37TH ANNUAL CONFERENCE, Toulouse, France, 30 March – 3 April 1998
WP No. 159
Interference with Air Traffic Control Radiotelephony (RT)
Following the 36th Annual Conference in Taipei, the problem of interference with ATC RT was raised by an MA who were experiencing interference from taxi companies. In order to investigate the legal implications of such a problem this topic was added to the work programme of SC 7. In order to fully appreciate the legal and professional aspects of this paper it is first necessary to explain certain fundamental technical aspects of the subject.
With an ever growing number of users making increased demands of the frequency spectrum, it should come as no surprise to learn that there has been a corresponding increase in the instances of interference. Every aspect of ATC Communications, Navigation and Surveillance (CNS) are susceptible to this problem. Most controllers receive little if any formal instruction on the subject of RT interference, this being left for individuals to experience first hand during the normal execution of their duties. Until recently this situation although not ideal, was considered acceptable due to the infrequent instances experienced. With this problem on the increase, the present lack of formal training should no longer be considered acceptable.
A programme of education for controllers on this subject should be researched which would provide guidance on appropriate actions and limits of responsibility in such matters.
Interference with RT can manifest itself in a variety of ways, but can be categorised as either electromagnetic, harmonic or atmospheric. A brief description of each follows:
- Atmospheric interference is caused when a number of natural phenomena coincide, usually during periods of high barometric pressure and high sunspot activity Radio signals may be affected by the ionosphere. They can bounce back to earth at unusual angles even repeatedly bouncing often referred to as ‘skip’. This can cause signals to travel much further than they would under normal circumstances, sometimes across continents. As atmospheric interference is generated by natural causes and generally is of only short duration, there is little that can actually be done about it. However, during periods of relatively prolonged activity, ATC watch supervisors should ensure that all staff are briefed accordingly and significant instances are logged.
- Harmonic interference within the aviation frequency bands is usually caused by the higher harmonics of equipment from industrial, scientific or medical establishments (ISM Interference) operating legitimately in the HF band. Factories use HF radio waves for instance to dry materials, hospitals use them in certain treatments and for the sterilisation of instruments. This form of interference can however, affect frequencies across different bandwidths and over considerable distances. Repeated cases of harmonic interference will need to be investigated and should in the first instance be reported to the Duty Telecommunications Officer at the facility concerned.
- Electromagnetic Field (EMF) interference is the category that is most frequently encountered. This category should be divided into; EMF caused by RT transmitting equipment, other electrical sources, or other phenomena. The many sources of EMF may be sub-divided into. Natural, intentional, unintentional and malicious. All pieces of electrical equipment passing current through copper wire will generate an EMF. This EMF if of sufficient power, can cause interference if not properly suppressed or screened.
Most ISM sources are usually well screened in the horizontal plane by the walls of the establishment housing them, however, the roofs of these establishments unfortunately do not offer the same degree of screening. This makes the source of any subsequent interference difficult to detect by conventional means. This can often result in pilots hearing the interference which is undetectable from the ground based station operating on the same frequency. It is for this reason that National Air Traffic Services UK is investigating the feasibility of using airborne detection equipment.
Each MA’s national administration should have in place regulations regarding the screening or suppression during manufacture, installation or subsequent operation of any equipment capable of generating either a harmonic or an EMF likely to cause interference. However, even with stringent regulations in place, it is impossible to completely eliminate the problem.
In order to illustrate the problem, the following are all actual cases of differing types of interference that have been experienced by controllers in recent years; some of which are ongoing:
- At a military airfield located within the London TMA, a car telephone with an illegal aerial was responsible for completely blocking the tower frequency each time the car was driven along a stretch of road adjacent to the airfield when the phone was in use. This situation persisted for 3 months before the individual was successfully identified by a process of elimination and the use of direction finding equipment.
- At a busy rural UK airport, a factory located within a few kilometres is in the process of being investigated for allegedly causing interference from an oven used in the production of foam for furniture. The interference occurs at different times of day in the form of a background noise on the tower frequency. However, it is only received by aircraft and not the tower itself.
- At the same airport two separate instances involving pirate radio stations, operating from a nearby city were responsible for causing considerable interference on various ATC frequencies. Music could clearly be heard by pilots making approaches to the airport, but again ATC were unable to receive these transmissions from the ground.
- At two adjacent UK airports, there is an ongoing problem of bogus RT calls. These are believed to be made by an individual using a portable transceiver. The bogus calls are a deliberate impersonation of air traffic control messages. The calls are on various RT frequencies and sometimes are only received by the pilots. In an attempt to minimise the interference, one airport has had a permanent change of one frequency.
- At locations world-wide, the airport ground management radio system is being disrupted by a wide range of users on similar bandwidths – Taxi companies and fishing trawlers for example.
- In the USA and Canada in 1994 during one particular sun-storm, which is associated with high sunspot activity. A plasma cloud, which is bound together by highly charged magnetic ions emanated from the sun, travelling at over 2 million miles per hour knocked out 2 communications satellites. One of these satellites was used for ATC communications, the damage was significant taking 5 months for the satellite to return to operational use. The effects of these sunstorms are not just restricted to lower earth orbit as the residents of Quebec in Canada in 1989 will vouch. On this occasion the entire power grid serving 8 million homes and industries was knocked out by a powerful EMF.
