Many teams also used inertial navigation for times when GPS went out. These can also be used for sensing tilt angle. I compete in a competition called IGVC (intelligent ground vehicle competition) which is a very close thing to darpa, but our robots have a minimum length of 3 feet and the course is only 10 feet wide at the widest parts. You may be able to find some info here
http://www.igvc.org/deploy/ We use Sick LMS (laser measurement system) now, however in the past we used an array of 8 sonar units from Polaroid cameras.
BTW our darpa vehicle can be found here
http://team.cartlink.org/
I hope to post some news about igvc soon, also igvc was mentioned in a post by Jack Jones
http://www.chiefdelphi.com/forums/showthread.php?t=38640&highlight=igvc and yes I am proud to be the member he spoke of.