SSR and ACAS

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SSR and ACAS

13

SSR and ACAS

Secondary Surveillance Radar (SSR)

13.1 Background. The technical requirements of the EASA Theoretical Knowledge syllabus for

SSR, is covered in 062 Radio Navigation. However, the operation of SSR in the ATC environment

is within the remit of Air Law. In the chapters of these notes concerning ATC, the use of radar

will be discussed and it will be implied that where required, the radar derived information

will be enhanced by the use of SSR. The modern SSR systems have been developed from the

equipment used in WWII as a means of identifying ‘friend’ from ‘foe’ (IFF). The basic system

was rather crude effectively giving the ground station a response from the aircraft indicating

that the aircraft was fitted with the then highly secret equipment. As only ‘friendly’ aircraft

had the equipment the radar operator could easily distinguish a friendly aircraft radar response

from an enemy response. The airborne equipment was given the code name ‘parrot’ which

explains some of the rather peculiar phraseology associated with the operation of a modern

SSR system. Current SSR systems enable individual aircraft to be identified on a flight by flight

basis with the additional facilities to pass data via the SSR system, indicate aircraft altitude

and, using the redundant capacity of the SSR system, provide effective collision avoidance

enhancement.

13.2 Carriage of Transponders. The airborne equipment is called a transponder (because

13

it transmits a response to an interrogation). PANS OPS requires that where a serviceable

transponder is carried in an aircraft, unless ATC instructs otherwise, it will be used at all

ACAS

times. Individual states may specify certain conditions where the carriage and operation of a

transponder is mandatory. In either case, the operation of the equipment will be regardless of

and

the provision of an ATC service.

SSR

13.3 Mode of Operation. The output codes of the SSR system consist of groups of 4

numbers in the range 0 - 7. The ground station (the interrogator) transmits a coded signal that

prompts the aircraft transponder to reply. The overall SSR system has several different ‘modes’

of operation. The response will be mode Alpha with addition of mode Charlie. In addition,

the military have other modes of operation of SSR which overlap with civilian usage. In order

to correctly indicate the required response code, the mode as well as code should always be

specified. The pilot is to respond with the mode and code. Typically a radar controller will

request the mode and code as follows:

“G-CD, squawk Alpha two one six one”

The response to this by the pilot will be (mode and code):

“Squawking Alpha two one six one G-CD”

13.4 Squawk Ident. The transponder has a facility to enable the radar controller to

automatically identify the specific radar contact using SSR other than by reliance on the specific

code transmission. This facility may be activated by selection of the ‘Ident’ feature on the

transponder controller in response to a request to “squawk Ident.” Pilots are not to squawk

Ident except on instruction from the radar controller.