Figure 6: The lockout process
Once a transponder is known to the interrogator and its track has been established, it can be ‘locked out’. This prevents the transponder from replying to any more All-Call interrogations from that or any other Mode S interrogator with the same identifier code. It will then only respond to Selective interrogations. However, it will continue to respond to interrogations from other Mode S interrogators with a different Identifier Code and also to Mode A/C interrogators.
The lockout period is controlled by an 18 second timer located in the transponder; however, during the roll-call period, lockout may be continually reset by the interrogator to its own IC by setting control information as part of selective surveillance interrogations. By this means, the all-call lockout is maintained as the target travels through the coverage of the interrogator.
To allow an interrogator to operate without co-ordination with adjacent Interrogators, a lockout override protocol is provided which allows the interrogator to force a transponder to reply to all-calls, regardless of the current lockout status to that interrogating IC. In order to avoid reply garbling, the lockout override is applied with a Probability of Reply value of less than 1.
The subject of reply garbling and stochastic (probabilistic) acquisition is covered later in the text.
(False Replies Unsynchronized to Interrogator) also known as (False Replies Unsynchronized In Time).
This occurs when the ground interrogator sees replies in response to interrogations from another interrogator. Defruiting replies using delay line and subtraction techniques can be effective. However, the probability of a reply is reduced, as FRUIT increases.
Adjacent Radars and FRUIT
Refer to Figure 7
. In order to allow effective operation of Mode S ground interrogators with overlapping coverage areas, a discrete identification code, known as an IC (or Interrogator Code), is allocated to each interrogator. The IC is included in selectively addressed interrogations and repeated back in the reply. This allows the interrogator to distinguish or defruit replies from individual targets.
The original ICAO provision was for 15 Interrogator Identifier (II) Codes. Amendment change 73 to ICAO Annex 10 resulted in an additional 63 codes being made available in the form of Surveillance Identifier (SI) Codes. SI codes may only be used when all aircraft in the coverage volume support SI operation. II or SI codes are contained in the UF11 4 bit IC field.
Figure 7: Use of IC Code
To ensure efficient surveillance, many factors need to be considered and often in coordination with adjacent radars, including:
* Pulse repetition frequency (PRF)
* Mode interlace pattern (A/C, All-Call A/C/S, All-Call Mode S only, Roll-calls)
* Interrogator Code (IC)
* Use of lockout (coverage and protocol)
* Use of datalink capability
* Types of transponders in the airspace