AIM ¶ 4-5-2 — ATCRBS Secondary Surveillance Radar
Learn how ATCRBS works for pilots: interrogator, transponder, and radarscope components, Mode C altitude, and discrete codes. AIM 4-5-2 explained for checkride prep.
In Plain English
ATCRBS (Air Traffic Control Radar Beacon System), also called secondary surveillance radar, works alongside primary radar to give controllers a stronger, clearer picture of traffic. Unlike primary radar, which relies on a signal physically bouncing off your aircraft, ATCRBS asks your transponder to actively reply with a coded signal.
The system has three main components:
- Interrogator — a ground-based transmitter-receiver that scans in sync with the primary radar and sends out discrete signals asking transponders to reply.
- Transponder — your airborne radar beacon transmitter-receiver. It automatically replies with a specific pulse group (code) when interrogated on the mode it's set to. Replies are much stronger than primary returns.
- Radarscope — displays both primary radar targets and ATCRBS replies side-by-side for the controller.
A decoder lets controllers assign discrete codes (usually one per flight, via the ARTCC computer and the National Beacon Code Allocation Plan) and receive Mode C altitude data. Advantages over primary radar alone include reinforcement of returns, rapid target ID, and unique display of selected codes. Bottom line: your transponder dramatically improves how well ATC can see and separate you.
AIM Source Text
FAA AIM ¶ 4-5-24-5-2. 4-5-2. Air Traffic Control Radar Beacon System (ATCRBS)
The 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: Reinforcement of radar targets. Rapid target identification. 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. REFERENCE- AIM, Para 4-1-20 , Transponder and ADS-B Out Operation.
Oral Exam Questions a DPE Might Ask
Q1What are the three main components of the ATCRBS?
Per AIM 4-5-2, the ATCRBS consists of the interrogator (ground-based radar beacon transmitter-receiver), the transponder (airborne transmitter-receiver), and the radarscope (which displays both primary and beacon returns to the controller).
Q2How does ATCRBS differ from primary radar?
Per AIM 4-5-2, primary radar relies on a signal being reflected back off the aircraft, while ATCRBS uses an interrogator that prompts the airborne transponder to actively reply with a coded pulse group. These transponder replies are independent of and much stronger than a primary radar return.
Q3Who assigns transponder codes and what additional information can ATCRBS receive from the aircraft?
Per AIM 4-5-2, discrete transponder codes are assigned by the ARTCC computer based on the National Beacon Code Allocation Plan, normally one code for the entire flight. The ground equipment is also designed to receive Mode C altitude information from the aircraft.
Related Paragraphs in AIM Chapter 4