44TH ANNUAL CONFERENCE, Melbourne, Australia, 17-22 April 2005WP No. 88Review policy on RTF Frequency UsagePresented by TOC |
Introduction
1.1. As part of the 2004/5 work programme, TOC undertook to review existing policy on RTF Frequency Usage.
1.2. The purpose of this paper is to review and, if applicable, amend IFATCA policy on the use of RTF frequencies.
Discussion
2.1. IFATCA policy on Radio Telephony (RTF) frequency usage dates from 1979, and states:
| “A controller should transmit on only one RTF channel. Where two or more ATC operating positions are provided from one common position only one RTF channel shall be designated for that position.
Alternatively where circumstances dictate that more than one RTF channel be used then suitable ‘talk through’ facilities must be provided to enable reception, by all the users of the combined channels, of all transmissions made on any individual channel.” |
2.2. Since the above policy has been produced a lot has changed. Traffic demand has risen dramatically and technical developments have advanced rapidly in many countries.
2.3. Ideally, a controller should use only one RTF channel, which is located on the one working position the controller is working at. Two or more combined frequencies are often used though. This can be a combination of a VHF and UHF frequencies; two or more UHF frequencies; or two or more VHF frequencies.
2.4. The use of combined frequencies is often the result of the combining of ATC sectors or functions:
- at times when traffic demand is low (i.e. between rush hours or at night time);
- for frequency coverage over large volumes of airspace;
- due frequency shielding due mountainous terrain or man-made barriers;
- to provide communications on both VHF and UHF in the same airspace; or
- during periods of staff shortages.
2.5. More advanced Voice Switching and Control Systems (VSCS) are capable of combining the receiver/transmitter combination of many RTF frequencies onto an output (speaker or headset) at one Controller Working Position (CWP). This is commonly called grouping or cross-coupling and means that any transmission made by a controller is emitted from the aerials of each of the frequencies. Any transmission made on any of the frequencies is heard at the workstation on which they are grouped as well as being retransmitted on all other frequencies included in the group. The latter is so that pilots on one frequency can hear when another frequency is being used helping them to avoid over transmissions. It can also assist in pilots’ situational awareness as they can hear the whereabouts of other traffic as well as having an indication of how busy the controller may be.
2.6. Despite these technical solutions, problems do exist with coupling of frequencies. Sometimes a slight delay occurs in the transmission, as a result of which other transmissions can be blocked. Another problem can be masking. This means that with simultaneous transmissions one or more can be masked by the other. Incidents as a result of these problems are known, and technology should be in place to warn the controller for these so called cross transmissions.
2.7. When traffic demand is low, sectors or areas of responsibility can be grouped onto one frequency and the responsibility for control of these functions is adopted at one CWP.
2.8. Where the geographical proportions of one sector are larger than the coverage provided by a single RTF, further frequencies can be provided and are often permanently grouped together. To prevent a loss of communication, techniques are applied so the controller knows when to transfer aircraft from one frequency to the next, without transferring control to another controller, so that aircraft don’t fly out of range of the frequency’s aerials. Man made barriers or terrain can prevent line of sight communications in a particular quadrant of airspace or manoeuvring area. To mitigate this, other aerials can be erected and grouped together.
2.9. Retransmitting can be utilised to provide communications between aircraft equipped with a different frequency band (i.e. UHF only) and those on the frequency band in use (i.e. VHF). This allows controllers and other aircraft on the appropriate frequency to communicate with aircraft within the airspace but using another frequency band.
2.10. Occasionally, sectors or functions are combined onto one CWP for non-operational reasons. This usually occurs when staff availability is reduced, and a decision has been made to provide Air traffic Services (ATS) by combining sectors or functions onto one CWP. Safety should dictate that a different course of action would be preferable, however if such an event was to occur, a number of facilities must be made available.
2.10.1. When frequencies are grouped together and an aircraft is transferred from one frequency to the next, an indication of the frequency on which the last incoming call was made should be provided. This can be a combination of the frequency button lighting up during the call and/or the button remaining dimly lit after the reception (in case the controller would be busy with other tasks during the call). A pilot calling on the wrong frequency within the group could go unnoticed, when this feature is not available to the controller.
