National IFF/SSR Committee

NISC Activities

IFF/SSR Interrogators

IFF/SSR Interrogators are defined in CAP 761 (formerly CAA Doc 714) as all equipment capable of transmitting in the 1030 MHz frequency band.  This includes Ground-Based, Airborne and Maritime Secondary Surveillance Radar (SSR), Airborne Collision Avoidance Systems (ACAS), some Collision Warning Systems (CWS) and Active Multilateration Systems (MLAT).  The application and approval procedures detailed within CAP 761 apply to all operators of IFF/SSR systems: civil or military, operational or T&D, domestic or visiting.

Approval of Mode S SSR Interrogators

The requirements for approval of Mode S SSR Interrogators are the same as that for any SSR interrogator and are described above. 

Applications for approval to operate Mode S radar  will normally require an additional application for a Mode S Interrogator Code (IC).  Details of the IC application process can be found in the Mode S Interrogator Code Allocation Process document produced by Eurocontrol.  Applicants are advised that there is a six-month allocation cycle, based on the AIRAC cycle, associated with IC applications and should therefore submit applications to the NISC Secretariat in a timely manner.

Operators of test, research and development (TRD) surveillance systems will be allocated a shared IC for which there are specific TRD operating procedures.

Approval of Mode A/C Interrogation Capability

In April 2006 the Chairman of the National IFF/SSR Committee (NISC) wrote to all Interrogator Approval holders, and other interested parties, advising that the NISC intended to cease renewal of approvals to operate for Mode A/C systems beyond December 2011. 

The need to cease approval of Mode A/C interrogators both on the ground and in the air (for certain military platforms) is borne from the requirement to sustain the already heavily congested IFF/SSR spectrum.  Furthermore, there is a need to ensure consistency between the ground and air.  With legislation already in place for the airborne environment, it is necessary to ensure that the ground environment can match this to provide sustainability and allow the RF benefits to be accrued.

Consequently, with effect from 31 December 2011, the NISC will no longer issue approvals for ground-based (including maritime) or airborne IFF/SSR Mode A/C interrogators.

Copies of the letters issued by the NISC in Spring 2006 and December 2007 are available from the Forms and Documents heading.

Details of current and future UK Mode S policy and regulations can be found on the CAA Mode S webpage. In addition the Eurocontrol website may provide some guidance on Mode S developments throughout Europe.



The approval guidelines detailed in CAP 761 also apply to Airborne Collision Avoidance Systems (ACAS), often referred to as Traffic and Collision Avoidance Systems (TCAS).  ICAO Annex 10 compliant ACAS II and specified ACAS I equipments have generic approval from the NISC.  However, all other ACAS I systems require approval on a case by case/individual basis.  Individuals/organisations intending to equip their aircraft with ACAS I equipment should read AIC 37/2006 (Yellow 199) 30 Mar 06, in addition to CAP 761 prior to buying and installing any system.

Application forms

Forms for approval to operate are available at Forms and Documents.

Contacts Details

020 7453 6536
020 7453 6565
mail: NISC Secretariat
Surveillance & Spectrum Management
Directorate of Airspace Policy
Civil Aviation Authority
CAA House
45-59 Kingsway