### Using AI objects in FlightGear

Starting with FlightGear version 0.9.4 you can place AI objects in the "FlightGear world". An example is the sailboat that moves through the waters just east of KSFO (take a look!). In version 0.9.4 the AI objects can be defined in the `preferences.xml` file, or in an airplane's `*-set.xml` file. In later versions of FlightGear they are defined in a "scenario file" only. Other types of AI objects are airplanes, thunderstorms, thermals and ballistic objects.
AI objects have some things in common: The have a location in the "FlightGear world", they can have an associated exterior 3D model, and they can move according to an internal FDM (flight dynamics model). As of now, these objects are created at simulator start-up by adding some XML code to a scenario file. The scenario file must be in the `data/Data/AI` directory. You select which scenario file you want to use by naming it in the `preferences.xml` file. The `preferences.xml` file has an entry that looks like this (FlightGear versions newer than 0.9.4, including CVS):
`<ai>
<enabled type="bool">true</enabled>
<scenario>demo_scenario</scenario>
</ai>
`
The scenario contains one entry for each AI object. The entry specifies what kind of object to create, what it's initial conditions will be, and (for aircraft and ships) a flight plan. The entry for a sailboat would look like this:
`<entry>
<type>ship</type>
<path>Models/Geometry/sailboat.xml</path>
<speed type="double">12.0</speed>
<altitude type="double">0.0</altitude>
<longitude type="double">-122.33333</longitude>
<latitude type="double">37.61667</latitude>
<heading type="double">20.0</heading>
<rudder type="double">-3.0</rudder>
</entry>
`
Most of the entries are self-explanitory. The "type" of object can be one of "aircraft", "ship", "storm", "thermal" or "ballistic". The rest of the items give the AI object a model, a starting location, and a starting speed and direction. You use the <path> item to give the object any valid exterior model. You can even make the ship look like an airplane if you want! The "ship" type can also have a <rudder> value specified, which will cause the ship to move in a circle (HINT: use small values, five degrees or less, and right rudder is positive). Here is an example of how to create an aircraft AI object:
`<!-- puts an A-4 north of KSFO, orbiting at 7000 ft -->
<entry>
<type>aircraft</type>
<class>jet_fighter</class>
<path>Aircraft/a4/Models/a4-blue.xml</path>
<speed type="double">320.0</speed>
<altitude type="double">7000.0</altitude>
<longitude type="double">-122.6</longitude>
<latitude type="double">37.9</latitude>
<heading type="double">210.0</heading>
<roll type="double">-15.0</roll>
</entry>`
It looks much the same as the ship AI code. There are two differences, the <class> item and the <roll> item. If the class is set to "tanker" the airplane will allow you to refuel if you can get close behind it (so far supported by JSBSim CVS version only) The "roll" will cause the airplane to fly in a circle. In the above example the A-4 will be orbiting to the left at 15 degrees of bank. You can also create a ship or airplane with a flight plan. In this case the object will follow the flight plan and then delete itself when it reaches the end. The flight plans are in `data/Data/AI/FlightPlans`. To create an airplane with a flightplan do this:
`<entry>
<type>aircraft</type>
<class>jet-transport</class>
<path>Aircraft/737/Models/737.xml</path>
<flightplan>KSFO_ILS28L.xml</flightplan>
</entry>
`
In this case you don't need to specify initial conditions because they are already defined in the flight plan. To make a thunderstorm, use this:
`<!-- puts a thunderstorm overhead OSI (Woodside VOR) -->
<entry>
<type>storm</type>
<path>Models/Geometry/thunderstorm.xml</path>
<speed type="double">20.0</speed>
<altitude type="double">4000.0</altitude>
<latitude type="double">37.3917</latitude>
<longitude type="double">-122.2817</longitude>
<heading type="double">90</heading>
</entry>
`
There's not much to it. No, they don't turn :) To create a thermal, use this:
`<!-- puts a thermal over the control tower at KSFO -->
<entry>
<type>thermal</type>
<latitude type="double">37.61633</latitude>
<longitude type="double">-122.38334</longitude>
<strength-fps type="double">8.33</strength-fps>
<diameter-ft type="double">4000.0</diameter-ft>
</entry>
`
The AI thermals don't move, they are invisible, and they don't "lean" downwind. The <strength-fps> defines the maximum vertical velocity of the airmass at the center of the thermal. The strength decreases to zero at the thermal's edge. A ballistic AI object starts with an initial azimuth, elevation and speed, then follows a ballistic path from there (with air resistance included). Try this:
`<entry>
<type>ballistic</type>
<path>Models/Geometry/rocket.xml</path>
<speed type="double">500.0</speed>
<altitude type="double">50.0</altitude>
<longitude type="double">-122.39</longitude>
<latitude type="double">37.62</latitude>
<heading type="double">200.0</heading>
<azimuth type="double">70.0</azimuth>
<elevation type="double">45.0</elevation>
</entry>
`
The AI storm objects can be displayed on weather radar. See the `Aircraft/Instruments/wxradar.xml` file for details, or fly the OV-10 for a demo. The AI aircraft objects can be displayed on radar. See the `Aircraft/Instruments/radar.xml` file for details, or fly the T-38 for a demo.

Dave Culp, 22 June 2004