OVERVIEW
Excerpts of HF Management Guidance Material
North Atlantic Systems Planning Group
2.1.1 The Aeronautical Mobile Service is a service reserved for air-ground communications related with the safety and regularity of flights, flying primarily along national or international civil air routes.
2.1.2 In areas like the North Atlantic, where VHF coverage is insufficient due to range limitation to cover all portions of the routes flown, the use of HF frequencies are necessary because they provide long range communications coverage, not only for air-ground voice communications, but also for the broadcast of ATS or Meteo information.
2.1.3 For various reasons, some technical, others economical, environmental, physical, natural, etc., coverage of a wide area by a single station with equipment located in a single place are impractical.
2.1.4 Taking these factors into account, the most practical option is to employ a number of stations sharing a range of frequencies and working as a network to provide the facilities and services required for the AMS.
2.1.5 To work as a network the AMS should follow appropriate principles of operation, in order to achieve the highest possible level of capacity and efficiency, otherwise, its purpose will not be achieved and the safety and regularity of flights will be affected.
PRINCIPLES OF NETWORK OPERATIONS
2.3.3.1 The aeronautical stations of a radiotelephony network should assist each other in order to provide the air-ground communication service required of the network by aircraft flying on the air routes for which the network is responsible.
2.3.3.2 When the network comprises a large number of stations, network communications for flights on any individual route segment should be provided by selected stations, termed “regular stations” for that segment. In principle, the regular station will be those serving the locations immediately concerned with flights on that route segment, i.e. points of takeoff and landing and appropriate flight information centres or area control centres.
2.3.3.3 In areas or on routes where radio conditions, length of flights or distance between aeronautical stations require additional measures to ensure continuity of air-ground communications throughout the route segment, the regular stations should share between them a responsibility of primary guard whereby each station will provide the primary guard for that portion of the flight during which the messages from the aircraft can be handled most effectively by that station.
SELECTIVE CALLING
2.4.1 With the selective calling system known as SELCAL, the voice call is replaced by the transmission of coded tones to the aircraft over the radiotelephony channels. A single selective call consists of a combination of four pre-selected audio tones whose transmission requires approximately two seconds. The tones are generated in the aeronautical station coder and are received by a decoder connected to the audio output of the airborne receiver. Receipt of the assigned tone code activates a cockpit call system in the form of light and/or chime signals.
2.4.2 There is a critical shortage of possible 4-letter codes, which has required re-use of the same code by more than one aircraft. Duplicate codes are usually assigned to aircraft operated in widely separated areas of the world, and usually do not have the same HF radio frequency assignment. However, there are occasions when two or more aircraft having the same code may be operating in the same general area, and may respond to the same transmission. Therefore, SELCAL should not be used as a substitute for proper voice identification procedures.
2.4.3 Any radio operator who detects two or more aircraft with duplicate SELCAL flying in the same area, should put the flights on separate frequencies and advise the downstream aeronautical stations which will assume the primary guard in the future