Redbird use FSX or the commercial licensed version of it, ESP. There's also another variant of ESP developed by Lockheed Martin called Prepar3d which they use for simulators - you can download it and have a play, but they want a monthly fee if you want to keep using it, for what is pretty much just FSX (although it does boast better stability and frame rates).
Last edited by Slopey; 2nd December 2011 at 13:32.