RTK (Real Time Kinematics)

Devices on the ground will discover where they are located to 1 cm of accuracy given enough time to receive GPS + GLONASS satellite data. This device then communicates with the sUAS to give the sUAS more accurate positioning data. This data is used to geolocate imagery.

DJI offers a system for some models of their sUAS: http://www.dji.com/d-rtk/info This system costs several thousand dollars.

Some RTK systems eliminate the need for GCPs, some are used in conjunction with GCPs. It depends on the system.