CNS: Communications, Navigation Aids and Surveillance
The CNS paper tests equipment knowledge and the ability to reason with the numbers behind navigation and surveillance aids.
Communications
VHF air-ground spans 118.000-136.975 MHz, amplitude modulated and line-of-sight. HF covers remote and oceanic areas, with SELCAL so crews need not monitor continuously. Safety-critical items require readback; the lost-communication transponder code is 7600.
Navigation
- NDB/ADF: bearing only, low frequency, prone to thunderstorm and terrain errors.
- VOR: 108-117.95 MHz, gives the magnetic radial from the station; DME gives slant range in nautical miles.
- ILS: localizer 108-112 MHz, glide path 329.15-335 MHz; CAT I minima 200 ft decision height and 550 m RVR, CAT II 100 ft and 300 m, CAT IIIA 50 ft and 200 m.
- GNSS, RNAV and PBN; RNP adds on-board performance monitoring and alerting.
Surveillance
PSR shows primary skin returns; SSR interrogates the transponder, Mode A giving identity, Mode C pressure altitude, Mode S a unique 24-bit address; ADS-B Out broadcasts position on 1090 MHz extended squitter; MLAT triangulates transponder replies.
Worked example
At 30,000 ft a DME reads 15 NM. Height in nautical miles is 30,000 divided by 6,076, about 4.94 NM. Ground distance is sqrt(15 squared minus 4.94 squared), that is sqrt(225 minus 24.4), about 14.2 NM. DME always over-reads when the aircraft is high and close, which matters when DME distance is used for a separation check.
Traps
Assuming localizer and glide path share a frequency band; treating DME as ground distance; confusing Mode C with Mode S; quoting CAT I minima as 100 ft; forgetting that VOR gives bearing from the station, not to it.