← Back to catalog

ASR-9 Airport Surveillance Radar

Aviation & Air-Traffic-Control Antennas

The rotating S-band primary radar that paints aircraft in the airspace around an airport.

Band
S-band (2.7-2.9 GHz)
Gain
~34 dBi
Polarization
Circular (selectable) for rain rejection

Photos

Real-world photo of a ASR-9 Airport Surveillance Radar in use
Real-world example. Source: Wikimedia Commons (CC BY-SA 3.0; WizardOfOz).

Radiation / wave patterns

Idealized radiation pattern of the ASR-9 Airport Surveillance Radar
Idealized azimuth radiation pattern (illustrative, generated). Radial scale in dB.

How & why it works

The ASR-9 is a terminal-area primary surveillance radar: a large rotating reflector antenna shaped to produce a 'cosecant-squared' fan beam—narrow in azimuth (about 1.4 degrees) for good bearing resolution, but deliberately shaped in elevation so aircraft at all heights out to ~60 nautical miles return similar signal strength. It transmits high-power S-band pulses (2.7-2.9 GHz) from a klystron and listens for the faint skin echoes reflected by aircraft, using circular polarization so rain (which reflects the opposite sense) can be rejected while aircraft echoes survive. The antenna turns roughly 12-13 times a minute, so every aircraft is revisited every few seconds.

Real-world uses

Primary surveillance of the terminal airspace at busy airports, feeding controllers' radar displays.