2.10.2. If two transmissions are made at the same time for the same duration, both parties can be unaware that their transmissions were unreadable. Often other frequency users will transmit “two together” as a warning. Technology is being included on present systems to alarm when crossed transmissions are made. Some systems even select the strongest signal and allow this transmission through while disallowing the other(s). Future systems should include technology that warns the controller in the event of a crossed transmission.
2.10.3. To mitigate for loss of communications by the ground system, all ATC equipment should be at least duplicated. This includes receivers, transmitters and the lines between the CWP and the aerial site. To mitigate against the failure of the VSCS, a handset should be provided which is wired directly to the receiver/transmitter equipment. This independent back-up equipment should be in place at the CWP, and should be tested regularly.
2.10.4. In some operations, frequency usage design requires that communications with aircraft should only be undertaken within the Designated Operational Coverage (DOC) for the frequency being used. This prevents interference on the same frequency when it is being used at different locations near each other, and ensures that under normal circumstances communications can always take place with aircraft.
2.10.5. In the event of inference, e.g. from an open microphone, VSCS should include facilities to ungroup (or mute) the individual frequency on which the problem is occurring. This should ensure that other transmissions can still be heard on the other frequencies within the grouping.
2.10.6. In the event of failure of the RTF equipment, controllers need to know immediately so they don’t assume it is a failure of the aircraft equipment. Otherwise the controller will spend time attempting to contact the aircraft instead of solving the problem straight away by selecting secondary equipment. The provision of equipment failure alarms is therefore necessary as well as the ability to acknowledge these indications to prevent distraction.
2.11. Other means of communication are currently being developed and implemented, e.g. Controller to Pilot Datalink Communications (CPDLC). However, this technology is currently not being used in airspace with many time-critical clearances and communications. The current systems are too limited to deal with environments in which the majority of clearances is time-critical, mainly because of response times. It is therefore very likely that RTF communications will be the sole or major means of communications in many areas in the world.
Conclusions
3.1. By reviewing present policy, TOC has found it to be outdated. New requirements are listed that TOC believes all controllers should be provided with, while providing ATS.
3.2. Several problems can occur with the use of (coupled) RTF frequencies. Technology has to be in place to warn controllers when these problems occur, and should be in place to assist the controller when channels are being blocked.
3.3. Other ways of communicating have to be found as the use of frequencies, in some areas, is on the limits of its operational boundaries.
3.4. It is expected that voice communications remain the prime way of communicating for the foreseeable future due to limitations of alternative means.
Recommendations
It is recommended that;
4.1 IFATCA Policy on page 3 2 4 10 of the IFATCA Manual:
“A controller should transmit on only one RTF channel. Where two or more ATC operating positions are provided from one common position only one RTF channel be designated for that position.
Alternatively where circumstances dictate that more than one RTF channel be used then suitable ‘talk through’ facilities must be provided to enable reception, by all the users of the combined channels, of all transmissions made on any individual channel.”
be deleted.
4.2 IFATCA Policy is:
4.2.1. If a controller is providing ATS for two or more areas, the relevant channels must be located on the Controller Working Position being used.
4.2.2. If more than one RTF Channel is being used, then suitable ‘retransmit’ facilities must be provided to enable all users to receive all transmissions. The ability to enable or disable ‘retransmit’ facilities should be provided.
4.2.3. Future systems should include technology that warns the controller in the event of a crossed transmission.
4.2.4. Independent backup equipment, should be provided.
4.2.5. Communications with aircraft should only be undertaken within the Designated Operational Coverage (DOC) for the frequency being used.
4.2.6. Voice switch systems must include facilities to:
- mute individual frequencies (due open mic, etc problems);
- presentation of equipment failure alarms and the ability to isolate these faults;
- provision of secondary equipment (i.e. receivers, transmitters and paths) at the Controller Working Position;
- an indication of the frequency on which the last incoming call was made on should be provided.
and be included in the IFATCA Manual page 3 2 4 10.
Last Update: September 29, 2020