How can the military help alleviate radio frequency congestion

The 1030/1090 MHz applications see its performance degraded when airborne transponders are interrogated beyond their designed capability and performance. Potential reasons include too many radio frequency (RF) interrogations on 1030 MHz triggering, too many replies from aircraft transponders on 1090 MHz, unexpected transmissions, or non-interoperability issues.

The RF spectrum protection of the 1030/1090 MHz frequencies and the monitoring of the performance of surveillance avionics are regulated by article 6 of EU regulation No 1207/2011 of 22 Novakober 2011 (Surveillance Performance and Interoperability) and article 7 of EU regulation 2019/123 of 24 January 2019 (Network Functions).

EUROCONTROL assists all stakeholders to ensure that their surveillance requirements are taken into account and that aeronautical surveillance using 1030/1090 MHz remains fit for purpose, now and in the future. EUROCONTROL delivers sound technical recommendations working closely with an extensive range of civil and military stakeholders, including air navigation service providers, aircraft operators, industrial partners as well as International Organisations and standardisation bodies.

Military operators can also contribute to alleviate and gain control of the potential 1030/1090 MHz RF congestion by taking into account a number of particular technical characteristics and usage of specific military systems.

Typically, the cooperative surveillance systems use two frequencies, 1030 MHz for interrogations to aircraft transponder and 1090 MHz for replies from the transponder or spontaneous message transmissions (squitters, e.g. ADS-B), in order to build the air situation picture.

The 1030/1090 MHz RF frequenciss are a critical resource for Air Traffic Control (ATC). Aircraft surveillance is based on cooperative surveillance systems, all relying on the operation of the 1030/1090 MHz link. This RF link is used by (see below):

• Mode A/C radars,
• Mode S radars (Elementary Surveillance - ELS and Enhanced Surveillance – EHS)
• Automatic Dependent Surveillance – Broadcast (ADS-B)
• Multilateration systems for airport surface surveillance (MLAT) or over a wide area (WAM)
• Military systems (Identification Friend or Foe – IFF, combat ID)
• Air-air airborne collision avoidance systems (ACAS) and future ADS-B IN applications.

The 1030/1090 MHz surveillance infrastructure sees its performance degraded when:

• transponders are interrogated beyond their designed capability and performance; because there are too many RF interrogations on 1030 MHz triggering too many replies from aircraft transponders on 1090 MHz;
• there are too many transmissions on the frequencies;
• there are unexpected transmissions on the frequencies;
• non-interoperability issues happen.

Air traffic increase, more aircraft types equipped (e.g. RPAS), increasing number of ground interrogators, more data exchanges (e.g. air-air applications) and dynamic systems with start-stop transmission and re-interrogation increase the risk of RF performance degradation.

An excessive high level of activity on the 1030/1090 MHz infrastructure can result in detection losses of aircraft surveillance, requiring possible traffic restriction, generating delays and reducing the network capacity.

It is essential to keep the usage of the 1030 and 1090 MHz frequencies at an acceptable level in order to maintain the safety and performance of surveillance systems and to avoid the need to deploy a new link. One of the key measures to reach this goal is through the mitigation of the number of ground initiated interrogations received by a transponder triggering too many replies.

To highlight the safety relevance of a loss of radar detection, it is important to recall the events of 5 and 10 June 2014 when there were several occurrences of unplanned and uncontrolled radar losses from ATC displays in central Europe. These events resulted in reduced capacity in some of the affected ATC sectors, the introduction of flow measures and, as a result, in flight delays. At that time, the technical investigation concluded that the source of the RF interference was a commercial surveillance system or technical installation which over-interrogated the transponders on board aircraft not only at rates beyond their requirements but also beyond design limits.

The need to prevent 1030/1090 MHz RF congestion was reflected in article 6 of EU regulation No 1207/2011 of 22NOV2011 (Surveillance Performance and Interoperability), on spectrum protection, which states:

1. By 2 January 2020 at the latest Member States shall ensure that a secondary surveillance radar transponder on board any aircraft flying over a Member State is not subject to excessive interrogations that are transmitted by groundbased surveillance interrogators and which either elicit replies or whilst not eliciting a reply are of sufficient power to exceed the minimum threshold level of the receiver of the secondary surveillance radar transponder.
2. For the purpose of paragraph 1, the sum of such interrogations shall not cause the secondary surveillance radar transponder to exceed the rates of reply per second, excluding any squitter transmissions, specified in paragraph 3.1.1.7.9.1 for Mode A/C replies and in paragraph 3.1.2.10.3.7.3 for Mode S replies of Annex 10 to the Chicago Convention, Volume IV, Fourth Edition.
3. By 2 January 2020 at the latest Member States shall ensure that the use of a ground based transmitter operated in a Member State does not produce harmful interference on other surveillance systems.

These obligations are to ensure that ground-based surveillance interrogators do not over-interrogate airborne aircraft transponders and do not trigger too many replies, thereby protecting the performance of ATC surveillance in Europe.

