AI Traffic

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AI Traffic (Artificial Intelligence), AI-Traffic, or Interactive Traffic came to life with FlightGear version 0.9.5. Since then FlightGear has an AI traffic system that is based on the AI-Models subsystem, and which is currently under active development. This page aims to provide the required documentation needed to set up some traffic.

In essence, setting-up the AI controlled traffic system involves three steps:

  1. Create Traffic files,
  2. Build ground networks for the Airports containing traffic.
  3. Create the appropriate Departure / Arrival procedures for each airport involved.

Notice that the "Download all AI aircraft" is no longer necessary, because most, if not all, AI aircraft are now part of the base package.

Building Traffic Files

Traffic patterns are stored in data files in extended markup language (.xml) format. The actual location of these files are version dependent. For FlightGear 0.9.x, traffic files are stored in a subdirectory called Traffic in FlightGear's main data directory, hereafter referred to with it's technical name:

$FG ROOT/Traffic/

In FlightGear 1.0, the traffic files can be found in a subdirectory of:

$FG ROOT/AI/Aircraft/

For example, traffic belonging to a United Airlines 737 would be located in: $FG ROOT/AI/Aircraft/737/737-UnitedAirlines-traffic.xml

FlightGear 1.9.0 has seen yet another move. In this version, the traffic files can be located in:

$FG ROOT/AI/Traffic/

For FlightGear 1.9.0 and later, a new traffic file format was introduced (henceforth referred to as "Traffic Manager II" format (TM-II), in which aircraft and flights are no longer directly coupled, leading to more flexibility.


Each traffic pattern is built around two entities: Aircraft and Flights. Before discussing the details, lets start by exploring these two concepts a little further.

Some Details: TrafficManager, Aircraft, and Flights

As in real life, AI traffic in FlightGear is centered around aircraft. In real life, a commercial airliner is put to use by operating on a series of scheduled flights, on a daily or weekly basis. Take for example the long haul routes that are flown by McDonnell Douglas MD11 aircraft. For instance, the MD11's operated by KLM fly regularly between Amsterdam and various distant destinations, such as San Fransisco or Minneapolis in the United States, Vancouver or Montreal in Canada, Accra and Lagos in Africa, or New Delhi in India. Some of these routes are too long to complete a return flight in one day, such as the trip between Amsterdam and San Fransisco, which is basically an 11-hour flight one-way. So it would take 22 hours of just the flying time, and at least an additional two hour turn-around time at each airport. Therefore, in real-life, it is necessary to operate this aircraft on a series of routes, because other routes are considerably shorter than the trip to San Fransisco. Therefore, the time lost on one route can be gained on another, averaging out to approximately one round-trip a day.

In FlightGear, we wouldn't have to be so strictly considerate about the turnaround times as a real-world airline would, but in the not-so distant future, we also want to be able to see realisticly crowded terminals at our simulated airports, so considering the turnaround time in the schedules is a good thing. Therefore, most of the long-haul aircraft will need to be scheduled to fly more than one route. Hence, each AI aircraft has one or more routes assigned to it, which repeat on a weekly, daily, or hourly basis.

The FlightGear traffic manager system, periodically checks the approximate position of a each aircraft in its database. This database was originally constructed on the basis of a fixed routing table that was assigned to each aircraft. As in the real-world, when a route ends at one airport the next one has to start from the same airport as well. An important difference between the real world and these FlightGear traffic patterns is that while in the real world aircraft schedules are frequently rotated, the FlightGear routes remain the same unless a major update to the database takes place. Real aircraft require maintenance, and are therefore taken out of service periodically. FlightGear aircraft have the advantage that they do not require maintenance in this respect.

There were still some significant drawbacks to this approach, nevertheless. Consider the MD11 a little further. In real life, KLM operates this aircraft on many routes that are serviced at relatively irregular intervals. For example, many flights to the Carribean are served only two or three times a week, making it extremely complicated to build completely accurate traffic files for these routes.

To circumvent these problems, a new database format was introduced in FlightGear 1.9.0. In this new format, flights are no longer directly and rigidly assigned to specific aircraft. Instead, a more generic description of a fleet is given, along with a series of flights that need to be carried out. The routing is then taken care of by FlightGear itself.

An example of a traffic file

Below is a complete and working example of a Traffic Manager II file, as it can be used with FlightGear 1.9.0 and later:

 <?xml version="1.0"?>
 <trafficlist>
    <aircraft>
        <model>Aircraft/MD11/Models/KLMmd11.xml</model>
        <livery>KLM</livery>
        <airline>KLM</airline>
        <home-port>EHAM</home-port>
        <required-aircraft>MD11KLM</required-aircraft>
        <actype>MD11/P</actype>
        <offset>25</offset>
        <radius>39</radius>
        <flighttype>gate</flighttype>
        <performance-class>jet_transport</performance-class>
        <registration>PH-KCA</registration>
        <heavy>true</heavy>
    </aircraft>
    <aircraft>
        <model>Aircraft/MD11/Models/KLMmd11.xml</model>
        <livery>KLM</livery>
        <airline>KLM</airline>
        <home-port>EHAM</home-port>
        <required-aircraft>MD11KLM</required-aircraft>
        <actype>MD11/P</actype>
        <offset>25</offset>
        <radius>39</radius>
        <flighttype>gate</flighttype>
        <performance-class>jet_transport</performance-class>
        <registration>PH-KCB</registration>
        <heavy>true</heavy>
    </aircraft>
  
