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NOTE: Typical PTT controls may be panel mounted switches, headset in-line switches, foot switches, switches incorporated into desk or handheld microphones and handsets.
6.1.13 The audio level of each audio outlet shall be independently adjustable (1103) and any communications shall still remain audible and intelligible to the operator when the minimum level is selected (1104).
6.1.14 Separate controls for the audio level of RTF Communications and Ground-Ground Communications shall be provided with the setting in use being apparent to the operator (1105).
6.1.15 Recommendation: All lines of communication should incorporate an automatic gain control function in order to maintain adequate speech signal levels (1106).
6.1.16 Where an automatic gain control function is used, only one device or function shall operate on any signal path (1107).
NOTE: Where such devices or functions are incorporated, a signal gain path memory or similar feature can be used to prevent distortion of the initial syllables of speech at the beginning of a transmission or after pauses in speech. The principle of operation being that the last dynamic gain/ attenuation setting is stored and used for subsequent transmissions.
6.1.17 The design and implementation of the voice switch shall be such that any input can be connected to any output without the possibility of blocking occurring (1108).
6.2 RTF Communications
NOTE: Reference should be made to COM 02 'VHF Aeronautical Radio Stations' and COM 06 'UHF Radio Equipment and Systems' (1109).
6.2.1 Air-ground Communications on appropriate frequencies shall be provided (1110).
6.2.2 Two-way radiotelephony communication facilities shall be provided for aerodrome (surface movement) control service for the purpose of controlling vehicles on the manoeuvring area, except where communication by a system of visual signals is deemed to be adequate (1111).
NOTE: This communication facility is normally provided by UHF radio equipment and systems but the use of VHF Aeronautical Mobile Service frequencies may be permitted for ground to ground communications in specific circumstances.
Recommendation: Where conditions warrant, separate communication channels should be provided for the control of vehicles on the manoeuvring area (1112).
Recommendation: VHF air-ground communications should be cross-coupled to UHF two-way radiotelephony communications for vehicles operating on the active runway (1113).
NOTE: Cross-coupling between the VHF air-ground communications used for the control of aircraft and the UHF two-way radiotelephony communications used for the control of vehicles provides situational awareness for the aircrew, controller and operator of the vehicle. The vehicle operator is aware of any aircraft transmissions by monitoring the cross-coupled VHF Air-Ground Communications and has direct two-way radiotelephony communications with the controller. In some cases the transmissions from vehicles are re-transmitted to aircraft. As separate transmit and receive frequencies are used at UHF between the base station and vehicles, talkthrough facilities may be used to enable vehicles to hear one another.
6.2.3 RTF communications which have been selected shall always be available irrespective of the state of other lines of communication (1114).
6.2.4 The operator shall be provided with a degree of assurance that Air-Ground Communications transmissions have been successful (1115).
NOTE: The normally accepted method of implementing this is to provide off-air sidetone to the operator’s headset derived from either the radio receiver associated with the transmitter for that radio channel, or from a separate receiver. It is recognised that the future widespread implementation of digital telecommunications systems may mean that off-air sidetone cannot be implemented in all situations, in which case alternative ways of indicating the successful transmissions may be used.
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CAP 670 Air Traffic Services Safety Requirements 2(15)
