Selective interrogations make use of the unique 24-bit aircraft address, overlaid on parity (AP field) and can be transmitted close to the azimuth where the aircraft is expected to be. Other aircraft that happen to be in the interrogator beam at that time will not reply.
The type of Mode S reply is determined by the interrogator uplink format (UF), but in theory, only a single reply is required because there is no ambiguity as to which aircraft the reply belongs. Typically, additional interrogations are transmitted to ensure that at least one reply is received and that tracking performance is maintained.
Refer to Figure 8
. Typically, the selectively addressed aircraft will be interrogated with a UF4 Surveillance Altitude Request, the RR field being configured for a 56 bit short reply. The 16 bit SD field is comprised of 13 subfields.
The 4 bit IIS (Interrogator Identifier Subfield), within the SD field, contains the self-identification code of the interrogator and is numerically identical to the II code transmitted by the interrogator in Mode S Only All Calls (UF11).
The 1 bit LOS (Lockout Subfield), contained in the SD field, if set to One, initiates an All Call lockout to Mode S Only All Calls (UF11) from the Interrogator, indicated in IIS subfield of the interrogation.
Figure 8: Surveillance altitude request
Refer to Figure 9
. The aircraft will reply with a DF4 Surveillance Altitude Reply, with altitude reported in the AC field. The AC field supports 25 ft resolution compared with the 100 ft resolution of Mode C altitude replies.
The 6 bit UM (Utility Message) field, is comprised of two subfields. One of these is the 4 bit IIS (Interrogator Identifier Subfield), which reports the identity of the interrogator.
Figure 9: Surveillance Altitude Reply
Refer to Figure 10. The aircraft will also be selectively interrogated with a UF5 Surveillance Identity Request, the RR field being configured for a 56 bit short reply.
The IIS and LOS subfields, contained within the SD field, operate the same as UF4.
Figure 10: Surveillance Identity Request
Refer to figure 11
. The aircraft will reply with a DF5 Surveillance Identity Reply, with aircraft identification and Mode A 4096 code (if assigned), reported in the ID field.
The IIS subfield operates the same as DF4.
Figure 11: Surveillance identity reply
Formats UF4 and UF5 may also be used by the interrogator to set the RR field to request a long reply, in which case, the reply will be a DF20 Comm B Altitude Reply or a Comm B Identity Reply, respectively. Refer to Figure 12
. Both of these replies will be 112 bits in length and contain the 56 bit MB message field. The use of the GICB (Ground Initiated Comm B) protocol allows the ground interrogator to extract DAP’s (Downlinked Aircraft Parameters) contained in the MB message field of the reply, which may include additional aircraft information such as aircraft identity, flight number, airspeed, heading, roll angle and status.
Figure 12: Comm B altitude reply