Identification

Aircraft identification is a fundamental task in air traffic control, ensuring accurate tracking, communication, and coordination between controllers and pilots. Before issuing any ATC clearance in a Surveillance Services environment, an aircraft must be positively identified.

Methods of Identification

Aircraft can be identified using various methods:

• Unique Call Signs – Each aircraft has a unique identifier associated with its flight plan.
• Transponder Codes (SSR) – Squawk codes assigned by ATC for tracking and separation.
• Aircraft Type & Registration – Helps verify aircraft identity, especially in mixed traffic environments.
• ADS-B Data – Automatic Dependent Surveillance–Broadcast, providing real-time aircraft information.

If a pilot selects an incorrect SSR code or transponder mode, ATC must instruct them to correct it.

Radar Identification

Surveillance radar systems provide position, altitude, and speed data, allowing controllers to track and correlate aircraft targets accurately. Radar identification is required before providing ATC services.

When an aircraft leaves radar coverage or enters uncontrolled airspace, controllers must terminate radar service and inform the pilot accordingly.

Level Verification

• Controllers must verify the displayed level information at least once on initial contact.
• The tolerance for verifying accuracy:
  • ±200 FT in RVSM airspace
  • ±300 FT in non-RVSM airspace

If the displayed altitude exceeds tolerance values:
✔ Ask the pilot to confirm the correct altimeter setting (QNH)
✔ If necessary, instruct the pilot to disable Mode C altitude reporting

Unlike tower controllers, who can visually observe aircraft, radar controllers rely entirely on surveillance data from various systems.

Primary Surveillance Radar (PSR)

How PSR Works

• PSR transmits electromagnetic waves and displays reflections on the radar screen.
• Each reflection represents an aircraft, but PSR does not transmit aircraft identity information.
• Identification must be performed manually using position correlation or maneuver-based methods.

Methods of Identifying Aircraft Using PSR

1. Position Reports – Correlating a radar target with a pilot's position report (distance & bearing from a known point).
2. Departing Aircraft – Assigning a radar target to an aircraft departing within 1 NM of the runway end.
3. Turn Method – Instructing an aircraft to turn by 30° or more and observing the corresponding radar movement.
4. Transfer of Identification – Another controller transfers a positively identified aircraft to your control.

Secondary Surveillance Radar (SSR)

How SSR Works

• Unlike PSR, SSR actively interrogates aircraft transponders, which reply with encoded data.
• Provides enhanced aircraft identification, reducing workload and increasing accuracy.

SSR Interrogation Modes

Mode	Transmitted Data
A	4-digit squawk code
C	Pressure altitude
S	Callsign, 24-bit aircraft address, selected altitude, speed, etc.

Modes A and C are often combined as Mode 3A/C.

Methods of Identifying Aircraft Using SSR

✔ Recognition of aircraft callsign in an SSR label
✔ Recognition of an assigned discrete squawk code
✔ Observation of a pilot-acknowledged squawk IDENT activation
✔ Transfer of identification from another controller

The most common method of identification on VATSIM is recognizing the aircraft ID (callsign) in an SSR label. If a pilot is unable to activate their transponder, they can be identified using PSR methods.

Reading and Deviations of Transponder Values

While transponder deviations are less relevant in a simulated environment than in real life, controllers should still monitor transponder readouts for accuracy.

• A flight level is considered "reached," "maintained," or "left" based on the transponder reading.
• A 200-foot tolerance is generally applied on VATSIM.
• Any deviation beyond tolerance should be addressed with the pilot.

Pilots should be reminded to check their QNH settings if an altitude discrepancy is detected.

SSR & ADS-B in ATC Operations

SSR (Secondary Surveillance Radar) vs. ADS-B

SSR and ADS-B (Automatic Dependent Surveillance–Broadcast) are complementary technologies enhancing ATC surveillance:

Technology	Function
SSR	Interrogates aircraft transponders to receive replies
ADS-B	Aircraft broadcasts its own position and data automatically

Transponder Use in Ground Operations

• Before takeoff: The transponder should be turned on before departure.
• At airports with ground movement radar (SMGCS): The correct squawk code should be set before taxiing.

For more details on Surface Movement Guidance and Control Systems (SMGCS), refer to Skybrary.

Controller Responsibilities in Identification

Before Providing ATC Services

✔ Aircraft must be positively identified.
✔ Identification must be confirmed before issuing clearances.
✔ Inform the pilot of radar identification unless the previous sector already identified them.

If Transponder Issues Occur

✔ Instruct the pilot to check transponder settings.
✔ If Mode C data is unreliable, request them to disable altitude reporting.

Loss of Radar Contact

✔ If an aircraft leaves radar coverage, radar service must be terminated and the pilot must be informed.
✔ Procedural separation may be required if radar service is lost.