The voice communication between the controller and the flight crew is necessary for air traffic control. For ensuring high reliability the voice communication system consists of three independent systems - primary, backup and emergency system.
Each radio channel of the main system is equipped with a pair of receivers (RX) and a pair of transmitters (TX) for VHF frequencies. For UHF frequencies then with a pair of transceivers (TRX). The receivers, transmitters and transceivers are automatically stored with each other in each pair.
For good radio coverage of the whole territory of the republic, the transmission and reception are performed from three different locations - Prague, and the Písek and Buchtův kopec hills. Radio channels of the primary system are split between these three points.
Radio channels of the backup system have one receiver and one transmitter, frequencies in the UHF band, one transceiver.
The backup system is located in Prague and contains all of the used channels. It is used only in case of a channel failure in the primary system.
The emergency system consists of a pair of transceivers that are tuned to emergency frequencies. One in the VHF (121.5 MHz) and the other one in the UHF (243.0 MHz) band.
The frequencies of the emergency system are located at workplaces in Prague, Karlovy Vary, Brno and Ostrava. Its use is restricted to the case of a simultaneous failure of the primary and backup system.