Radio Detection and Ranging


  • ATC uses Radio Detection And Ranging (RADAR) which create radio waves, transmitted into the air that are then received when they have been reflected (echo) by an object in the path of the beam
  • Range is determined by measuring the time it takes (at the speed of light) for the radio wave to go out to the object and then return to the receiving antenna
  • Direction of a detected object from a radar site is determined by the position of the rotating antenna when the reflected portion of the radio wave is received
Precipitation Attenuation
Figure 1: Preceipitation Attenuation

Radar Limitations:

  • The characteristics of radio waves are such that they normally travel in a continuous straight line unless they are:
    • "Bent" by abnormal atmospheric phenomena such as temperature inversions
    • Reflected or attenuated by dense objects such as heavy clouds, precipitation, ground obstacles, mountains, etc.
    • Screened by high terrain features
  • The bending of radar pulses, often called anomalous propagation or ducting, may cause many extraneous blips to appear on the radar operator’s display if the beam has been bent toward the ground or may decrease the detection range if the wave is bent upward
    • It is difficult to solve the effects of anomalous propagation, but using beacon radar and electronically eliminating stationary and slow moving targets by a method called moving target indicator (MTI) usually negate the problem
  • Radar energy that strikes dense objects will be reflected and displayed on the operator’s scope thereby blocking out aircraft at the same range and greatly weakening or completely eliminating the display of targets at a greater range
    • Again, radar beacon and MTI are very effectively used to combat ground clutter and weather phenomena, and a method of circularly polarizing the radar beam will eliminate some weather returns
    • A negative characteristic of MTI is that an aircraft flying a speed that coincides with the canceling signal of the MTI (tangential or "blind" speed) may not be displayed to the controller
  • Relatively low altitude aircraft will not be seen if they are screened by mountains or are below the radar beam due to earth curvature
    • To mitigate this, some areas strategically install multiple radars to cover these blind spots
  • There are several other factors which affect radar control:
    • There are several other factors which affect radar control. The amount of reflective surface of an aircraft will determine the size of the radar return. Therefore, a small light airplane or a sleek jet fighter will be more difficult to see on radar than a large commercial jet or military bomber. Here again, the use of radar beacon is invaluable if the aircraft is equipped with an airborne transponder. All ARTCCs’ radars in the conterminous U.S. and many airport surveillance radars have the capability to interrogate Mode C and display altitude information to the controller from appropriately equipped aircraft. However, there are a number of airport surveillance radars that don’t have Mode C display capability and; therefore, altitude information must be obtained from the pilot
  • At some locations within the ATC en route environment, secondary−radar−only (no primary radar) gap filler radar systems are used to give lower altitude radar coverage between two larger radar systems, each of which provides both primary and secondary radar coverage. In those geographical areas served by secondary−radar only, aircraft without transponders cannot be provided with radar service. Additionally, transponder equipped aircraft cannot be provided with radar advisories concerning primary targets and weather
  • The controller’s ability to advise a pilot flying on instruments or in visual conditions of the aircraft’s proximity to another aircraft will be limited if the unknown aircraft is not observed on radar, if no flight plan information is available, or if the volume of traffic and workload prevent issuing traffic information. The controller’s first priority is given to establishing vertical, lateral, or longitudinal separation between aircraft flying IFR under the control of ATC
  • FAA radar units operate continuously at the locations shown in the Airport Facility Directory (A/FD)
    • Their services are available to all pilots both civil and military
    • Contact the associated FAA control tower or ARTCC on any frequency guarded for initial instructions, or in an emergency, any FAA facility for information on the nearest radar service

Air Traffic Control Radar Beacon System (ATCRBS):

