Linux in Air Traffic Control

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

Initially, the reason for the port to Linux was to allow developers to test and debug the systems at their desks before testing on the target hardware. The target hardware is Motorola OEM boards in a VME chassis running LynxOS. The systems are relatively expensive, so neither the FAA nor Lockheed-Martin wants to have a bunch sitting around. Instead, several test systems are used almost full-time for integration testing and development.

Because the IS department gives developers a Microsoft Windows NT PC, an attempt was made to port the software to NT. Most of the port was completed when I started working for the company. For testing some things, NT worked fine. An adapter layer was used to make the POSIX threads, file I/O and graphics behave like the target system, so it wasn't good at testing those features.

When I started contracting at Lockheed-Martin, I was placed in the messaging layer group, the group that maintains the communications, threading and file I/O for the system. Basically, none of my testing could be done on the desktop, and I had to use the target hardware. Initially, a side project was working to see if it was possible, and I was given one old (200MHz Pentium) PC for development purposes.

Most of the code compiled just fine, although there were some issues with POSIX standards. LynxOS 2.4 and 3.0 used an older standard, whereas Linux uses the current one. I initially was doing the development on Red Hat 7.0 with a 2.2 kernel, and it didn't support named semaphores or named shared memory segments. In a distributed system like ours, it is easier to use standard names within a processor than some other communication mechanism to know where the shared memory and semaphores are. I did cobble together a named shared memory compatibility layer, and I found a Russian site with a named semaphore compatibility layer.

During development I moved to Red Hat 7.1, which was supposed to support named shared memory, but there was an incompatibility between glibc and the header files. I was able to look in the source to find this problem, and I posted a note to the kernel mailing list. Someone beat me to it, though. To keep things stock, allowing anyone to pick up any Red Hat version without this fix, I left my cobbled-together version in the code.

The target hardware was all big endian (Motorola 68K and PPC) and the Linux PC was an x86 little endian, so I needed to do some byte swapping to make the whole system work. Many of the files are stored in binary (maps, adaptation, and so on) format. The networking layer already had a byte-swapping mechanism built in, and it worked great.

Once I had all the messaging code compiled and running, I needed an application. The FAA agreed to fund the further development of the TP, CP, SMC and DPS systems for desktop testing and debugging. The systems all ported well, but the DPS had some issues with X displaying. Normally the large 2048×2048 pixel display is run on special hardware with two or three pseudo-color physical planes. If the maps and menus are drawn on the bottom plane, the weather on another plane and the aircraft on the top plane, the whole display doesn't need to be redrawn if an aircraft moves. To make this plane idea work, the color map was split into three parts. Being pseudo-color (8-bit) limited the number of colors in each plane. The map and menu plane got one color (white), the weather got another color (brown) and the primary display plane got 78 colors.

So, we needed to have more color table adjustments, as normally the primary display used 250 colors. The large display has an animated fading history trail that emulates the fading phosphors on the vector display. It takes 128 color cells to make the animation work. For this application I made one cell and no animation. It looks amazingly good. Between finding all the reds that were similar, greens that were similar, and yellows and blues and whites and grays, I trimmed the table down to the 78 available.

Once all this was done, I was given a two-plane video card to see if the system would still work. A change to one compile-time flag had the whole thing working. The two-plane card puts the weather maps and menus on the same plane.

Two things happened about this time. I delivered the Linux code to the FAA, and a couple other developers were tasked with getting Linux working on the target PowerPC hardware. The FAA found a few updates I hadn't kept up with in the baseline of the code, and we were able to work together on that. The other developers found most of my #ifdefs were specific to Linux and not the machine architecture. Therefore, I was able to hold back my changes to the FAA and make the proper #ifdefs when the FAA finally took it.

The PowerPC Linux Project was an attempt to improve the data recording tasks that the SMC handles. The current system uses consumer-grade, off-the-shelf optical disks that aren't suitable for 24/7 writing. The new system incorporates SAN disks, which are more suitable to air traffic control needs. Although technically a success, the project is on hold for now.

In the spring of 2003, the FAA began using the Common ARTS on Linux for a noncritical subsystem, an inexpensive gateway system feeding ARTS data to other systems. Full certification may happen eventually.

Tom Brusehaver is a coder grunt who has been writing code since before the PC. He mostly does contract work these days, preferring embedded systems. He is married and has grown kids, two cats and a dog. He is building an airplane when the weather is nice.