Long Ca

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):

• ATCRBS, sometimes referred to as secondary surveillance radar, consists of three main components:

• Interrogator:

  • Primary radar relies on a signal being transmitted from the radar antenna site and for this signal to be reflected or “bounced back” from an object (such as an aircraft)
  • This reflected signal is then displayed as a “target” on the controller’s radarscope
  • In the ATCRBS, the Interrogator, a ground based radar beacon transmitter-receiver, scans in synchronism with the primary radar and transmits discrete radio signals which repetitiously request all transponders, on the mode being used, to reply
  • The replies received are then mixed with the primary returns and both are displayed on the same radarscope

• • Transponder:

    • This airborne radar beacon transmitter-receiver automatically receives the signals from the interrogator and selectively replies with a specific pulse group (code) only to those interrogations being received on the mode to which it is set
    • These replies are independent of, and much stronger than a primary radar return

  • Radarscope:

    • The radarscope used by the controller displays returns from both the primary radar system and the ATCRBS
    • These returns, called targets, are what the controller refers to in the control and separation of traffic
• The job of identifying and maintaining identification of primary radar targets is a long and tedious task for the controller
• Some of the advantages of ATCRBS over primary radar are:
  1. Reinforcement of radar targets
  2. Rapid target identification
  3. Unique display of selected codes
• A part of the ATCRBS ground equipment is the decoder
  • This equipment enables a controller to assign discrete transponder codes to each aircraft under his/her control

  Normally only one code will be assigned for the entire flight

  • Assignments are made by the ARTCC computer on the basis of the National Beacon Code Allocation Plan
  • The equipment is also designed to receive Mode C altitude information from the aircraft
• It should be emphasized that aircraft transponders greatly improve the effectiveness of radar systems
• ATCRBS displays differ between NAS Stage A (en route), ARTS III (terminal), and other non-automated (broadband) radar systems have different displays [Figure 2/3]

• 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

• (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)

Limitations:

• 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