GM1 Article 6 Spectrum protection
CAA ORS9 Decision No. 1
EXCESSIVE INTERROGATIONS
To maintain an effective surveillance service that supports the provision of air traffic services (ATS) and the safe separation of aircraft, the ground-based interrogators should not excessively interrogate aircraft. Excessive interrogation may result in the loss of transponder data and the subsequent loss of the surveillance service; therefore, surveillance service providers should maintain the interrogation rates as low as necessary and the surveillance coverage to the minimum required for the safe operation of the system.
Surveillance service providers are required to comply with Annex VIII (Part-CNS) to UK Regulation (EU) 2017/373 by demonstrating that their working methods and operating procedures comply with the international Standards and Recommended Practices (SARPs) of Annex 10 ‘Aeronautical Telecommunications’ to the Chicago Convention on International Civil Aviation. Surveillance service providers should compute the reply rates per second of the transponder on board aircraft flying over the UK and compare them to the minimum reply rates of International Civil Aviation Organization (ICAO) Annex 10, Volume IV. As specified in Section 3.1.1.7.9.1 for Mode A/C replies, and in Section 3.1.2.10.3.7.3 for Mode S replies, the minimum reply rates are as follows:
— for Mode A/C replies:
‘3.1.1.7.9.1 All transponders shall be capable of continuously generating at least 500 replies per second for a 15-pulse coded reply. Transponder installations used solely below 4 500 m (15 000 ft), or below a lesser altitude established by the appropriate authority or by regional air navigation agreement, and in aircraft with a maximum cruising true airspeed not exceeding 175 kt (324 km/h) shall be capable of generating at least 1 000 15-pulse coded replies per second for a duration of 100 milliseconds. Transponder installations operated above 4 500 m (15 000 ft) or in aircraft with a maximum cruising true airspeed in excess of 175 kt (324 km/h), shall be capable of generating at least 1 200 15-pulse coded replies per second for a duration of 100 milliseconds.
Note 1. A 15-pulse reply includes 2 framing pulses, 12 information pulses, and the SPI pulse.
Note 2. The reply rate requirement of 500 replies per second establishes the minimum continuous reply rate capability of the transponder. As per the altitude and speed criteria above, the 100 or 120 replies in a 100-millisecond interval defines the peak capability of the transponder. The transponder must be capable of replying to this short-term burst rate, even though the transponder may not be capable of sustaining this rate. If the transponder is subjected to interrogation rates beyond its reply rate capability, the reply rate limit control of 3.1.1.7.9.2 acts to gracefully desensitize the transponder in a manner that favours closer interrogators. Desensitization eliminates weaker interrogation signals.’; and
— for Mode S replies:
‘3.1.2.10.3.7.3 Minimum reply rate capability, Mode S. A transponder capable of transmitting only short Mode S replies shall be able to generate replies at the following rates:
50 Mode S replies in any 1-second interval
(…)11
In addition to any downlink ELM transmissions, a level 2, 3 or 4 transponder shall be able to generate as long replies at least:
16 of 50 Mode S replies in any 1-second interval
(…)12
(…)13
In addition, a transponder within an ACAS installation shall be able to generate as ACAS coordination replies at least 3 of 50 Mode S replies in any 1-second interval.’
11 Not applicable. This section is omitted as this GM specifies only rates of reply per second. However, surveillance service providers may consider other time intervals as specified in Annex 10.
12 Not applicable. This section is omitted as this GM specifies only rates of reply per second. However, surveillance service providers may consider other time intervals as specified in Annex 10.
13 Section related to ELM is omitted as not relevant due to ELM not being implemented in the Single European Sky airspace.
Therefore, the reply rates of aircraft transponders should be computed per second, in compliance with ICAO Annex 10, Volume IV, and satisfy the following conditions:
— be less than or equal to 500 Mode A/C replies per second;
— be less than or equal to 50 Mode S replies in any 1-second interval; and
— be less than or equal to 16 Mode S long replies in any 1-second interval. To verify compliance, several methods may be used:
— simple theoretical calculation;
— simulation of the 1030/1090-MHz RF environment;
— periodic (offline) analysis of the 1030/1090-MHz RF environment recorded on the ground; and
— permanent (online) analysis of the 1030/1090-MHz RF environment on the ground.
For the analysis of the 1030/1090-MHz RF environment, airborne recordings may complement the analysis of the 1030/1090-MHz RF environment on the ground.
The choice of the most appropriate method depends on the estimated reply rates of transponders on board aircraft that fly within a given service area.
Further information may be found in EUROCONTROL-GUID-178.
To maintain low interrogation rates outside the ANSPs’ area of surveillance interest, the interrogator output power should be as low as possible and as necessary for the safe performance of the system as specified in ICAO Annex 10, Volume IV, paragraph 3.1.1.8.2. In addition, the provisions of UK Regulation (EC) No. 262/2009 regarding Mode S surveillance and lockout restrictions should be observed. If an active MLAT system is utilised, the spectrum protection measures as defined in ICAO Annex 10, Volume IV, paragraph 6.2, should be observed.
As per Article 7(3)(g)(iii) of UK Regulation (EU) 2019/123, the Network Manager is required to monitor the performance of the infrastructure relevant for the execution of the network functions, i.e. surveillance interrogators and avionics. The application of this central analysis tool, which is intended to provide statistics and alerts with respect to transmission rates, does not relieve surveillance service providers of the responsibility to ensure initial and continued compliance with Annex VIII (Part-CNS) to UK Regulation (EU) 2017/373. Such statistics and alerts provided by the Network Manager could be used to monitor the continued operation of the interrogators.