With this app, you can monitor dBm levels at specific frequencies using RTL-SDR dongle(s). E.g. you can detect emergency services (ambulance, firetrucks, police) in case TETRA is used by the emergency services in the country you are located in.

To receive real-time signal power levels, you need one or more RTL-SDR dongles (starting from EUR 10 each), that need to be connected to your phone using a USB OTG cable/hub (approx. EUR 5).

Also an antenna is required, which can vary in costs. However, even with a cheap 1 euro antenna reception is good. The dongles, USB cable/hub and antenna can be ordered at various websites, just search for "RTL-SDR dongle".

* App can be used to be a detector for emergency services, in case police or ambulance is nearby. Only when the country is using TETRA mobile devices.

** Reception can not be guaranteed, the signal to be received depends on, among other things, the environmental factors and the transmission of the signals, within the frequency range to be received.