    <flight>
        <callsign>KLM0765</callsign>
        <required-aircraft>MD11KLM</required-aircraft>
        <fltrules>IFR</fltrules>
        <departure>
            <port>EHAM</port>
            <time>0/12:35:00</time>
        </departure>
        <cruise-alt>330</cruise-alt>
        <arrival>
            <port>TNCM</port>
            <time>0/21:15:00</time>
        </arrival>
        <repeat>WEEK</repeat>
    </flight>
  
    <flight>
        <callsign>KLM0769</callsign>
        <required-aircraft>MD11KLM</required-aircraft>
        <fltrules>IFR</fltrules>
        <departure>
            <port>TNCM</port>
            <time>3/01:25:00</time>
        </departure>
        <cruise-alt>330</cruise-alt>
        <arrival>
            <port>EHAM</port>
            <time>3/10:50:00</time>
        </arrival>
        <repeat>WEEK</repeat>
    </flight>
 </trafficlist>

Dissecting the traffic file

Here I will discuss the general structure of a traffic file in more detail. As discussed, the traffic patterns are centered around aircraft and flights. Therefore, a minimal traffic file will consist of two sections: the aircraft definition and the flights section. In the next two sections, I will discuss each of these sections.

General layout

The general layout of each file should look like the above example.

 <?xml version="1.0"?>
 <trafficlist>
   
 </trafficlist>

The general layout of each file should look like the above example. The first line contains a generic xml header, which is followed on the second line with a <trafficlist> statement. Also, the last line in the file should close off the trafficlist section using the </trafficlist> statement. As will be illustrated below, between these two statements can be one or more aircraft definitions.

Defining the aircraft

 <?xml version="1.0"?>
 <trafficlist>
     <aircraft>
         <model>Aircraft/MD11/Models/KLMmd11.xml</model>
         <livery>KLM</livery>
         <airline>KLM</airline>
         <home-port>EHAM</home-port>
         <required-aircraft>MD11KLM</required-aircraft>
         <actype>MD11/P</actype>
         <offset>25</offset>
         <radius>39</radius>
         <flighttype>gate</flighttype>
         <performance-class>jet_transport</performance-class>
         <registration>PH-KCA</registration>
         <heavy>true</heavy>
     </aircraft>
     <aircraft>
     ...
     </aircraft>
 </trafficlist>

The first lines inside the <aircraft> definition specify some of the aircraft's performance characteristics.

  • <model> Here is a path specified to the 3D model that should be used in FlightGear.
  • <livery> This line is currently unused, and will likely remain unused. The original idea was to combine this with the <model> line (see above) to load a specific combination of aircraft type (<actype>) and paint scheme, but I abandoned that idea, because it didn't turn out to be very compatible with the existing FlightGear model loading code.
  • <airline> This line refers to the airline operating the aircraft. This information is currently used by FlightGear to handle gate/parking assignments. Use the official 3-letter ICAO airline code here, in case of commercial traffic. This keyword is unlikely to be used for general aviation and military traffic.
  • <home-port> Each FlightGear aircraft is assigned to a home airport. Internally, this is used to ensure that routes are setup that will eventually lead back to the home airport. This is done to maintain some sensibility in the routing algorithm. [New for Traffic Manager II]
  • <required-aircraft> This value is a key that binds aircraft and flights together. In case of this example, the key MD11KLM. Indicates that this aircraft will only carry out flights containing the same key. Usually, a combination of aircraft type, and airline will suffice for this key. In some cases, in particular, when specific aircraft / airlines are distributed across multiple hubs (i.e. home ports), it may be necessary to specify a key containing the home airport as well. For example, Delta Airlines operates 767's out of Atlanta, as well as out of New York. To separate between these two cases, it would be advisable to use two keys; one for KATL (e.g. 767DALKATL), and one for KJFK (e.g., 767DALKJFK). [NEW for Traffic Manager II]
  • <actype> A description of the aircraft type reserved for future use in ATC.
  • <offset> Ground offset of the 3D model. Not all aircraft 3D models are built using the same convention. Use this parameter to align the wheels with the ground. Notice that this parameter will probably become obsolete in the near future, because model view point references can also be done in the XML file that the <model> keyword refers to.
  • <radius> An estimate of the aircraft's size. This is mainly used at airports for gate assignments.
  • <flighttype> In the near future, this keyword will be used for runway assignments, so that general aviation, commercial, and military traffic will use different runways if that is part of the airport's operational procedures. This line is also used for gate assignments and should be one of the following:
    • ga (general aviation),
    • cargo (cargo)
    • gate (commercial passenger traffic)
    • mil-fighter (military fighter)
    • mil-cargo (military transport)
  • <performance-class> This line is used to determine the performance characteristics of AI aircraft. This should match one of the performance classes that are predefined in FlightGear. Currently, the following performance classes are supported:
    • light_aircraft (prop driven single or twin),
    • ww2_fighter (world war two fighter),
    • jet_transport (modern commercial jet),
    • jet_fighter (modern fighter aircraft)
    • tanker (tanker aircraft), or.
    • ufo (allows extreme accel/decel capabilities).
  • <registration> The aircraft's tail number. Future versions of FlightGear will use this registration in ATC for general aviation traffic. For commercial traffic the registration number will likely remain unused.
  • <heavy> Can be true or false. Reserved for future use by ATC, to determine whether the postfix "Heavy" should be appended to the aircraft's callsign.

