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.
(f) 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.
REFERENCE-
Pilot/Controller Glossary Term- Radar.

(g) 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.
c. FAA radar units operate continuously at the
locations shown in the Chart Supplement U.S., and
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.
4-5-2. Air Traffic Control Radar Beacon
System (ATCRBS)
a. The ATCRBS, sometimes referred to as
secondary surveillance radar, consists of three main
components:
1. Interrogator. Primary radar relies on a
signal being transmitted from the radar antenna site
and for this signal to be reflected or "bounced back"

4-5-2

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.
2. 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.
3. 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.
b. 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.
c. 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.
NOTE-
Refer to figures with explanatory legends for an illustration
of the target symbology depicted on radar scopes in the
NAS Stage A (en route), the ARTS III (terminal) Systems,
and other nonautomated (broadband) radar systems. (See
FIG 4-5-2 and FIG 4-5-3.)

d. It should be emphasized that aircraft transponders greatly improve the effectiveness of radar
systems.
REFERENCE-
AIM, Paragraph 4-1-20 , Transponder Operation

Surveillance Systems