DARPA has provided a basic software radio physical layer implementation that allows the ground control station to control the SDR-enabled 3DR Solo drone. The source code can be found here:

https://github.com/deptofdefense/gr-uaslink

The drone will host a USRP B200mini as the SDR and a Raspberry Pi 3 (RPi3) as the embedded computer to run GNU Radio and the other software packages. The operating system running on the RPi3 is a Linux distribution created using OpenEmbedded (OE). The recipes and image files for the OE distribution can be found here:

https://github.com/deptofdefense/meta-darpahackfest

Information on setting up the ArduPilot software-in-the-loop:

http://ardupilot.org/dev/docs/setting-up-sitl-on-linux.html#setting-up-sitl-on-linux