The Mode S Only All-Call
During the all-call period, targets will normally be interrogated with Mode A/C Intermode (all-call short) interrogations, which inhibits Mode S aircraft from replying. Refer to Figure 4
. Targets will also be interrogated with 56 bit Mode S only, all-call (UF11) interrogations, with the ICAO aircraft address of 1111 1111 1111 1111 1111 1111, to which Mode S equipped aircraft reply with their own 24-bit ICAO Aircraft Address. The 3 bit CL field determines the content of the 4 bit IC field. For example, if the CL field is 000, then the IC field contains the II code; if the CL field is 001 through 100, then the IC field contains SI codes 1 to 63.
Figure 4: Mode S Only All-Call Interrogation
All Mode S aircraft in the beam, that receive this interrogation, would normally send a reply. However, this would cause many garbled and lost replies at the interrogator receiver and other problems for the RF environment. The issue of all-call FRUIT and garble is resolved through the use of the lockout principle, as well as stochastic acquisition and an ability to override lockout already set.
Refer to Figure 5. The 24 bit AA field contains the Aircraft Address. The first 20 bits of the 24 bit PI field have zero value. The last 4 bits are a replica of the II field contained in the UF11 and establish the Interrogator Identity. The 3 bit CA field provides information about transponder capability.
Figure 5: All-Call reply