Understanding Mode S technology

This article provides an introduction to some important SSR (Secondary Surveillance Radar) concepts that lie at the heart of Mode S.

Introduction
One of the primary purposes of the Mode S system is to resolve the operational problems associated with the legacy Mode A/C or ATCRBS (Air Traffic Control Radar Beacon System). Use of Monopulse interrogators allow the separation of closely spaced targets in azimuth, and reduce the number of transponder replies required, for determining the azimuth angle of the target. This in turn reduces transmission occupancy, allowing more targets to be tracked within a given airspace. However, several problems still remain, and these are now addressed in more details.

Mode S interrogation and reply formats
Refer to Figure 1. The Mode S selective interrogation transmitted on a 1030 MHz carrier comprises a two pulse preamble, followed by a data lead in period. A sync phase reversal in the 1030 MHz carrier references the start of a 56-bit or 112-bit DPSK (Differential Phase Shift Keying) data block. The second preamble, pulse P2, will suppress Mode A/C transponders from replying.

Figure 1: Mode S selective interrogation format

Refer to Figure 2. The Mode S reply transmitted on a 1090 MHz carrier consists of a four pulse preamble, followed by a 56 bit (short) or 112 bit (long) PPM (Pulse Position Modulation) reply.

Figure 2: Mode S reply format

Refer to Figure 3. The ATCRBS/Mode S All-Call format consists of two 0.8us pulse P1 and P3, that identify the Mode group A or C, with a 0.8us P2 ISLS (interrogation Side Lobe Suppression) or control pulse, followed by a P4 pulse that may be 0.8us (all-call short) or 1.6us (all-call long). Mode S transponders only reply to the all-call long, whereas Mode A/C (ATCRBS) transponders will ignore the P4 pulse and reply to both all-call short and all-call long.

Figure 3: ATCRBS/Mode S All-Call format