Defining a flight

The original traffic file format contained a series of flights enclosed within the <aircraft> section, e.g.,

 <aircraft>
    ...
    <flight>
    </flight>
 </aircraft>

One major difference from this format is that the Traffic Manager II file formats now contain separate sections for aircraft and flight information, with the common denominator being a shared key that links aircraft and flight information together. In other words, the general layout of a Traffic Manager II file looks something like this:

 <aircraft> 
     ...
     <required-aircraft>MD11KLM</required-aircraft>
     ...
 </aircraft>
  
 <flight>
     ...
     <required-aircraft>MD11KLM</required-aircraft>
     ...
 </flight>

Each flight is defined between the <flight> and </flight> statements. For example, let's have a look at a randomly selected flight from our KLM.xml example.

 <flight>
     <callsign>KLM0769</callsign>
     <required-aircraft>MD11KLM</required-aircraft>
     <fltrules>IFR</fltrules>
     <departure>
         <port>EHAM</port>
         <time>2/12:35:00</time>
     </departure>
     <cruise-alt>330</cruise-alt>
     <arrival>
         <port>TNCB</port>
         <time>2/22:15:00</time>
     </arrival>
     <repeat>WEEK</repeat>
 </flight>

The <flight> section is slightly more complex than the aircraft definition itself, because it has a few nested levels, however, these should mostly be self-explanatory. The following keywords should be specified:

  • <flight> All the relevant parameter should be specified between the <flight></flight> keywords.
  • <callsign> The airline callsign used for ATC. If this is an airline flight it should be combination of the airline callsign (e.g. KLM for KLM, or Springbok for South African Airways), and the flight number (e.g. KLM0605, or Springbok0033).
  • <required-aircraft> The key that links this flight to a particular aircraft. The see explanation in the aircraft section.
  • <fltrules> Can be IFR or VFR. This is required for use in ATC, but currently not used. This is likely to change, however.
  • <departure> Definition of the departure airport. This section should contain the <port> and <time> keywords.
  • <cruise-alt> Cruising altitude for this flight. This is a bit of a simplification from the real world, where aircraft usually don't stay at the same cruise altitude throughout the flight. This behavior will probably also change in future versions. Values are in flightlevels
  • <arrival> Same as <departure>, but now defining the <port> and <time> of arrival.
  • <repeat> Repeat period. This can be either the keyword WEEK, or a number followed by Hr. WEEK means that the whole schedule repeats itself on a weekly basis. Hr means that the whole schedule repeats after the number of hours specified directly before it (e.g. 24Hr means that the schedule repeats after 24 hours). With Traffic Manager II, it is generally recommended not to mix schedules that rotate on different frequencies. In general, the best results are obtained when using only weekly rotating flights. For flights that are in reality operated on a daily basis, it is recommended to just specify multiple entries, i.e. one separate flight for every weekday.
  • <port> This should be the international ICAO airport code. This keyword should be specified only within the <departure> or <arrival> sections. As far as I know, here it should still be in upper case, although the FlightGear command line currently also supports lower case ICAO codes.
  • <time> Used to specify the <departure> or <arrival> time. The format of this string is hour:minute:second. Notice that departure day is optional and is specifically intended to be used in combination with a weekly repeating schedule. When used in combination with other schedules, results may be unpredictable. Times should be in UTC. Weekdays start on Sunday (0) and end on Saturday (6).

Putting it all together: Including a traffic file

After creating a traffic file, all we need to do is make sure FlightGear knows how to use it. In earlier versions of FlightGear, this used to involve quite a bit of editing, because every file had to be referenced in a master traffic file, named fgtraffic.xml.

Those days are gone, however, since FlightGear 1.0.0, when a directory scanning mechanism was put in place. Both FlightGear 1.0.0., and 1.9.0 and beyond use this directory scanning mechanism, however, in order to separate between the Traffic Manager I format used by FlightGear 1.0.0, and the Traffic Manager II format used by later versions, these files are located in two different locations. In FlightGear 1.0.0, traffic files should be located in a subdirectory of the $FG_ROOT/AI/Aircraft directory. For example, the traffic definition for a Boeing 737 would be located in a XML file in $FG_ROOT/AI/Aircraft/737, and traffic for a Boeing 747 would be located in a XML file in $FG_ROOT/AI/Aircraft/747.

