The TETRA (Terrestrial Trunked Radio) digital radio network is operated by authorities and organizations responsible for security, such as police, emergency services, fire brigade, and technical relief organizations. In Germany, it is also used by transport and energy supply companies. As in the case of the public mobile phone networks, the TETRA network has a cellular design, but with larger cells. Introduced already in over 100 countries, the standard TETRA is similar to the GSM standard, but is operated in Germany at lower frequencies in the range of 380-400 MHz. It includes additional features, such as encryption for safety from interception, group communication, alert function, and GPS-assisted deployment control and emergency positioning. TETRA base stations typically radiate with a transmission power of 20 watts per channel and thus cover a range from 10 up to 25 km on land and 4-5 km in urban areas (Source: Landesamt für Natur, Umwelt und Verbraucherschutz, p. 8). TETRA handsets are power controlled and transmit with 30 mW up to 1 W (see exposure source database). More information is provided by the German Federal Agency for Public Safety Digital Radio (BDBOS).