The EU regulation 2019/123 of 24JAN2019 (Network Functions), in its article 7 (Tasks of the Network Manager), states: “3. (g) …monitor the performance of the infrastructure relevant for the execution of the network functions, i.e.: (iii) surveillance interrogators and avionics”.

In summary, it is essential to prevent transponder overcapacity through the limitation of the number of interrogations received by each transponder, to support the operators in their actions to verify compliance and to perform monitoring actions to determine cases when the performance of the infrastructure is not satisfactory.

A number of verifications and analyses could be performed to demonstrate compliance of each system with SPI IR Article 6. Such verifications consider as a fundamental aspect the occupancy of an aircraft transponder in relation with the number of ground interrogations, eliciting or not a reply, that such airborne transponder needs to process and the resulting replies.

The transponder occupancy is a state of unavailability of the transponder from the time it detects an incoming signal that appears to cause some action or from the time of a self-initiated transmission, to the time that it is capable of replying to another interrogation. The transponder occupancy is typically expressed as a percentage of time and depends on the number of interrogations and pulses received by a transponder, which are of sufficient power to exceed the design criteria minimum threshold level (MTL) of the transponder (–74 dBm ±3 dB for a Mode S transponder).

ACAS interrogations and replies are transmitted over the same surveillance RF link. Although ACAS interrogations are not within the scope of the SPI IR, they need to be taken into account as they impact the transponder occupancy. Typically, the transponder occupancy of an aircraft flying above a Member State is expected to be below 20% (10% of occupancy due to replies and 10% of occupancy due to interrogations without reply).

It is also required to verify that the number of replies of an aircraft transponder is below the minimum reply rate capabilities specified in ICAO Annex 10, volume IV (i.e. below 500 Mode A/C replies per second, 50 Mode S replies per second and 16 Mode S long replies per second). The document “EUROCONTROL Guidelines on the Assessment of Ground-based Surveillance Interrogations” provides additional information on the criteria to verify SPI IR compliance in terms of transponder occupancy and number of replies.

States have to ensure that aircraft transponders flying over a State are not over-interrogated, that the transponder occupancy is below 20% and that the number of transponder replies does not exceed the minimum reply rates specified in the Annex 10, volume IV. Those verifications can be done using a RF Model, or analysing 1030/1090 MHz RF measurements made on the ground or on-board a test aircraft.

A continuous monitoring mechanism on the use of 1030/1090 MHz frequencies may need to be in place to detect any problems in real time in order to be prepared and react to stop the cause. In the longer term, there is a need to understand the increase of on 1030/1090 MHz and to propose ways, means and actions to maintain acceptable levels of performance.

Such 1030/1090 MHz monitoring system, to be ensured in the context of the regulated Network Functions, must be able to issue “1090 MHz RF alerts”, to identify possible sources of saturation and to trigger remedial measures like the request for operators to stop tests or exercises or to prepare for possible sector restrictions. Monitoring results will also provide the basis for longer-term strategic actions. A network of monitoring receiver stations is to be used in coordination with local authorities.

Verification of compliance and monitoring

As for all other aeronautical surveillance operators, the military can also provide a specific contribution to alleviate 1030/1090 MHz RF congestion. Such contribution must be organised taking into account all national prerogatives and responsibilities related with defence and security roles on the basis of processes and coordination mechanisms that safeguard in all circumstances sensitive information related with the military surveillance infrastructure.

It is up to the Member State to take adequate measures to discharge the responsibility to ensure that military fixed or mobile interrogators, for permanent or temporary operation, are correctly configured. Members States competent authorities¹ should implement the appropriate coordination processes and information repositories to ensure the assessment of the configuration of those interrogators. It is assumed that the Military authorities are aware of the criteria to be followed when assessing the surveillance sensor configuration. Military authorities should naturally confirm to their Member State authority that their fixed and mobile interrogators do not produce harmful RF interference on other aeronautical surveillance systems.

An RF Model can be used to show the compliance of surveillance systems with the requirements defined in the SPI IR Article 6. The RF Model can be tailored locally by a Member State. However, abnormal and unexpected events are not simulated. That is why this method is acceptable if the results are not close to the limits.

Real time 1090 MHz monitoring is out of scope of SPI IR but may be needed to detect the deterioration of the RF environment to enable a quick reaction. Unexpected increase in the number of transponder replies within a short period of time should be detected, reported and analysed. When such occurrences are identified, there must be a quick process in place to contact military operators if military surveillance infrastructure is involved. The socalled STOP buzzer procedure, already in place mostly for systems operated on non-interference basis under article 4.4 of the ITU Radio Regulations ie. Electronic Warfare training and exercises.

In case of a military exercise, military mobile interrogators may be temporary deployed and operated on Interrogator Identifier II=00. No Interrogator Code (IC) allocation is required for such mobile military interrogators, MLAT and WAM as these systems operate on II code 00. No IC allocation is required for Mode A/C radar neither. Therefore, no information will be maintained in the centralised code allocation repositories for these systems. This should be in line with Military authorities requirements to potentially conceal detailed information about the mode of operation of deployed interrogators.