Since traffic files for FlightGear 1.9.0 are, strictly speaking, no longer tied to one particular aircraft, or aircraft type, the traffic files were moved to a different directory, namely $FG_ROOT/AI/Traffic. Again, the actual traffic files should be stored in a subdirectory one level below /AI/Traffic. In the demo that is provided with FlightGear 1.9.0, all traffic is organized by airline, and stored in a single letter directory. For example, KLM traffic can be found in $FG_ROOT/AI/Traffic/K/KLM.xml, and United Airlines traffic is stored in $FG_ROOT/AI/Traffic/U/UAL.xml. Although currently no formal definition exists for military or general aviation traffic, a similar scheme could be adapted. For instance, military traffic could be placed in $FG_ROOT/AI/Traffic/mil/USAF.xml

Tools

One major motivation for introducing the Traffic Manager II file format is to make it easier for FlightGear users to contribute to the traffic database. To further motivate this, some tools are currently in development that aim to make user interaction even easier. Although most of these tools are not ready for public use yet, it is probably worth mentioning some of these developments. First of all, the custom scenery project is working on extending their scenemodel database to store traffic. A web based front end will allow users to enter their favorite flight, and thus collect an extensive amount of traffic data. Users will be able to download the collected results and place the downloaded files in their traffic directory or get it via Terrasync.

Secondly, the author of the traffic manager code has written some scripts, mainly for private use, that will allow one to input the flight data into a simple text format and then convert the resulting text file to XML. The most important of these scripts can be found in the FlightGear source package under scripts/perl/traffic/.

Creating AI traffic files this way is easy. First, you need to define the aircraft for each fleet (data taken from the June 2010 Malaysian project):

###HOMEP RegNo  TypeCode    Type           AirLine  Livery  Offset  Radius  FltType  Perf.Class     Heavy   Model
######## ###### ########### ############## ######## ####### ####### ####### ######## ############## ####### ######################################
AC WMKK  TF-ARN 747         747-2F6B       MAS      MAS     19      39      gate     jet_transport  true    Aircraft/747-400/747-400-Malaysian.xml
AC WMKK  TF-ARJ 747         747-2F6B       MAS      MAS     19      39      gate     jet_transport  true    Aircraft/747-400/747-400-Malaysian.xml
AC WMKK  TF-ATZ 747         747-2F6B       MAS      MAS     19      39      gate     jet_transport  true    Aircraft/747-400/747-400-Malaysian.xml


Then, you need to define all the flights according to the following layout:

########Flt.No       Flt.Rules Days    Departure       Arrival         FltLevel A/C type
#################### ######### ####### ############### ############### ######## ##########

# London Heathrow (EGLL; UTC+0100 ) to Kuala Lumpur (WMKK UTC+0800)
FLIGHT Malaysian0001 IFR       0123456 21:00   EGLL    09:25   WMKK    360      747MAS
FLIGHT Malaysian0002 IFR       0123456 15:40   WMKK    04:50   EGLL    360      747MAS
FLIGHT Malaysian0003 IFR       0123456 11:00   EGLL    23:25   WMKK    360      747MAS
FLIGHT Malaysian0004 IFR       0123456 09:05   WMKK    08:15   EGLL    360      747MAS

# Frankfurt (EDDF; UTC+0200 ) to Kuala Lumpur (WMKK UTC+0800)
FLIGHT Malaysian0005 IFR       0.2.456 10:30   EDDF    22:25   WMKK    360      777MAS
FLIGHT Malaysian0006 IFR       .1.3456 15:50   WMKK    04:25   EDDF    360      777MAS


Note that the flight information laid out above resembles that of a standard airline schedule table as close as possible, with a few known exceptions:

  • Weekdays are in the range 0 to 6, but do follow standard airline conventions (0 = Monday; 6 is Sunday)
  • All times in the table are in UTC; to convert from local time, as given in the airline table, subtract the airport in question's UTC offset.
    • eg1: KJFK is UTC-0400 so a flight leaving at 20:00 local New York time is written as 00:00 UTC.
    • eg2: EHAM is UTC+0100 so a flight leaving EHAM at 20:00 local Amsterdam time is written as 21:00 UTC.
  • The last entry in each line (A/C type) should correspond to the combination of the TypeCode and Airline entries in the aircraft sections.

The resulting table can now easily be converted to XML output by running the conf2xml.pl (conv2xml.pl - check) script.

For Operating Systems of the Unix family, simply run conf2xml.pl from the directory where you unpacked the trafficdata.zip archive:

./conf2xml.pl

will give the output:

Writing to ADR.xml
Writing to AFL.xml
...
Writing to VLG.xml
Writing to VLM.xml

When the script encounters an error, it will stop prematurely.

The AI Schedule Manager is a GUI based application that uses a database backend to store and perform operations on flights, fleets and aircraft for airlines.

Ground networks

Using a Groundnet, AI aircrafts can park precisely at the Terminal gates

Using the traffic files information above, the AI Traffic Manager knows which AI aircraft should land at (and take off from) each airport and when. It will automatically place the relevant aircraft models in the scenery and animate them so they navigate from the runways to the gates and vice versa, according to their individual schedule.

Although the physical layout of each airport is stored in FlightGear's APT.DAT master file, the information is not accurate enough to determine which specific routes AI models can use; Instead, Traffic manager will rely on a dedicated file containing a simple wireframe/network of taxiways and gates AI aircrafts can follow whilst on the ground ie a GroundNet. A groundnet is made of 3 different elements: Parking Positions, Nodes and Segments (to join nodes and Parking together).

This routing information is aggregated, per airport, in a XML, stored and distributed by Terrasync as /Terrasync/Airports/[I]/[C]/[A]/[ICAO].groundnet.xml where ICAO stands for the 4 letter ICAO code of the relevant airport.

