I've been developing homebrew flight instruments for recreational paragliding and hanggliding for the past few years. These have included altimeters/variometers as well as GPS displays.

The first instruments were based on the Nintendo Gameboy Advance (GBA) as a platform. It is easy to develop for - the hardware has been reverse engineered, and there are GNU based free software development environments with libraries to ease the pain of developing graphical user interfaces. I used the GBA multi-player link port to interface with GPS modules and digital pressure sensors. The application software is on a flash memory cartridge that fits into the game ROM slot. So there is no need to modify the GBA hardware at all.

I then did some experiments with the Charmed Labs XPort 2 embedded GBA development system. This is a card with an FPGA and flash memory that fits into the game ROM slot. The FPGA can be reconfigured with your own add-on hardware logic, and there are a large number of I/O pins on the card available for interfacing to the outside world. With this I was able to develop a GPS/altimeter/vario with GPS tracklog save capability.

The XPort 2 system is however not suitable for outdoors applications with exposure to heat, humidity, dust and hard knocks. And the FPGA eats into battery life.

I then tried out the Nintendo DS Lite. This needs a BIOS reflash for homebrew code execution. After this is done, you can execute code from a flash cartridge in the GBA cartridge slot to take advantage of the new hardware - ARM9 CPU, two screens with improved resolution, touch screen, more RAM, upgraded graphics and audio hardware, etc.

I used a DSerial2 interface card in the DS cartridge slot to interface with a GPS and pressure sensor, and developed a mapping altimeter/variometer application. Unfortunately, the system had a fatal flaw - unlike the standard GBA and the earlier GBA SP models which used a reflective LCD Screen, the DS Lite uses a backlit screen. This is washed out in bright sunlight, making it useless for paragliding/hanggliding applications. The use of the backlight decreases battery life as well. So that ended up as an expensive experiment ...

NOTE : I found that the newer Gameboy Advance SP units have a backlit screen as well. So if you do plan to construct any of these instruments, make sure you test the display outdoors or get yourself an older GBA SP model with the reflective screen.

I am now developing small form factor, low cost, low power instruments, using inexpensive microcontrollers, LCD displays from cellphones and standard cellphone batteries.

I have provided circuit schematics and full source code for these projects, so you should be able to build your own instrument using the information on this site. The only experience you need is the ability to solder fine pitch SMD components and some understanding of C programming. Sorry, I'm not assembling or selling any units.