How could a light or buzzer on the ground be triggered from an aircraft? I work at a Skydiving company in New Zealand and we are looking for a light switch or buzzer that is triggered in the plane and can effectively alert the staff on the ground that the plane is 3 minutes away from dropping the Parachutes. The plane can not have any external alterations and the device has to be completely portable, we currently have a small wifi router inside the plane but it is very unreliable. I'm completely oblivious to the world of electrical engineering so any help would be greatly appreciated
People on the ground should have a radio tuned to listen to the local frequency. Then the pilot can simply radio a call to notify the people on ground (and any other planes that may be nearby) about the skydivers.

Having this radio also means that during emergency (like a jumper getting caught on the plane) there can be coordination between pilot and ground crew.