What technology is used for real-time tracking of balloons carrying amateur radio transmitters?
Why APRS, the Automatic Packet Reporting System, was designed exactly for this: a GPS receiver feeds position, altitude and telemetry into a packet TNC, which beacons short AX.25 bursts (1200 baud on 144.390 MHz in the US). Digipeaters and internet gateways relay those beacons so anyone can watch the balloon's track on a map in near real time. The same system tracks vehicles, weather stations and public service events.
Watch out FT8 is a weak-signal mode for very short exchanges (call signs, grid square, signal report) and carries no live telemetry, while PACTOR III is an HF ARQ mode used for message and email forwarding, not position beacons.
APRS = Automatic Packet Reporting System. Position beacons on 144.390 MHz: balloons, cars, weather stations.