Aircraft with ACARS can exchange data messages via a network of automated ground stations incorporating internal computers. Airlines first used the data link system to send movement reports to the ACARS service processors using the telex formats that operators had previously used to send those reports. ACARS are widely used today with airborne installations exceeding 10,000 aircraft.
ACARS units are connected to a VHF radio and in many cases, interfaced with satellite systems. This type of data communications is sent via conventional VHF radio waves that are received through a network of ground stations linked via a terrestrial network to a centralized data link service processor. This is what provides the connection to the ground systems of the users.
Data communications can also be sent via satellite networks but will ultimately link to the service processor that supports the VHF ACARS service. The function of the service processor is to route messages automatically between the user aircraft and ground systems, using mostly a fixed configuration of delivery addresses by message type for downlink messages and by memorizing the ground station to be used for uplink messages.
The main restriction on the ACARS system is that it uses character codes representing only printable characters. This limitation applied to all early generation data communications systems. This did not prevent the ACARS system from becoming the foundation of airline operations efficiency. However, the development of new radio communications technology and the need to support air traffic management, calls for newer technologies to be implemented.
Aircraft have been able to carry out voice and data communications via the Inmarsat satellites for more than 25 years. Until then, this satellite constellation was intended to provide communication services to ships. The number of aircraft currently equipped to use Inmarsat has exceeded 3,500 and is made up of airliners, business jets, and government aircraft.
Four satellites placed in geo-stationary orbit above the equator are centralized over the Pacific Ocean, Indian Ocean, Atlantic Ocean-East, and Atlantic Ocean-West. This constellation provides coverage through a “global beam” between 80 degrees above and below the equator.
The original Inmarsat Aeronautical service provides two modes, circuit mode supporting voice communications and packet mode supporting “always-on” data communications.
Aircraft operators use the Inmarsat circuit mode to offer voice service to passengers and flight deck crew. Aircraft operators use the Inmarsat packet mode, which provides a data rate approaching that of some home high-speed lines.
The move of aircraft communications from voice to data has motivated some operators of HF radio ground stations to install “HF data link” (HFDL) computers that enable the transport of ACARS data. Manufacturers of aircraft HF radios have added capabilities to support ACARS.
The new HF radios can switch between voice and data mode using the same components, but they are required to give voice communications precedence over data link. This has a tendency to limit the HFDL availability. This isn’t a commonplace application.
High Frequency radio data link has been found to provide better availability than HF voice on trans-Polar routes beyond the 80-degree North/South limit of Inmarsat satellite coverage. The HFDL capacity is only limited by the frequencies available in the HF band. The allocation of HF frequencies to data link has required a very complex co-ordination process and the system will quickly reach its limits.
The addition of data link capability to HF radio is a way for aircraft operators to get additional use out of the radios they still carry in order to meet ATC rules when most communications migrate from voice to data. However the HFDL system provides delivery of 95 percent of transmitted messages in three to four minutes compared to 20 to 30 seconds via satellite communications — so it is likely to be limited to providing a safety net in case of failure of satellite avionics, rather than a good alternative to satellite communications. The FAA Central Reporting Agency report as of July 2003 found 95 percent of uplink messages took four minutes, 20 seconds and 96 percent took up to 10 minutes.
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