SIGINT sweep
...fitting a SIGINT (SIGnals INTelligence) system on a light balloon offers two advantages. Placed at altitude, electromagnetic sensors are less constrained by the effects of terrain- masking (trees, buildings and the roundness of the Earth). They thus gain in detection capabilities. A balloon also offers a very long endurance at altitude, at a lower cost than a fleet of aircraft or drones.
http://www.dmitryshulgin.com/2019/11...gint-aerostat/