Radio Altimeter
The radio altimeter (radar altimeter) measures height above the terrain directly below the aircraft by timing the reflection of a frequency-modulated continuous wave (FMCW) signal, typically in the SHF band (4.2-4.4 GHz). It is used for precision approach, autoland, and terrain awareness, and displays height above ground (AGL), not altitude above mean sea level.
Candidates must understand the operating range (typically 0-2500 ft, with accuracy best at low height), the difference between radio altitude and barometric altitude, and the use of radio altitude for decision height and autoland. Know that the radio altimeter is affected by terrain reflectivity, steep terrain, and water, and that it may give erroneous readings over uneven ground.
Traps: confusing radio altitude (AGL) with barometric altitude (AMSL); assuming radio altimeter works at high altitude (it does not, typically above 2500 ft); forgetting that it measures the nearest reflecting surface, not the runway threshold. Another trap is the belief that radio altimeter is a navigation aid; it is a height sensor.
Rule of thumb: radio altimeter = height above ground; barometric = height above datum. Use radio altitude for decision height, not for cruise level.
SSR Transponder and TCAS
The SSR transponder replies to ground interrogations. Mode A gives identity (squawk), Mode C gives altitude, and Mode S gives selective addressing and data link. Candidates must know the squawk codes (7500 hijack, 7600 radio failure, 7700 emergency), the use of IDENT, and the meaning of Mode S and ADS-B. Understand that the transponder should be operated as directed by ATC, and that altitude reporting requires Mode C.
TCAS (traffic alert and collision avoidance system) uses transponder replies to build a traffic picture. TCAS II provides traffic advisories (TA) and resolution advisories (RA); TCAS I provides TAs only. Know the RA manoeuvres (climb, descend, adjust vertical speed) and that the pilot must follow an RA, even against ATC, unless it endangers safety. Understand the coordination between TCAS-equipped aircraft and the limits of TCAS (no guidance for aircraft without transponders).
Traps: confusing TA and RA; forgetting that an RA must be followed promptly; assuming TCAS works without a transponder; misreading the emergency squawks. Another trap is the belief that TCAS provides terrain warning (that is TAWS/EGPWS).
Rule of thumb: 75 = hijack, 76 = radio, 77 = emergency. TCAS RA = act now; TA = look out.