Replies on II=0 represent one of the potential sources of 1030/1090 MHz RF pollution. The military is however not the root cause of this phenomena. Possible mitigation in this case include the removal of spurious replies (2020 long P4 removed) and optimization/tailoring of the interrogation pattern configuration of mobile platforms. Other sources of 1030/1090 MHz RF pollution, that are less-military specific, but that concern civil and military operators alike comprise:

• Mode S all call (acquisition) – to be mitigated through better configuration (e.g. reduced all-call interrogation frequency, probability of reply) of interrogators and passive/hybrid acquisition
• Long Mode S replies (EHS) – to be mitigated through reduction of extraction rate and sharing of data
• TCAS (local) - to be mitigated through use of Extended Hybrid Surveillance on new aircraft

Specific cases involving military interrogators

It is also recommended to limit the number of mobile Mode S radars deployed in the same area, (as they use stochastic lockout override to acquire aircraft on II = 0).

Mode S radars operated on an Interrogator Code (IC) different from II=00 may generate IC code conflicts in case of uncoordinated transmission, i.e. operation without a coordinated IC allocation issued by the EUROCONTROL MICA Cell, or if the IC allocation issued for the radar is not correctly implemented.

An interrogator code conflict is encountered in case of uncoordinated lockout coverage overlap of two or more Mode S interrogators operating on the same interrogator code, potentially resulting in aircraft remaining undetected by at least one of the Mode S interrogators.

In this case, it is important to consider carefully the following points when procuring and using Mode S radars:

• No operation on IC different from II=00 if an IC allocation has not been issued for the Mode S radar by the EUROCONTROL MICA (Mode S Interrogator Code Allocation) Cell.
• When an IC allocation is issued for the radar, the IC, the surveillance coverage and the lockout coverage provided with the IC allocation have to be programmed in the radar.
• Ensure that the latest IC allocation issued for the Mode S radar is programmed.
• Ensure that the surveillance/lockout coverage can be programmed either in range (per sector) or in using coverage maps in EMS map format.
• Ensure that the lockout is performed only within the defined range or coverage map provided in the IC allocation.
• Ensure that the Mode S radar supports II/SI2 code operation:
  • to acquire aircraft which are not SI capable on the matching II code when the Mode S radar operates on SI code
  • to not lock aircraft which are not SI capable on II code
• When a conflict is encountered, it should be reported to the EUROCONTROL MICA Cell
• Solution could be to use II=0 BUT that is not good for the 1090 RF environment (compromise needed).

When fulfilling the SPI IR obligations and constructively contributing to alleviate 1030/1090 MHz RF pollution the military are recommended to:

• consider the existing risk of disrupting the operation and safety impact that exist due to SSR Frequencies (1030/1090 MHz) saturation and transponder overload and take adequate mitigating measures;
• actively pursue verifications and analyses on their surveillance systems to demonstrate compliance of each system with SPI IR Article 6;
• consider the use of an RF Model or the analysis of 1030/1090 MHz RF measurements made on the ground or on-board a test aircraft, to show the compliance of surveillance systems with the requirements defined in the SPI IR Article 6;
• take adequate measures to ensure that military fixed or mobile interrogators, for permanent or temporary operation, are correctly configured;
• the configuration of interrogator has to be assessed and approved prior to start operation in order to ensure that it does not lead to excessive interrogations and/or replies from aircraft transponders;
• the number of All-Call interrogations and triggered replies, and the number of selective interrogations per aircraft have to be evaluated during the assessment;
• be aware of the criteria to be followed when assessing the radar configuration to be sure that their fixed and mobile interrogators do not produce harmful interference on other surveillance systems;
• maintain information repositories related with the interrogation pattern configuration of military interrogators;
• identify a point of contact inside the military organisation (preferably 24-7) to contact in case of 1030/1090 MHz problem or for any other related topic;
• support state in the implementation of a process to handle over-interrogation report suspected to come from a ground-based surveillance interrogator located inside their own State;
• envisage real time 1090 MHz monitoring as needed to detect the deterioration of the RF environment and to enable a quick reaction by contacting relevant POCs (STOP buzzer procedure);
• when necessary, implement coordination mechanisms regarding aeronautical surveillance infrastructure configuration that safeguard in all circumstances related sensitive information;
• consider specific guidance on the use of mobile interrogators with II=00 assigned including the removal of spurious replies (2020 long P4 removed) and better configuration of mobile platforms;
• deploy a limited number of mobile interrogators operating on II=00 in the same area
• identify and appropriately address the risks for 1030/1090 MHz RF pollution associated with Mode S all call (acquisition), Long Mode S replies (EHS) and TCAS (local);
• consider the measures related with the specific cases involving military interrogators described in 5.2 above.