aircraft model (example see conf/simulator/gazebo/models/ardrone/):
Place the aircraft model in the conf/simulator/gazebo/models/ folder, this folder is added to the search path of Gazebo when NPS is launched.
Gazebo uses a Front, Left, Up coordinate system for aircraft, so make sure the +x axis points forwards.
The model should include a link for each motor with the same names as those listed in NPS_ACTUATOR_NAMES (see below), e.g. 'nw_motor'. Camera links should have the name specified in .dev_name in the
 corresponding video_config_t struct, see sw/airborne/boards/pc_sim.h and sw/airborne/modules/computer_vision/video_thread_nps.c
Additional models can be found in the sw/ext/tudelft_gazebo_models. You will have to run git submodule init, git submodule update to pull in the models.
===World===
Prepare the world (see conf/simulator/gazebo/worlds/ardrone.world or any other world file you might find there). 
NOTE: Pay attention to the following:
The real-time update rate should be set to zero, as the simulation back-end is already handled by Paparazzi:
 <physics type="ode">
 <max_step_size>0.001</max_step_size>
 <real_time_update_rate>0</real_time_update_rate><!-- Handled by Paparazzi! -->
 </physics>
Spherical coordinates should be provided for navigation. At this moment, there is an issue where Gazebo incorrectly uses a WSU coordinate system instead of ENU. This can be fixed by setting the heading to 180 degrees as shown below:
 <spherical_coordinates>
 <surface_model>EARTH_WGS84</surface_model>
 <latitude_deg>51.9906</latitude_deg>
 <longitude_deg>4.37679</longitude_deg>
 <elevation>0</elevation>
 <heading_deg>180</heading_deg><!-- Temporary fix for issue https://bitbucket.org/osrf/gazebo/issues/2022/default-sphericalcoordinates-frame-should -->
 </spherical_coordinates>
Additional world models can be found in the sw/ext/tudelft_gazebo_models. You will have to run git submodule init, git submodule update to pull in the models.
===Airframe===
Enhance your Paparzazzi airframe file to be able to use Gazebo (see examples/ardrone2_gazebo.xml):
Select Gazebo as the FDM (Flight Dynamics Model) by adding it to the aircraft file
 <target name="nps" board="pc">
 <module name="fdm" type="gazebo"/>
 </target>
Add actuator thrusts and torques to the SIMULATOR section:
 <section name="SIMULATOR" prefix="NPS_">
 <define name="ACTUATOR_NAMES" value="nw_motor, ne_motor, se_motor, sw_motor" type="string[]"/>
 <define name="ACTUATOR_THRUSTS" value="1.55, 1.55, 1.55, 1.55" type="double[]"/>
 <define name="ACTUATOR_TORQUES" value="0.155, -0.155, 0.155, -0.155" type="double[]"/>
 ...
 <section>
The thrusts and torques are expressed in SI units (N, Nm) and should be in the same order as the ACTUATOR_NAMES.
In the same section, bypass the AHRS and INS as these are not supported yet, so add this
 <section name="SIMULATOR" prefix="NPS_">
 ...
 <define name="BYPASS_AHRS" value="1"/>
 <define name="BYPASS_INS" value="1"/>
 ...
 <section>
 If you want to use visual based behaviour, enable video thread simulation:
 <section name="SIMULATOR" prefix="NPS_">
 ...
 <define name="SIMULATE_VIDEO" value="1"/>
 ...
 <section>
Specify the Gazebo world and aircraft name:
 <section name="SIMULATOR" prefix="NPS_">
 ...
 <define name="GAZEBO_WORLD" value="my_world.world"/>
 <define name="GAZEBO_AC_NAME" value="my_uav"/>
 <section>
 4. Make sure all included modules work with NS. 
At the current state of Paparazzi code (20180206), most of the modules that depend on video_thread are only built when the target ap (autopilot hardware) is selected as the target. 
As a quick 'n dirty fix, try to remove the target attribute from the makefile element in the module xml, e.g.:
<makefile target="ap"> ---> <makefile>
It would be great if as a user you would improve this and make a Pull request of your code improvements to the main Paparazzi codebase, TIA
=Reality=
Never forget... A sim is not reality... validate via testflights in the real world after you think everything you created works perfectly.