gps might be an option, but inav is far too inaccurate for the required precision. the possible prob with gps is that the a/c would have to transmit its position, thereby tying up a frequency which could be otherwise better used.
the fact that LIN didn't have an operational groound surveillance radar points to the fact that a change in technology isn't necessary, as these work reasonably well where they are installed. having said that, apparently JFK get primary targets on theirs when a truck goes past on the freeway.
i thought that thermal image cameras might be a good solution, and they'd give the benefit of being entirely autonomous. maybe someone with a bit more (operational) experience could suggest why this hasn't been explored?
regards,
BGPM.