Linux in Air Traffic Control

Linux is the natural development platform for the Federal Aviation Administration's air-traffic control software.

Many people say Linux isn't ready for air traffic control, but in reality, it is ready and already is being used. Recently I was involved in a project that ported the FAA's Common ARTS software ( to Linux.

A Controller in Atlanta

When most of us think of air traffic control systems, we usually see round screens with a sweep. Some people know what the RADAR antenna array looks like. Between the RADAR antenna and the display are a few computers to make things easier. The Digital Signal Processors (DSPs) are in the antenna building, and automation systems are used as well. This article describes how the automation system works, and where Linux was used.


The Automated Radar Terminal System (ARTS) got its start back in 1964 on Univac computers. The system eventually went nationwide in 1973. The original computers have been upgraded and continue to be used today. Many of the larger sites moved from the Univac computers to microprocessors during the 1980s. All of the legacy ARTS software was ported or rewritten in C to run under the real-time LynxOS during the move to microprocessors.

Full digital ARTS display. The ACDs (right side) are replacing these vector displays. All the knobs are now menus on the display.

The move to LynxOS was fortuitous, as it provided a POSIX base to which further porting could be done. Using LynxOS also allowed developers to choose which microprocessors to use. Initially, the software ran on Motorola 68K CPUs, and it currently runs on PowerPCs.


The Common ARTS system is a highly distributed, networked, multithreaded, real-time system. Absolute reliability is a requirement. Dual networks are used, and under normal conditions, two backups are assigned to each specific task. The software is designed such that some functions of one subsystem can be taken over by another subsystem.

The RADAR data comes in over serial lines to each of the track processors (TPs). Normally, four serial lines go from each RADAR to each TP. The data on the RADAR include the raw RADAR signal, a transponder beacon (each aircraft should have a transponder that returns an ID and an altitude in the beacon each time the RADAR pings the aircraft) and weather data. The same antenna array usually receives all three signals. Any of the RADAR and beacon data can go down any of three serial lines, with weather transferring on its own serial line.

The TP is divided into at least two subsystems, the serial message assembly and the actual track processing. Serial message assembly converts each message type (raw RADAR, beacon and weather) into a network message. Track processing involves correlating RADAR and beacon messages (targets) into a single track. A track is the known history of the aircraft. Targets can be RADAR, beacon signals or both. Once a target is correlated to a track, another track message is put on the network.

Messages usually are broadcast on the network. They are sent using UDP on both trunks, and each message has a unique packet ID. Each computer on the network listens to both trunks and keeps a record of the packet IDs it has heard from each unique network ID. If there is a gap in packet IDs, a chassis may request a rebroadcast. Duplicate packet IDs are ignored, assuming it was the message on the other trunk or some other chassis requesting a rebroadcast.

Each system must broadcast heartbeats on the network. If a heartbeat is missed, it is assumed the system is down, and one of the other systems sends a message to have a standby system take over.

The next processor to deal with the network messages is the common processor (CP). The CP does many things, such as matching flight plans to tracks, sending conflict alerts (CAs), minimum safe altitude warning (MSAW) and monitoring some of the Common ARTS system heartbeats. The biggest thing the CP does is determine airspeed and direction of a track.

CA uses the speed and direction to look at other tracks to determine the possibility of a conflict. If a conflict is detected, the CP broadcasts a CA message indicating the aircraft in conflict. When aircraft are traveling at 300KTS—about five miles a minute—it is important to be looking a minute or two ahead.

MSAW uses a site-adapted map to learn terrain in the area. The terrain can be hills and mountains as well as towers and buildings. For transponder-equipped aircraft, the MSAW system looks at the altitude and the position and determines if the aircraft may be too low. If an aircraft is determined to be too low, an MSAW message is broadcast.

The last major system in the back room is the system monitor and control (SMC). The main purpose of the SMC is to monitor and control the other systems. It is a gateway to the SMC display PC, a GUI for monitoring the network and its current state. Current state indicates the systems that are on-line, off-line, standby or idle. If a heartbeat is missed, the SMC instructs a standby system to take over. A system operator can instruct a manual switch at anytime, load new software or reboot systems from this PC. The SMC also is used for recording all the data that crosses the network.

What most of us think of when we think of air traffic control are the displays: a room full of round vector displays and guys in white shirts watching them. More and more sites are using large 20"-square color displays. The new color displays are 2048×2048 pixel, X window displays. The design of the display processing software (DPS) is such that parts of it can be used by any display type, the color square display, like an ARTS Color Display (ACD) or the round vector display, like a Full Digital ARTS Display (FDAD).

Actual screenshot of the 8-bit VGA Linux DPS software. Compared to the ACD display, only the all-brown weather is different. The blue history trails look similar, and the green beacon and RADAR images are the same. The menu is transparent to the map, but not weather.

A real controller watching an ACD.

The DPS receives the broadcast messages and displays appropriate images depending on the state of the system. In normal operation, the display includes a track indication, a direction history indication, a full or partial datablock and the state of the various systems.

The flight plan information message from the CP is displayed in the full datablock near the aircraft for which the TP created the track. The broadcast SMC overall state can be displayed as well. The network can use one or several hundred displays.

In addition, each system can be run on one or more CPUs. If the CPU is powerful enough, all the systems and subsystems can be run on a single CPU.