[gravypod]

GPS isn't too great in skyscrapers. In most current gen smartphones GPS can only really pin you to a 16ft resolution on a current gen smartphone in ideal conditions [0]. This gets worse indoors and if the target is moving.

[0] - https://www.gps.gov/systems/gps/performance/accuracy/

[sbr464]

It wouldn’t necessarily need to come from GPS. You could use beacons (ble) or similar to augment the raw dataset.

https://estimote.com (etc)

[joshvm]

For airspace it doesn't matter too much. Allowable Mode-C accuracy is something more than 100 feet (I think) to allow for poor sensors, and controllers simply add a buffer.