All-Call and Selective (Roll-Call) periods
The Mode S ground interrogator provides surveillance of both Mode A/C and Mode S equipped aircraft, with minimal mutual interference. The 1030 MHz interrogation channel is divided into distinct and non-overlapping periods of Mode A/C and Mode S activity, known as the ‘all-call’ period and Mode S selective interrogation activity, known as the ‘roll-call’ period.
Interrogations during the all-call period elicit replies from Mode A/C aircraft and acquire Mode S aircraft via acquisition of the 24-bit ICAO aircraft address and the position of the aircraft. During the roll-call period (also known as the Mode S period), selective surveillance interrogations are sent to Mode S aircraft. Once an aircraft has been acquired during the all-call period, surveillance is then carried out discretely during the roll-call period.
Figure 3: All-Call/Roll-Call Periods
Refer to Figure 3. The interrogations consists of a regular interleave of All-Call periods with Mode S (Roll-Call) periods, typically repeating with a frequency of anywhere between 40 Hz and 150 Hz. The repetition frequency and duration of the All-Call period is a local implementation issue, the duration of either period depending on the characteristics of the system such as the interrogator antenna revolution rate, the antenna beam-width and the maximum range. Normally there will be several all-call and roll-call periods available, to interrogate all targets in range, during one revolution.
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