AIM Text
- Surveillance radars are divided into two general categories: Airport Surveillance Radar (ASR) and Air Route Surveillance Radar (ARSR).
- ASR is designed to provide relatively short-range coverage in the general vicinity of an airport and to serve as an expeditious means of handling terminal area traffic through observation of precise aircraft locations on a radarscope. The ASR can also be used as an instrument approach aid.
- ARSR is a long-range radar system designed primarily to provide a display of aircraft locations over large areas.
- Surveillance radars scan through 360 degrees of azimuth and present target information on a radar display located in a tower or center. This information is used independently or in conjunction with other navigational aids in the control of air traffic.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-5-3.
Research Notes
AIM 4-5-3 covers radar limitations — the conditions under which ATC radar may not detect or properly track an aircraft.
Radar limitations:
- Line of sight: Radar is straight-line propagation. Terrain, the Earth's curvature, and altitude floor limit coverage. Low altitudes and remote areas often have no radar.
- Distance: Even with line of sight, radar range is limited (typically 60-100 NM for SSR; less for primary radar).
- Weather: Heavy precipitation can degrade radar returns and create clutter that obscures aircraft.
- Aircraft attitude: Banked aircraft may have reduced radar cross-section.
When you might lose radar coverage: Mountain valleys, remote areas (Alaska, parts of the West), low altitudes near terrain, when in heavy weather. ATC will tell you "radar contact lost" and you'll need to make position reports.
ADS-B coverage: Generally broader than radar coverage because ADS-B doesn't require ATC radar — the aircraft broadcasts directly, and ground receivers process the broadcast. ADS-B has gradually filled in radar gaps in remote areas.