Land Vehicle

Real Time Kinematics - RTK - is computation done upon regular GPS signals to determine location to within 5 cm by solving the integer ambiguity problem of carrier GPS wavelengths [1]
Beagle Bone Blue for specialized rovers [2]
https://ardupilot.org/rover/
1 cm accuracy for under $300 [3]
Rover accuracy discussion - few centimeters! [4]
GNSS modules cost $30 and are 2 meters accurate https://junipersys.com/support/article/6614].

Autopilot Hardware