Radar Scope with Alphanumeric Data
Figure 2: Radar Scope with Alphanumeric Data
1. Areas of precipitation (can be reduced by CP) 2. Arrival/departure tabular list
3. Trackball (control) position symbol (A) 4. Airway (lines are sometimes deleted in part)
5. Radar limit line for control 6. Obstruction (video map)
7. Primary radar returns of obstacles or terrain (can be removed by MTI) 8. Satellite airports
9. Runway centerlines (marks and spaces indicate miles) 10. Primary airport with parallel runways
11. Approach gates 12. Tracked target (primary and beacon target)
13. Control position symbol 14. Untracked target select code (monitored) with Mode C readout of 5,000'
15. Untracked target without Mode C 16. Primary target
17. Beacon target only (secondary radar) (transponder) 18. Primary and beacon target
19. Leader line 20. Altitude Mode C readout is 6,000' (Note: readouts may not be displayed because of non-receipt of beacon information, garbled beacon signals, and flight plan data which is displayed alternately with the altitude readout)
21. Ground speed readout is 240 knots (Note: readouts may not be displayed because of a loss of beacon signal, a controller alert that a pilot was squawking emergency, radio failure, etc.) 22. Aircraft ID
23. Asterisk indicates a controller entry in Mode C block. In this case 5,000' is entered and “05” would alternate with Mode C readout 24. Indicates heavy
25. “Low ALT” flashes to indicate when an aircraft's predicted descent places the aircraft in an unsafe proximity to terrain. (Note: this feature does not function if the aircraft is not squawking Mode C. When a helicopter or aircraft is known to be operating below the lower safe limit, the “low ALT” can be changed to “inhibit” and flashing ceases.) 26. NAVAIDs
27. Airways 28. Primary target only
29. Non-monitored. No Mode C (an asterisk would indicate non-monitored with Mode C) 30. Beacon target only (secondary radar based on aircraft transponder)
31. Tracked target (primary and beacon target) control position A 32. Aircraft is squawking emergency Code 7700 and is non-monitored, untracked, Mode C
33. Controller assigned runway 36 right alternates with Mode C readout (Note: a three letter identifier could also indicate the arrival is at specific airport) 34. Ident flashes
35. Identing target blossoms 36. Untracked target identing on a selected code
37. Range marks (10 and 15 miles) (can be changed/offset) 38. Aircraft controlled by center
39. Targets in suspend status 40. Coast/suspend list (aircraft holding, temporary loss of beacon/target, etc.)
41. Radio failure (emergency information) 42. Select beacon codes (being monitored)
43. General information (ATIS, runway, approach in use) 44. Altimeter setting
45. Time 46. System data area

  • A number of radar terminals do not have ARTS equipment. Those facilities and certain ARTCCs outside the contiguous U.S. would have radar displays similar to the lower right hand subset. ARTS facilities and NAS Stage A ARTCCs, when operating in the non-automation mode, would also have similar displays and certain services based on automation may not be available
  • This figure illustrates the controller's radar scope (PVD) when operating in the full automation (RDP) mode, which is normally 20 hours per day

NAS Stage A Controllers View Plan Display
Figure 3: NAS Stage A Controllers View Plan Display
  • (When not in automation mode, the display is similar to the broadband mode shown in the ARTS III radar scope figure. Certain ARTCCs outside the contiguous U.S. also operate in “broadband” mode.)
  • Target symbols:

    • Uncorrelated primary radar target [O] [+]
    • Correlated primary radar target [¢]:See note below
    • Uncorrelated beacon target [ / ]
    • Correlated beacon target [ \ ]
    • Identing beacon target [≡]: Note: in Number 2 correlated means the association of radar data with the computer projected track of an identified aircraft
  • Position symbols:

    • Free track (no flight plan tracking) [∆]
    • Flat track (flight plan tracking) [◊]
    • Coast (beacon target lost) [#]
    • Present position hold [ ¢ ]
  • Data block information:

    • Aircraft ident:See note below
    • Assigned altitude FL 280, Mode C altitude same or within 200' of assigned altitude:See note below
    • Computer ID #191, handoff is to sector 33 (0-33 would mean handoff accepted):See note below
    • Assigned altitude 17,000', aircraft is climbing, Mode C readout was 14,300 when last beacon interrogation was received
    • Leader line connecting target symbol and data block
    • Track velocity and direction vector line (projected ahead of target)
    • Assigned altitude 7,000, aircraft is descending, last Mode C readout (or last reported altitude) was 100' above FL 230
    • Transponder code shows in full data block only when different than assigned code
    • Aircraft is 300' above assigned altitude
    • Reported altitude (no Mode C readout) same as assigned. (An “n” would indicate no reported altitude.)
    • Transponder set on emergency Code 7700 (EMRG flashes to attract attention)
    • Transponder Code 1200 (VFR) with no Mode C
    • Code 1200 (VFR) with Mode C and last altitude readout
    • Transponder set on radio failure Code 7600 (RDOF flashes)
    • Computer ID #228, CST indicates target is in coast status
    • Assigned altitude FL 290, transponder code (these two items constitute a “limited data block”):Note: numbers 10, 11, and 12 constitute a “full data block”
  • Other symbols:

