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.
A slightly different approach would be to monitor the ADSB broadcasts and use either the distance from the airfield or the crossing of a particular height to sound an alarm.
For the 'spectators' at our DZ we have a tv monitor showing the current aircraft position and altitude in realtime.
To do this we bought a raspberry pi + dvbt stick + tuned antenna (approx 70EUR altogether). Antenna is installed on the hangar. Installed piaware on the raspberry, did a few tweaks to the tracking page to highlight our plane.
A few more tweaks could the be used to sound an alarm. No modifications to your aircraft as she should already have a transponder.
Many glider pilots use the SPOT GPS messaging device to alert their ground support when they land out. It allows the user to send their GPS location and send a short message to someone. It is completely satellite based so no need for a cell phone network. This may be useful for your skydivers to alert you when they are approaching.
For more information check out the Wikipedia Article