Surveillance and Warning Systems (Radar, TCAS/ACAS, TAWS/GPWS, Weather Radar)
Radar and SSR
Primary surveillance radar detects skin returns and gives range and azimuth; secondary surveillance radar interrogates the transponder, receiving Mode A identity, Mode C pressure altitude, Mode S addresses and ADS-B data. Controllers assign squawks; special codes 7500, 7600 and 7700 retain their standard meanings.
TCAS/ACAS
TCAS II (ACAS II) interrogates nearby Mode C and Mode S transponders and builds a three-dimensional picture from range and altitude. Traffic advisories are amber; resolution advisories are red and command a vertical speed. The pilot response is due within about 5 seconds at 0.25 g, and the manoeuvre should not be reversed. RAs are inhibited below about 1,000 ft AGL, so the system becomes advisory-only close to the ground.
TAWS/GPWS
Classic GPWS uses radio altitude and has modes for excessive descent rate, excessive terrain closure, altitude loss after take-off, unsafe terrain clearance (gear and flap) and excessive glide-slope deviation, plus advisory callouts and windshear. TAWS/EGPWS adds a terrain database and GNSS position, allowing look-ahead alerts.
Weather radar
Airborne weather radar uses X-band, about 9.3 GHz. Returns depend on precipitation size and water content; attenuation can hide a severe cell behind a close intense one, and clear-air turbulence is not detected. Tilt and gain must be managed, and the antenna is stabilized in pitch and roll.
Worked example
An aircraft level at FL350 receives a 'Descend' RA. The crew responds within 5 seconds and descends at 1,500–2,000 fpm, tells ATC, and returns to the cleared level only when 'Clear of conflict' is announced.
Traps
Ignoring an RA because of an ATC clearance; reversing an RA; treating a TA as an RA; assuming weather radar shows all hazards; and misreading attenuation shadowing.