Similarly to Terrain and Objects, Terrasync groundnets can be overridden by placing a personalized version in your custom scenery folder, using the same path structure: /Custom Scenery/Airports/[I]/[C]/[A]/[ICAO].groundnet.xml.

Older versions of FlightGear store groundnets on your computer as $FG_SCENERY/Airports/[I]/[C]/[A]/[ICAO].groundnet.xml

Groundnets are not mandatory but, in absence of this routing information, AI Aircrafts cannot park anywhere; they will still try to stick to their schedule, appearing magically at the centre of the airport and taxiing directly to the runways’ thresholds, over grass, buildings and static objects, on time.

Groundnets rely on the runway threshold information stored in /Terrasync/Airports/[I]/[C]/[A]/[ICAO].threshold.xml to calculate the point at which an AI aircraft has reached a runway and can initiate take off, along the headings stored in this very same file.


A technical perspective

A ground network xml file consists of four tables:

  • <frequencies> The Airport’s radio frequencies (Optional). As of v1.9.0, FlightGear uses these to display some ATC messages like start-up approval requests. You can "hear" them by tuning to the first ground frequency listed in the section.
  • <parkingList> The details of each parking/gate at the airport and the characteristics of which AI aircrafts can use them.
  • <TaxiNodes> The list of all the nodes in the ground network.
  • <TaxiWaySegments> A list of all links/segments (or "arcs" as David Luff called them initially) existing between all nodes and Parking Positions.

Each Parking and Node element in the groundnet has a unique index number, allowing routes be formed (from one ID to the next via a segment/arc) between parking positions and runways

Parking Positions Parameters:

  • index Unique ID, internal to the file structure
  • lat latitude in decimal minutes format, for example N50 56.988)
  • lon longitude in decimal minutes format, for example W01 21.756)
  • name & number (gate identification as found on airport charts, for example D23 or C1).
  • type: The type of aircraft which can use this space. Matched to the <flighttype> parameter found in traffic files:
    • ga (general aviation),
    • cargo (cargo/freighter)
    • gate (commercial passenger traffic)
    • mil-fighter (military fighter)
    • mil-cargo (military transport)
  • heading: The heading at which the aircraft parks in this space.
  • radius: The size of the parking spot. Matched to the aircraft <radius> parameter in traffic files to determine if a given AI Aircraft model will fit the position. See aircraft radii
  • airlineCodes: a comma-separated list of ICAO airline codes allowed to park at this gate. Matched to the aircraft <airline> parameter of traffic files. Leave blank for all airline to be able to use the gate.
  • pushBackRoute: The ID of the next node in the network along a pushback route. In a correctly configured network, the AI aircraft will taxi to this node in reverse, thus simulating being pushed back. See the documentation for the PushBack hold point type below.

Nodes parameters:

  • isOnRunway' A logical value that is 1 when the node is on the runway, 0 otherwise. Aircrafts waiting to take off will line up behind the last node marked “not on runway” (“0”) until the runway is clear
  • holdPointType can have the following values:
    • None Not a holding point (normal taxi-through node)
    • PushBack The node marks the end of a pushback route, where the aircraft stops rolling backward and start rolling forward upon clearance. A pushback Holding Point can be part of more than one pushback route. See Pushback below.
    • Normal (Not yet supported): a regular taxiway holding point.
    • CAT II/III (Not yet supported): a special holding point for IFR conditions.

segments parameters:

  • begin The id of the parking space or AINode where this segment starts
  • end The id of the parking space or AINode where this segment ends
  • IsPushBackRoute a logical value. Should be 1 if the current segment is part of a route connecting a gate to a push back hold point, or 0 otherwise.
  • name Name of the taxiway.

Tools

Groundnets can be built using either Taxidraw (legacy software) or FG Airports (currently under development).

TaxiDraw pre dates WED and allowed the creation of airport layouts from basic geometrical shapes. Its ground network module is somewhat separate from the rest of the airport project code and allows the creation and export of fully functional groundnets. It is the tool behind all of the ground networks you see in Flightgear today.

FGAirports is currently under development. Its philosophy is airport centered rather than file specific and allows you to visualize at a glance all of the existing groundnets as well as editing them.

See the TaxiDraw and FGAirports articles for instructions on how to obtain the tools and operating instructions.

Reference material

Most Civil Aviation authorities make available on the web, electronic versions of their Aeronautical Information Publication (Lookup 'eAIP') which often contains precise Airport Charts but also lists of parking stands with their precise Latitude/Longitude as well as usage (Cargo, Gate, Maintenance) and the category (radii) of aircraft they can accommodate. Gates Numbers and Airlines operating them can often be found on Airports website. Most flight tracking sites/apps will also allow you to monitor a flight all to the way to its gate to identify who parks where. In essence, all the information you need can be compiled from multiple sources, including Wikipedia, airport diagrams published on the net, in flight airline magazines, etc. etc. In other words, be creative!

