Communication, Surveillance and Navigation (CNS) Systems
What is examined
CNS tests the communication, navigation and surveillance chain that supports ATM: VHF/HF voice, CPDLC, radar, ADS-B, GNSS and PBN. Expect questions on coverage, redundancy, data integrity and contingency procedures.
Core concepts
- VHF is line-of-sight; HF supports long-range oceanic/remote communication but is noisy and slower.
- CPDLC exchanges text clearances via data link; it needs an end-to-end connection and a fallback to voice.
- Primary radar measures position from reflections; secondary radar uses transponder replies (Mode A/C/S). ADS-B broadcasts position and velocity, typically from GNSS, on 1090 MHz extended squitter.
- GNSS is the position source for many PBN routes; RAIM, SBAS/GBAS and RNP monitoring provide integrity.
- PBN specifies RNAV/RNP navigation performance; RNP requires on-board performance monitoring and alerting.
Worked example
An aircraft at 35,000 ft contacts a VHF ground station at 100 ft above sea level. Use line-of-sight range ≈ 1.23√h(ft) for each station. Aircraft horizon = 1.23 × √35,000 = 230 NM; ground station = 1.23 × √100 = 12 NM; total ≈ 242 NM. Beyond that, VHF voice may be unreliable, so HF or CPDLC/satellite communication is needed. The calculation explains why VHF coverage is not unlimited even at high altitude.
Traps
Assuming ADS-B is radar; forgetting that ADS-B position quality depends on the onboard source; confusing RNAV with RNP; treating CPDLC as a substitute for all voice communication; ignoring HF/CPDLC contingency.