This Technical Basis for Regulation (TBR) specifies the technical requirements to be provided by terminal equipment of the Terrestrial Flight Telecommunications System (TFTS) capable of, and intended for, connection to a public telecommunication network. The TFTS operates in the frequency spectrum 1 670 MHz to 1 675 MHz (ground to-air) and 1 800 MHz to 1 805 MHz (air to ground). The modulation technique is p/4 DQPSK. The system is cellular in nature and the available spectrum supports up to 164 radio channels which may be reused between cells. Each radio channel supports four voice telephone circuits with a 9,6 kbit/s voice coding algorithm. TFTS mobile stations are airborne equipment and they are also subject to a separate certification process for aircraft.