Warnings and Limitations

The complexity of building a fully functional groundnet (and the time spent on it) grows exponentially with the size of the airport but very small airports, on Pacific Islands for example, pose even larger challenges. An ideal project to start with is a Metropolitan airport with one or two runways, and two dozen of parking positions.

eAIP showing a Parking Stand accommodating more than one aircraft category

FG Scenery and Traffic manager have their limitations and create specific challenges of their own. For example:

  • TM does not handle the spacing of arriving aircraft and you will most likely see packs of aircrafts landing at once on your runways, sometimes from both ends.
  • TM does not understand Holding Points so you cannot prevent an AI aircraft to cross an active runway if you have placed a route across it.
  • TM cannot handle the conditional usage of gates which accommodate more than one aircraft type (See Image on the right) so you should always assume that all the Parking position you set will be occupied.
  • AI Aircrafts cannot "pivot" on their parking positions so the route followed to reach a parking position must have the same heading than the parking position itself (Park Straight)
  • Groundnets, terrain and scenery objects are not technically interdependent and you may see your aircrafts rolling on grass or sitting on buildings. Before blaming your groundnet work, consider that the airport terrain/layout may be outdated or misplaced. Similarly, Building/Object may have been placed at certain Lat/Lon but since demolished to make space for a new taxiway as airports expand and change layout regularly.

Your candidate groundnet will need to be tested thoroughly by running it in FG, at different time of the day as wind conditions and traffic patterns will impact which runways are used and how many AI aircrafts are handled. You should have log/debug enabled as traffic manager will create :ai entries, allowing easier troubleshooting.

On the bright side, both Taxidraw and FG Airports contain a validation tool which will allow you to detect any structural problem with your groundnet. You will also find a lot of resources and tips in the AI section of the Flightgear forums.

Finally, the approach to creating a groudnet is described below as three separate phases, each one including its own specific testing which will hopefully simplify your journey.


Creating the network

The current documentation describes the procedure for the TaxiDraw NEW_GUI_CODE branch, which is rather drastically different from the procedure used by the original TaxiDraw, previously documented here. This section describes some of the general procedures involved in building a ground network.

Note: export your work regularly: Although ground network editing capabilities have improved dramatically, this part of TaxiDraw is still somewhat separate from the rest of the code, and ground networks are not automatically saved together with the project. When exporting, TaxiDraw asks whether the ground network should be saved directly into FlightGear's data directory. Usually, this is okay, so just clicking "yes" takes care of saving the file in the correct location. Likewise, while importing an existing network, TaxiDraw first looks in FlightGear's data directory, to check for an existing default file to be imported.

[Scenery v2]: For 850 Airports, a python script is available, to automatically create a basic ground net out of WED's earth.wed file. It converts Yellow Markings and Park Positions into a ground net which can be read, and refined by Taxidraw. Note that both Centerline and borders are converted (remove border nodes once in Taxidraw) See HERE


Set up a rough outline

To make best use of the background image, right click, choose the "view" menu, and unselect "taxiways", this way, all network drawing can be based on the real airport location. First, select the "Insert startup location" edit mode. This way, each left mouse click will place a startup location at the position of the mouse cursor. Once the major startup locations have been placed, switch back to "selection" mode, by clicking the little arrow button.

Next, start editing all the relevant startup location parameters, Parking heading can be changed by dragging the little red circular heading indicator. The parking space's radius can be changed by clicking anywhere on the big red circle. Then you can change the parking space's radius by dragging the mouse. Although this usually works for setting up a rough draft. these parameters can directly be changed in the properties dialog. If the properties dialog UI is not visible, press [ctrl-p] to bring it up. It is possible to select multiple startup locations by pressing the shift key while selecting, and the parameters for each selected startup location can be changed at the same time. This is a powerful feature for editing the parameters of a whole series of parking locations, and also for equating and orienting gate radius and heading.

With the startup locations in place, select one of them. The next step will be to connect this parking area to each runway end. After selecting the parking, click on the "insert bidirectionally connected network nodes" (the two green dots icon), and start drawing. Place nodes at strategic locations, such as intersections, corners, and arcs. finally, place one node at the centerline of one of the runways. Once this is finished, one gate is connected to one end of one runway. The next job is to extend the network by connecting more gates and more runways.

Traffic Manager 'calculates/understands' each runway centreline as the line between the two threshold points stored in the matching airport ICAO file in Terrasync/Airports/[I]/[C]/[A]/[ICAO].threshold.xml. Whenever placing a route end node on a runway, you should ensure the node sits on or close to this line. Traffic manager will then use the vector between the two threshold to trigger the AI aircraft take off sequence (deciding that the aircraft has reached the centreline and can start speeding up) as well as setting its take off heading. This is most important in case of displaced thresholds.

Backtracking

AI Groudnet configuration for backtracking in absence of parallel taxiway

Certain airports do not have taxiways along the runway and aircrafts will 'backtrack' on the runway itself to reach the threshold (and pivot on a turnaround area to align for take off). In this scenario, you must place nodes and segments to form a route on the runway to guide the AI aircrafts all the way from their access point to the threshold, including on the turnaround area

Create a second branch from the existing route, which goes to another runway. To do so, press [ctrl-a] to deselect all objects. Then, select the point where you want to branch off from. Select this point using a left mouse click, while holding down the shift key. Since TaxiDraw is still in node connect mode, simply left-clicking would have resulted in placing a new node. With this node selected, continue drawing. Repeat this procedure until all runways are connected.

Then, start adding the additional gates to the network. Press [ctrl-a] to deselect all objects. Then select a gate by pressing [shift-left mouse button], and start drawing until the the gate in question is one segment away from being connected. Then, [shift left-click] the network node that this route segment should be connected to, and make sure that the correct node is selected. Then, right click, and select "connect nodes bidirectionally" from the AI nodes menu.

