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Mode S Interrogator Codes |
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The introduction of Secondary Surveillance Radar (SSR) Mode S within Europe has identified the need for a co-ordinated allocation of the relevant Interrogator Codes used by ground-based, airborne and ship borne platforms.
Whilst traditional SSR stations continuously interrogate all aircraft within their range, Mode S establishes selective and addressed interrogations with aircraft within its coverage.
In order to avoid ambiguity in the operation of the system it is essential that each Mode S interrogator is allocated an Interrogator Code (IC) to protect it from interference by other Mode S interrogators operating in coincident or contiguous airspace.
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Selective addressing of aircraft and ‘selective replies’ |
A fundamental principle of Mode S (Select) is that it supports selective addressing.
Every airframe is allocated a unique 24-bit ICAO Aircraft Address by the appropriate registering authority. Mode S aircraft are therefore selectively addressable.
The 24-bit address is included in Mode S only interrogations and in the Mode S replies. So, interrogations are directed at specific aircraft and replies generated by that aircraft will include the unique identifier address so that ground systems can identify them. Targets are then tracked unambiguously.
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Each Mode S interrogator is allocated a specific Interrogator Code (IC) which it can use to uniquely identify itself in an interrogation. This Interrogator Code may also be included in the reply from a Mode S transponder to indicate the intended interrogator for that specific reply.
The IC has to be unique in so much that it cannot be used in a coverage overlapping with other (non-connected) interrogators using that same code. The use of the same code by two sensors with overlapping coverage could result in loss of detection in the overlapping coverage for one of these sensors.
The design of the Mode S system limits the number of Interrogator Codes available. ICAO Annex 10 defines 16 Interrogator Identifier (II) codes and 63 Surveillance Identifier (SI) codes. The term IC is used to define either an SI or an II code.
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The introduction of SSR Mode S in the European region has identified the need for a co-ordinated approach to the allocation and implementation of the Interrogator Codes used by ground-based, airborne and ship borne platforms.
A European co-ordination process has been agreed by the ICAO European Air Navigation Plan Group (EANPG). The Mode S IC Co-ordination Group (MICoG) is established within EUROCONTROL in order to provide an internationally recognised forum to oversee the allocation of Mode S Interrogator Codes. The process is managed by EUROCONTROL (Mode S IC Allocation Cell).
Discrete ICs will only be allocated to fixed position Mode S interrogators which are used operationally.
Currently, only II-codes are used. In order for an SI-code interrogator to operate effectively, all targets visible to the SI-interrogator need to be equipped with SI-capable transponders.
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Interrogator Codes (IC) and the military |
Discrete ICs will only be allocated to fixed position operational stations.
Operators of fixed ground stations must request an IC through the National Regulatory Authority. Positional information for the station is required for running simulations to check conflicts of IC with adjacent stations.
Fixed position ground based radars should be capable of operating with an SI-code.
Mobile interrogators should use II=0. This is a special case II-code used for non-selective operation. The use of II=0 can be approved at national level.
NATO has been allocated II=15 for its use and management. This code could be used by a fixed or mobile sensor. Certain AEW platforms, including the NATO AEW&C aircraft, will be fitted with a Mode S interrogator.
If, for security reasons or due to the unsuitability of the interrogator (e.g. mobile platforms), military operators do not envisage acquisition of Mode S targets using discrete IC and the lockout protocol, other Mode S mechanisms to acquire targets can be used.
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Last validation: 31/03/2009
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