Mode S Uplink Formats
A conventional SSR interrogator may have a typical sequence of Mode A interrogation, followed by Mode C interrogation or other modes. This would be repeated continually at a high rate to ensure that a position/identity plot can be produced for all targets in line-of-sight range of the interrogator during each antenna revolution.
The Mode S ground station produces a larger variety of interrogation types. These types can be roughly classified into two types:
- All-call interrogations
- Roll-call interrogations
All-call interrogations obtain replays from all aircraft in the beam dwell, although, under certain circumstances, Mode S aircraft can be “locked out” to all interrogations so that they do not reply.
Roll-call interrogations are selectively addressed to acquired Mode S equipped aircraft using the unique 24-bit address assigned to each aircraft. Only the addressed aircraft produce replies.
The first problem for the Mode S system is to find the addresses of aircraft that are in radar cover so that selective addressed interactions can be made with them. This is achieved by Mode S all-call interrogator witch are made periodically from the radar.
| pulse width of P4 | Mode S inter mode | |
| 0,8 µs | no replay | When the short P4 is used by Mode S radar, an aircraft fitted with a Mode S transponder will be detected solely by the Mode S formats - i.e. aircraft will be first detected by the Mode S formats all-call and subsequently followed by Mode S selective address interrogations. |
| 1,6 µs | all-call reply | Although the long p4 may be used to obtain Mode S all-call replies, its use is expected to be rather limited, in particular because the interrogator identity (ii) code and associated all-call lock features of Mode S are not relevant for this interrogation. |
| no P4 pulse | Mode A reply | This is the downwards compatible modus providing the Mode S transponder to reply on early interrogators. |
| none P4, but large P2 |
Mode S reply | The interrogator sends a P2 with full amplitude as for the P1 pulse. This activates a Mode S transponder to then look for a following P6 pulse containing the Mode S specific information. An older Mode A/C transponder seems this as an ISLS- condition and don't reply. |
Table 1: Mode S Uplink Formats