    • Navigational aid
    • Airway or jet route
    • Outline of weather returns based on primary radar. “H” represents areas of high density precipitation which might be thunderstorms. Radial lines indicated lower density precipitation
    • Obstruction
    • Airports (Major/Small)

Airport Surface Detection Equipment - Model-X:

  • Airport Surface Detection Equipment - Model-X (ASDE-X) is a multi-sensor surface surveillance system the FAA is acquiring for airports in the United States
  • ASDE-X provides controllers with high resolution, short-range, clutter free surveillance information about aircraft and vehicles, both moving and fixed, located on or near the airport surface under all weather and visibility conditions
  • The combination of multiple sensors ensures that the most accurate information about aircraft location is received in the tower, thereby increasing surface safety and efficiency
  • The system consists of four main components:
    • Primary Radar System:

      • Covers surface to up to 200'
      • Typically located on the Air Traffic Control Tower or other strategic location on the airport
      • Able to detect and display aircraft that are not equipped with or have malfunctioning transponders
    • Interfaces:

      • Contains an automation interface for flight identification via all automation platforms and interfaces with the terminal radar for position information
    • ASDE-X Automation:
      • A Multi-sensor Data Processor (MSDP) combines all sensor reports into a single target which is displayed to the air traffic controller

    • Air Traffic Control Tower Display:

      • A high resolution, color monitor in the control tower cab provides controllers with a seamless picture of airport operations on the airport surface
  • A list of facilities that have been projected to receive ASDE-X can be found within the AIM, Chapter 4-5-5, Airport Surface Detection Equipment - Model X

Doppler Radar:

  • Doppler Radar is a semi-automatic self-contained dead reckoning navigation system (radar sensor plus computer) which is not continuously dependent on information derived from ground based or external aids
  • The system employs radar signals to detect and measure ground speed and drift angle, using the aircraft compass system as its directional reference
  • Doppler is less accurate than INS, however, and the use of an external reference is required for periodic updates if acceptable position accuracy is to be achieved on long range flights

Surveillance Radar:

  • 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
  • Surveillance radars are divided into two general categories:
    • Airport Surveillance Radar
    • Air Route Surveillance Radar
  • Airport Surveillance Radar (ASR):

    • 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
    • Short range
    • Provides radar vectors and azimuth in conjunction with approaches
  • Air Route Surveillance Radar (ARSR):

    • Long-range system designed primarily to provide a display of aircraft locations over large areas
    • Used for en-route traffic
    • May be used for terminal operations (approach)
  • Center Radar Automated Radar Terminal Systems (ARTS) Processing (CENRAP):

    • Developed to provide an alternative to a non-radar environment at terminal facilities should an ASR fail or malfunction
    • CENRAP sends aircraft radar beacon target information to the ASR terminal facility equipped with ARTS
    • Procedures used for the separation of aircraft may increase under certain conditions when a facility is utilizing CENRAP because radar target information updates at a slower rate than the normal ASR radar
    • Radar services for VFR aircraft are also limited during CENRAP operations because of the additional workload required to provide services to IFR aircraft

Precision Approach Radar (PAR):

  • Precision Approach Radar is a highly accurate system designed for use as a landing aid rather than an aid for sequencing and spacing aircraft but may be used to monitor other types of approaches
  • PAR is designed to display range, azimuth, and elevation information
  • Two antennas are used in the PAR array, one scanning a vertical plane, and the other scanning horizontally
    • Since the range is limited to 10 miles, azimuth to 20°, and elevation to 7°, only the final approach area is covered
    • Each scope is divided into two parts
      • The upper half presents altitude and distance information
      • The lower half presents azimuth and distance information


  • May be unable to issue traffic advisories for aircraft not in control
  • Can have interference (clouds, terrain, weather)
  • Dense objects can cause blind spots
  • Low altitude aircraft may not be seen
  • Smaller aircraft have smaller returns


  • More reliable maintenance and improved equipment have reduced radar system failures to a negligible factor
    • Most facilities actually have some components duplicated, one operating and another which immediately takes over when a malfunction occurs to the primary component
  • It is very important however, for the aviation community to recognize the fact that there are limitations to radar service and that ATC controllers may not always be able to issue traffic advisories concerning aircraft which are not under ATC control and cannot be seen on radar