Identification

Unlike tower controllers, radar controllers cannot look out the window to provide air traffic services. They have to rely on data collected by what are called surveillance systems. Examples of these are Primary Surveillance Radar (PSR) and Secondary Surveillance Radar (SSR).

Primary surveillance radar (PSR)

When radar was first invented it only existed as a primary radar. A primary radar sends out electromagnetic waves in all directions and displays a dot on the screen for each reflection it detects. However, there is no way to know which dot on the screen belongs to which aircraft - this is where identification comes into the picture.

An aircraft is identified if we can see its target on the radar screen and are certain which aircraft the target belongs to.

But if the aircraft cannot send any data to us, how do we know which target is which aircraft? When using PSR, there are multiple so called methods of identification [1]:

position report: correlating a target with an aircraft that reports is position over or its distance and bearing from a significant point on your screen and making sure the track of the target is consistent with the aircraft's path / reported heading,
departing aircraft: correlating a target with a departing aircraft within 1NM of the end of the runway,
turn method: instructing an aircraft to change the heading by 30 degrees or more, and observing the change
transfer of identification: more on this later.

Secondary surveillance radar (SSR)

Modern surveillance systems use a transmitter/receiver combination that interrogate transponders onboard the aircraft which then send data back to the ground station. This is the fundamental difference to PSR systems where the ground station receives passive signals (reflections). There are different interrogation modes which transmit different data [2]:

Mode	Data transmitted
A	4-digit octal identification code, i.e. Squawk
C	Aircraft's pressure altitude
S	Callsign, unique 24-bit address, selected altitude, ground speed, indicated airspeed, etc. [3]

Modes A and C are often combined to mode 3 A/C. Using SSR, there are additional methods of identification:

recognition of the aircraft identification in the label

recognition of an assigned discrete code, the setting of which was confirmed by the pilot

observation of the setting/changing of an instructed individual code

observation the compliance with an instruction to squawk IDENT

Before providing surveillance services (any service provided directly by means of an ATS surveillance system, such as primary or secondary radar) the controller must establish identification and inform the pilot.

[1] ICAO Doc 4444, Procedures for air navigation services - Air traffic management, Sixteenth edition, 2016
[2] Aviation transponder interrogation modes, Wikipedia
[3] Skybrary Mode S