What is radar identification?
Before a controller can issue any instructions to an aircraft, they must first positively identify that target on the radar screen. This process - radar identification - ensures the controller is talking to the right blip. Getting it wrong means issuing instructions to the wrong aircraft, which is a serious safety issue.
Primary vs secondary radar
ATC uses two types of radar:
Primary radar sends out radio pulses that bounce off aircraft and return. It shows a "skin paint" - a raw blip on the screen with no extra information. The controller can see something is there, but not who it is or what altitude it's at. Primary radar works on anything, including aircraft with no transponder, birds, weather, and even terrain.
Secondary radar (SSR) interrogates an aircraft's transponder. The transponder replies with a squawk code and altitude. This gives the controller a data block next to the target showing callsign, altitude, speed, and destination. Secondary radar is far more useful than primary, but it requires a working transponder.
Most radar sites have both primary and secondary antennas co-located. The primary provides backup coverage; the secondary provides the useful data.
How controllers identify targets
A controller can identify an aircraft in several ways:
Departures are often identified by observing the target depart the airport. If you see a target lift off runway 31L and the strip says UAL452 is departing 31L, that's your identification.
Position correlation works when an aircraft reports their position and the controller sees a target in that exact spot. "UAL452, radar contact" means the controller has matched the voice to the blip.
Squawk code assignment ties a unique 4-digit code to a specific aircraft. When the target shows that code, the controller knows who it is.
Ident is the quick method. The controller says "squawk ident," the pilot presses a button on the transponder, and the target flashes or blooms on the screen for several seconds. Hard to miss, even in dense traffic.
"Radar contact"
When the controller says "radar contact," it means two things: the aircraft has been positively identified, and ATC is now providing radar separation services. From this point on, the controller is responsible for keeping that aircraft separated from other traffic.
The opposite call is "radar contact lost" - the target has disappeared from the scope, usually due to low altitude, terrain masking, or radar coverage limits. The controller must then switch to procedural separation (time and distance-based) until radar contact is re-established.
Why it matters
Without positive identification, a controller can't provide radar separation. In busy terminal areas, this means an aircraft might be stuck at an inefficient altitude or route until the controller can confirm who they are. In low-radar-coverage areas (oceanic airspace, remote regions), procedural separation with much larger spacing becomes the standard.
In radarcontrol.io
All aircraft in radarcontrol.io are automatically identified with callsigns and full data blocks on the radar screen. There's no manual identification process - you never need to say "squawk ident" or correlate a target with a flight strip.
This is a significant simplification from real-world operations. In practice, controllers must positively identify each target before providing services. Misidentification events, while rare, are taken very seriously. The sim skips this step so you can focus on the core separation and sequencing challenges.
Data blocks in the sim show callsign, altitude, speed, and aircraft type - similar to what a Mode S transponder provides in the real world.
Related: What is a squawk code? | What is TCAS? | How does ATC separation work?
Guides: Quick start guide | Scoring system
Play New York Center - all aircraft pre-identified and ready for your instructions.