The history of wireless communications dates back to 1898, when Guglielmo Marconi, transmitted the first wireless message. In the United States, mobile radiophones have been in operation since the early 1920s when the Detroit Police Department started using them in police cars. The British invention of radar (1935) was a significant step forward in understanding higher radio frequency portions of the electromagnetic spectrum. During World War II radiotelephony was further developed for military purposes. In 1946, the first commercial mobile radiotelephone service was tested by Bell Systems in St. Louis, and it was introduced into several US cities by the end of the 1940s. This service was intended to connect mobile users in cars to the public landline network. The system was based on a single powerful transmitter that used signals based on frequency modulation that could cover an area up to some 50 miles (about 80 km) from the transmitter. These FM mobile telephones used the 120 kHz frequency band to transmit voice over a bandwidth of just 3 kHz. In the 1960s, a new mobile telephone system was launched by Bell Systems called the Improved Mobile Telephone Service (IMTS). It brought many improvements, such as full-duplex, direct dialing, and FM channel bandwidth of 25-30 kHz.