Chapter

14

Secondary Surveillance Radar (SSR)

Introduction				227
Advantages of SSR . . . . . . . . . . . .	. . . . . . . . . . . .	. .	.	227
SSR Display				227
SSR Frequencies and Transmissions . . . . . .	. . . . . . . . . . . .	. .	.	230
Modes . . . . . . . . . . . . . . . . .	. . . . . . . . . . .	. .	. .	230
Mode C . . . . . . . . . . . . . . . .	. . . . . . . . . . . .	. .	.	232
SSR Operating Procedure				232
Special Codes . . . . . . . . . . . . . .	. . . . . . . . . . . .	. .	.	233
Disadvantages of SSR				233
Mode S				234
Pulses . . . . . . . . . . . . . . . . .	. . . . . . . . . . .	. .	. .	234
Benefits of Mode S . . . . . . . . . . . .	. . . . . . . . . . . .	. .	.	235
Communication Protocols . . . . . . . . . .	. . . . . . . . . . .	. .	.	.236
Levels of Mode S Transponders . . . . . . . .	. . . . . . . . . . .	. . .		.236
Downlink Aircraft Parameters (DAPS)				237
Future Expansion of Mode S Surveillance Services .	. . . . . . . . . . .	. .	. .	237
SSR Summary . . . . . . . . . . . . . .	. . . . . . . . . . . .	. .	.	238
Questions . . . . . . . . . . . . . . . .	. . . . . . . . . . .	. . . .239
Answers . . . . . . . . . . . . . . . .	. . . . . . . . . . . .	. .	.	240

225

14 Secondary Surveillance Radar (SSR)

(SSR) Radar Surveillance Secondary 14

226

	Secondary Surveillance Radar (SSR)		14
	Introduction
	Primary radar relies on the reception of a reflected pulse i.e. the echo of the transmitted pulse.
	Secondary radar, on the other hand, receives pulses transmitted by the target in response to
	interrogation pulses. Secondary surveillance radar (SSR) is one type of secondary radar system;
	DME is another such system that will be discussed in Chapter 15.
	Both primary and secondary surveillance radars are used to track the progress of an aircraft.
	Primary radar provides better bearing and range information of an aircraft than SSR but its
	biggest disadvantage is the lack of positive, individual aircraft identification; this is required
	for adequate safe control by ATC, particularly in crowded airspace. Primary radars also require
	higher transmitter power outputs for the two-way journey of the single pulses.
	SSR requires an aircraft to be fitted with a transmitter/receiver, called a transponder. The
	pilot will set a four-figure code allocated by ATC and the transponder will transmit information
	automatically, in pulse coded form, when it is interrogated by the ground station called the
	interrogator. The transmissions are therefore only one way from transmitter to receiver.
	Advantages of SSR
	SSR has the following advantages over primary radar:
	• requires much less transmitting power to provide coverage up to 200 to 250 NM.	14
	• is not dependent on an aircraft’s echoing area or aspect.		(SSR)
	• gives clutter free responses as it is does not rely on returning reflected pulses.		Radar
	• positively identifies an aircraft’s primary response by displaying its code and call sign		Surveillance
	alongside.
	• indicates an aircraft’s track history, speed, altitude and destination.		Secondary
	• can indicate on a controller’s screen that an aircraft has an emergency, has lost radio
	communications or is being hi-jacked.
	Thus when SSR is used in conjunction with primary radar, the advantages of both systems are
	realized. The two radars are therefore usually co-located as shown in Figures 14.1 and 14.2.
	SSR Display
	The SSR information is displayed in combination with the primary radar information on the
	same screen as shown in Figure 14.3. This includes the call sign or flight number, pressure
	altitude or flight level, ground speed and destination.

227

14 Secondary Surveillance Radar (SSR)

(SSR) Radar Surveillance Secondary 14

Figure 14.1 SSR aerial mounted on top of a 23 cm primary radar aerial

Figure 14.2 Primary and secondary radar used for ATC

228

Secondary Surveillance Radar (SSR) 14

Secondary Surveillance Radar (SSR) 14

Figure 14.3 A radar display showing positions of aircraft in the London TMA

229