13.4(b) Surveillance systems — transponder, TCAS, weather radar, radio altimeter, ADS-B, ARINC

Scope

13.4(b) is level 3 for B2 and is the whole exam for the B2L Surveillance rating: ATC transponder and SSR; TCAS; weather radar; radio altimeter; ADS-B and associated services (FIS-B, TIS-B, multilink); INS (see the navigation note); ARINC communication and reporting.

Primary and secondary radar

• Primary radar — ground radar detects the echo reflected by the aircraft's skin; no aircraft equipment needed.
• Secondary surveillance radar (SSR) — the ground interrogator transmits on 1030 MHz; the aircraft transponder replies on 1090 MHz with coded information. Stronger, more reliable returns with identity and altitude.

Transponder modes

Mode	Interrogation (P1–P3 spacing)	Reply contents
A	8 µs	Identity code: four octal digits 0–7 → 4,096 codes (0000–7777)
C	21 µs	Pressure altitude (referenced to 1013.25 hPa) in 100 ft increments, from an encoding altimeter or ADC
S	Selective, addressed to one aircraft	Unique 24-bit ICAO aircraft address, identity, altitude in 25 ft steps, flight ID, and data link messages (Enhanced Surveillance: selected altitude, heading, speeds)

• The P2 pulse is for side-lobe suppression: if P2 is stronger than P1 the aircraft is in a side lobe and does not reply.
• SPI / IDENT button — adds a special position identification pulse for about 15–20 s.
• Special codes: 7500 unlawful interference, 7600 radio failure, 7700 emergency.
• Mode S transponders send squitters (unsolicited broadcasts) used by TCAS and ADS-B (extended squitter, 1090 ES).
• Diversity antennas (top and bottom) on Mode S installations.
• Maintenance: avoid transmitting codes 7500/7600/7700 during tests, keep transponder in standby on the ground unless required, periodic tests of reply frequency, power, sensitivity and altitude correspondence with a ramp test set.