Repeat this procedure until all parking locations are connected to each runway.

Refining the network: Mark points on the runway as such

With the basic infrastructure in place, the next step is to fine-tune the network. The first step in this process is to mark the network nodes that are located on a runway. This is important for a number of reasons: TaxiDraw needs these OnRunway points for it's network verification tool. Secondly, the use of the type of runway marking will be used in future versions of FlightGear for runway entrance / exit calculations, in addition to a host of other possible uses in routing (such as blocking a certain node then a crossing runway is in use, etc, etc).

Taxidraws's verification tool will automatically detect and report discrepancies (Node not marked 'on runway' where it should and vice versa) for as long as the runway(s) elements are included in the project file.

Refining the network: Set Holding points

Holding points are nodes in the network that do something special. They can be used by FlightGear to make traffic wait at certain specific locations. As a matter of fact, the 'normal' and 'Cat II/III' hold points are not yet used by FlightGear, but are part of the development plan. Currently, hold points are implicitly assigned by the FlightGear AI code. For instance, traffic waiting for takeoff clearance is holding at the first point before the runway. Likewise, traffic can receive a hold position instruction just before an intersection in the taxiway system, when traffic is approaching at the other leg. While these implicit hold points work reasonably well, there are some limitations. Consider for example runway 07 at CYYC (Calgary Intl, Canada). In the FlightGear rendition of this airport, there are no taxiways leading to this runway, so departing aircraft should back track down the runway from an entrance point at the middle of the runway. Currently that point will also become the runway entrance holding point. This is clearly undesirable, because it means that traffic is actually waiting on the runway in order to get clearance to enter it. For this reason, airport ground network designers should have a means to have explicit control over holding points. The normal hold point is meant for just that.

The Cat II/III hold point type is meant as an extension to this scheme for special IFR conditions. Support for these types in FlightGear is planned for a later stage of development though.

Refining the network: Pushback routes

With the above-mentioned refinements, the ground network should be fully working with one notable exception. Aircraft will be driving forward when leaving the gate, making a sharp turn (while probably destroying themselves and the terminal building in the process). To prevent this, a push back route should be created. A push back route consists of at least one or more taxiway segments that have the "PushBack Route" property set to true. The last of these segments should be terminated by a PushBack HoldPoint network node. Pushback routes are optional (if you like the terminal crashing scenario described above).

Examples of simple valid pushback routes (Left: If the taxiway is bidirectional | Centre: If the taxiway is not bidirectional | Right: With shared Pushback Holding point)

Valid AI Groudnet pushback for  multi directional taxiway Valid Pushback Route on Un directional taxiway Shared Pushback Holding Point

Invalid Park Pos Exist route : at and angle with Parking Position
Pushback Holding Points must be unique per Parking Position

Each Parking space (ParkPos) can't have more than one push back route and one pushback holding point at the end of the route. Nevertheless, multiple Parkpos can share part of their pushback routes and a single Pushback Holding Point.

The formal criteria for a valid push back route is that each gate should have a maximum of one push back holding point associated with it, which can be reached using one route only. From an editing point of view, mark all segments between your parking position and its final holding point as "push back", do not forget to mark the ending node as Pushback Holding Point.

The AI code does not handle sharp angles

Add nodes to smoothen your routes so that no consecutive segments form an angle of less than 90 degrees. This rule applies to both pushback and roll forward / taxiways routes

The Inbound route and the outbound routes are parallel
Invalid Groudnet Pushback : Holding Point on Taxiway

The last segment of the pushback route will condition the aircraft heading:

1. During its travel along the pushback route

2. At the Pushback Holding point (whilst waiting for clearance)

3. On departing the pushback holding point, rolling forward to the runway.


You can not control/force an AI aircraft to leave a Pushback Holding point at a different angle/heading than the one it came onto its final holding node at, whatever the number of routes you add.

As a result, the last segment of the pushback route must align with the segment you expect the aircraft to use when starting to roll forward after clearance. The simpler set up is to have this final pushback segment overlapping the first segment of the roll forward route.

Alternatively, the last segment of the pushback route can -itself- be the first segment of the roll forward route but this means Traffic Manager will consider this segment 'reserved' whilst the aircraft is pushing back and no other aircraft will be able to use the taxiway.

For the same reason the Pushback Holding point should NOT be directly placed on the taxiway as shown in the image on the right 'On Taxiway Pushback Holding Point in Taxidraw'. Using this particular configuration, once the aircraft 'cleared for taxi', it will ignore the taxiway segments on its left and right and keep its heading, rolling forward towards the original parking position, then 'get lost' and start spinning around, looking for a node to re-anchor to.

The "Roll Forward on my current heading" rules also applies to Parking position with no pushback route. These are often used for smaller propeller aircrafts which "pivot" on their parking position before rolling forward. You cannot currently replicate this pivot behaviour in Flightgear AI


It is important to note that the Taxidraw "Verify Ground network" process should be run in order to get a correctly working push back system, because this function runs some internal consistency checks. Push back routes can be very simple, from just one route segment, to fairly complex, as illustrated below with all aircraft from one side of the E terminal are being linked to one shared push back point.

