In 1 μs light travels 300 m (984 ft) in a vacuum, or 225 m (738 ft) in air (roughly). That's not good enough. 1 nanosecond is about a foot. A 100MHz clock give

In 1 μs light travels 300 m (984 ft) in a vacuum, or 225 m (738 ft) in air (roughly). That's not good enough. 1 nanosecond is about a foot. A 100MHz clock gives us a resolution of 10 ns, which translates to 2.25 meters away by light in air. As a backup if your fob battery is dead, you could use inductive power in the door, or just use that to begin with (still with distance-bounding).