Verbal communication on Very High Frequency is used for rapid communication in range of direct visibility (up to 350km). The main purpose of verbal Very High Frequency communication is provide bilateral communication between aircraft and air traffic controller and also communication with other aircraft. Frequency range which is used in aviation for VHF is 118-135.975 MHz and distance between channels is equal to 25 kHz. However, the need for using a larger number of digital radio channels for data transmission is lead to narrow of distance between channels up to 8.33 kHz.
The significance in such type of communication is that channel can be organized only in line of sight of receiver and transmitter antennas. (Fig.114)
Fig. 114. Linesign of radio VHF communication
Control of available communication facilities on board of aircraft executes with the help of control panel.
High reliability of organizational communication channel on VHF frequencies is allowed to use his for information transmission in digital form. For organization of digital data transmission in VHF range is used two systems-ACARS and VDL.
In modern aero navigation systems the biggest role is intended for digital channel data link between aviation facilities and ground infrastructure. Digital data exchange on VHF data link is most desirable in case of communication implementation in small distances. ICAO intended special standards on building of digital data link:
- VDL mode 1 is based on protocol of data exchange ACARS, which allows implement speed of data exchange equal to 2.4kbit/s.
- VDL mode 2 ensure data exchange with speed is equal to 31.5kbits/s. This protocol is more effective than ACARS and support data exchange from pilot of aircraft till air traffic controller (Controller-to-Pilot Data Link Communication – CPDLC).
- VDL mode 3 is realized procedure of multiple accesses with time division channel (Time Division Multiple Access – TDMA) for digital and verbal building.
- VDL mode 4 is self-organized protocol data transmission that allows intending digital data exchange between aircraft and ground station or other aircraft. Consider like a base for organization of data exchange in concept ADS-B.