Given the current push back system allows for fairly complicated behavior it is advisable to test extensively your groundnet and play with various configurations before sharing it to the community.

Taxidraw note: TaxiDraw versions prior to February 5, 2009 did export the pushBackRoute attribute correctly.

TaxiDraw2.png

Verifying the network

Finally, with a network complete, it is important to verify it! The verification function not only detects obvious problems with the network, it also updates some internal states that FlightGear relies on. Presumably, an automatic verification process will be added to the export function, but until that is the case, make sure to do this manually. The verify network function can be found in the "Tools" menu.

Notice that TaxiDraw does not automatically fix problems. It is left to the user to fix the problems manually. TaxiDraw does select the offending node(s) / segments(s) for easier identification. Also note that TaxiDraw stops verifying at the first problem encountered, so it is worthwhile to continue checking until no further errors are found. Currently, the following checks are performed.

  • On runway points Added on January 24, 2009, this check is most likely not yet available in any distributed version. This is currently just a very lame check to see if any point in the network has been marked as such. This check is not exhaustive, but simply meant as a reminder to the editor that the OnRunway points should still be marked. Ultimately, this check should be replaced by the aforementioned automatic geometry function.
  • Duplicate Taxiway Segments It's easy to connect two network nodes twice. While this doesn't really hurt, it does add dead weight, so checking for duplicates is not a bad thing.
  • Routing One of the most persistent headache causing problems is that of a disconnected parking space in the network. FlightGear will happily place an aircraft there, but bail out when that aircraft cannot reach the runway. The routing check attempts to prevent this. Notice that it is of utmost importance that the OnRunway points are set correctly, because TaxiDraw relies on these points for it's route finding algorithm. Because the route finding algorithm is rather computationally intensive, some progress information is currently written to the console (a proper progress bar would be nice). When a disconnected parking space is found, TaxiDraw selects both the parking and the runway node. It is still left to the user to trace a route between these two points and find where the two pieces are disconnected.
  • Check and set pushback nodes this function verifies whether any specified pushback nodes adhere to the above specifications and updates some internal consistency.

Copying the ground network into FlightGear's scenery directory

Finally, once you have finished creating a groundnet project, you can test it in FlightGear. Create a directory in your $FG_SCENERY/Airports/[I]/[C]/[A]/ directory, with the three first letters of the ICAO code of your airport. For example $FG_SCENERY/Airports/E/H/A/.

In Taxidraw menu select File, Export AI Network and save the file in the above folde as ICAO.groundnet.xml. Please note that the File / Save project menu applies only to the airport layout and does NOT save your groundnet.

Testing the network

Startup FlightGear at the airport for which you have just created the network, and make sure you have traffic for that aircraft. FlightGear will check the network and report errors on the console. Aircraft that can't be placed at one of the parking locations will be placed at a default location.

If the FlightGear AI system can't find a valid route between startup location and runway, it will list which nodes are not connected and exit.

Airports with ground networks

As of Sep 2015 Flightgear has 90 airports that were made with Taxidraw. Here is a List: Airports with ground networks

user:laserman is offering additional groundnet.xml files for 7976 airports that were automatically converted from apt.dat at this URL: http://media.lug-marl.de/flightgear/7976groundnets.tgz and on github https://github.com/mherweg/Airports

900 airports have a complete groundnet, the rest have only parking positions. They (and those 90 airports that were made with TaxiDraw) are distributed via Terrasync since January 2016.

SIDs / STARs

SID is an acronym for Standard Instrument Departure. Likewise, STAR is an acronym for Standard Terminal Arrival Route. Directly after takeoff, in particular at busy airports, aircraft follow a standard flight path, that will keep them safely separated from arriving traffic, avoid ground obstructions, and also keeps traffic away from populated areas as much as possible. Currently, steps are in progress to allow FlightGear traffic to follow SIDs. Ultimately, the plan is to provide SID and STAR data in a format that can also be used by the user controlled Aircraft's Flight Management Computer. Currently, some sample data exist in the form of a PropertyList formatted XML file that contains a list of way points.

This section of the AI Traffic documentation is meant as a stub that keeps track of the current development. At the moment of writing, some proof-of-concept data for EHAM SIDs will be committed to the World Scenery Repository, along with some experimental code in FlightGear to make use of this data. Note that this data is meant to be just that; a proof-of-concept. User contributions will be accepted as soon as the format has stabilized.

Appendix: Special Notes

Realism

With FGFS 1.0.0 Interactive Traffic also responds to other Non-AI aircraft. The A320 ( and maybe some others) has working flaps and gears.

Perfomance

With recent versions of FlightGear, we have much better performance than before so realistic density of traffic is currently possible even for slower computers. For those who are still afraid, there is an, as of yet, unpublished feature: set a new property "/sim/traffic-manager/proportion" to any value between 0 and 1 in your preferences.xml. During program start up, the traffic manager draws a random number (between 0 and 1) for each aircraft, and if that random number is smaller than the value specified in /sim/traffic-manager/proportion, the aircraft is added, otherwise it is discarded. In essence, only the specified proportion of aircraft will be loaded.

Unfortunately you can't change this at runtime yet, so you need a little bit trial and error! However, it should allow the combination of slower computers and dense traffic files.

Related content

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