Gps and an on board computer with lots of sensors and coding used for safety. Bump sensors, ir and sonar sensors for safety and object avoidance. Camera for remote monitoring.

Then have it mow about 1/4 acre at a time programming for what it is to expect in that 1/4 acre.

Or, you could do like the Commercial ones do and just let it randomly mow an area, which works well, but is unpredictable and needs a lot of attention payed to safety.

I would do remote control... something like that could kill somebody if it malfunctions.... I would consider a high end commercial bot for that project....