[rasz]

Is there really GPS radio in the transmitter, or are they using drone takeoff location? or GPS coordinates supplied by the phone used for video feed?

DJI drones use external GPS modules (separate pcb + ribbon connection), it would be trivial to make modchips spoofing current coordinates by offsetting them by some fixed vector.

[sbierwagen]

The DJI transmitter is a joystick module that doesn't have a screen. You plug in a smartphone and install the DJI app to see what the drone sees. If they're being smart, they would be comparing GPS location from the phone against the GPS chip in the drone to see if you're doing anything funny.

[rasz]

Spoofing on the phone side should be even simpler (Mock Location App).