Complex ATC scenarios may exist when a problem of this nature arises. The interference may be only an intermittent arid minor background noise or in the worst case a bogus operator may deliberately attempt to disrupt flight safety by transmitting malicious instructions. Problems caused by these incidents have so far been minimised due to the experience of pilots and controllers receiving these communications. However, the danger to flight safety posed by this problem cannot be understated.
Each MA’s national administration has a responsibility to fully investigate interference problems and should devise contingency plans to cater for such scenarios. It is not feasible to address how this can best be achieved in this paper as each MA and every ATC facility within it will have differing requirements. However, the following points should be considered:
- A reduction in traffic loading;
- Provision of different frequencies to be available.
The following advice is issued to UK pilots and controllers regarding identification of malicious transmissions and the action that should be taken:
- Pilots and controllers should be aware that illegal transmissions can occur; the malicious messages transmitted may sometimes be identified by:
- A change in the individuality of the transmissions, i.e. the controllers voice characteristics are not those previously experienced;
- The message transmitted is out of context with the expected next message; it is obviously in error or uses incorrect phraseology;
- The messages are usually transmitted for a short time only and are not repeated when queried;
- The transmission is sometimes received only by the pilot or controller, not both.
- The local Air Traffic Control Officer in charge should be made aware that illegal transmissions are suspected. Details of the content, character, type and location of where the transmissions were received should be recorded. Action should then be taken to confirm where possible that these transmissions have occurred. The Air Traffic Controller in charge, once made aware that illegal transmissions are suspected, should notify the appropriate Government Agency responsible for the investigation of interference with wireless telegraphy. It is important that the best available evidence be collected as soon as possible in order that a prosecution can be brought. Original tapes of communications received by the ground must be preserved where possible. They can be copied to enable transcription and afterwards be ‘proved’ by formal witness statement from the Air Traffic Control Officer whose voice appears on the tape.
Aircraft voice cockpit recorders may provide additional information, however, any action to retain information of this sort would conflict with the primary intention of these devices and aircraft operational procedures. Therefore, cockpit voice recorder information is not considered to be part of the procedure for obtaining evidence. Whatever the type of evidence, it is vital to protect its ‘integrity’ and ‘continuity’. Integrity means that the evidence is genuine and obtained in a proper manner. Continuity means that there is a continuous record of each stage in the gathering of evidence and its handling, including steps taken to ensure it is not tampered with and that it is securely kept, from the time the evidence is obtained until the moment it is presented in court.
Any air traffic controller in the normal execution of their duty encountering any form of RT interference which disrupts the safe, orderly and expeditious flow of air traffic, should take all reasonable steps available to minimise the disruption and notify the watch supervisor without delay. This should be considered to be the limit of the controllers responsibility towards the occurrence. In the event of an incident resulting from such a situation the controller should not be held liable.
Once satisfactory RT communication has been established between the pilot and controller existing regulations normally require all safety critical clearances issued to be acknowledged by a readback. This system is well established and providing controllers obtain and verify readbacks, they ensure that all clearances are safely implemented. In the event that interference results in the non-delivery or corruption of any transmission, or a pilot responds to a bogus instruction, either of which results in an incident or accident. The controller providing the Air Traffic Service (ATS) and observing all proper regulations relating to the provision of that service, should not be held liable in such an event.
Modem ATC CNS systems are increasingly more complex. With continual demand for expansion of bandwidths across the frequency spectrum; our own introduction of 8.33khz spacing is one case in point, these systems will be more vulnerable to interference. It is imperative therefore that controllers are made filly aware of the problems that may occur.
Each MA’s national administration should have in place contingency plans for frequency loss due to interference. These plans, depending on the severity of the scenario may need to include quite draconian measures.
National administrations have a responsibility to investigate the source of any RT interference originating within their own borders. They should ensure that appropriate regulations exist to safeguard the integrity of the aviation frequency spectrum within their own country and any delegated airspace.
In the event of an incident or accident occurring as a result of RT interference from any source, Boards of Inquiry (BOI) should not consider the controller liable if the correct RT procedures have been adhered to.
The requirement for controller training on the subject of RT interference be included in the work programme of SC4.
It is recommended that the following be adopted as IFATCA policy and entered in the IFATCA manual on page 4413 as paragraphs 1.2.9 – 1.2.11:
1.2.9 In the event of corruption or non delivery of any ATC clearance or flight safety critical transmission due to interference of RT from any source, which results in an incident or accident, the controllers concerned shall not be held liable, providing that proper procedures have been followed and that all reasonable measures to overcome the problem if known to exist have been taken.
1.2.10 Member associations shall bring to the attention of their national administrations details of any persistent, disruptive RT interference in order that a full investigation may be conducted, and appropriate action taken.
1.2.11 Member associations shall urge their National Administrations to devise contingency plans for the continued safe operation of ATC within their own or delegated airspace when a frequency or frequencies are unavailable due to interference.
Last Update: September 28, 2020