Radar: What ATC Sees—and What ATC Knows
When ATC issues a traffic advisory, the information provided depends on what the controller can see and what they know about the aircraft.
Primary Radar
A primary radar return is simply a reflection of radar energy from an object. It might be an aircraft, a large balloon, or even a flock of birds. The return may be intermittent, and the controller may have little information about what is producing it.
Radar processing filters out much of the stationary or slow-moving clutter from terrain, buildings, trees, and other objects.
If ATC observes an unidentified primary radar target, a traffic advisory might sound like this:
“Traffic, twelve o’clock, five miles, opposite direction, altitude unknown.”
Secondary Radar
Secondary surveillance radar receives information transmitted by an aircraft’s transponder. Mode A provides a transponder code, while Mode C adds pressure-altitude information. Mode S provides additional identification and data capabilities.
If an aircraft is operating a transponder with altitude reporting but is not radar identified, ATC may see its reported altitude without having verified its accuracy or knowing the aircraft type.
A traffic advisory might sound like this:
“Traffic, twelve o’clock, five miles, opposite direction, altitude indicates four thousand.”
Radar-Identified Aircraft
When ATC has identified an aircraft and has the associated flight information, the controller may have considerably more information available, including the aircraft’s callsign, type, and an altitude.
If the reported altitude has been validated, ATC can use that information for applicable separation services.
A traffic advisory might sound like this:
“Traffic, twelve o’clock, five miles, opposite direction, Cessna One Seventy-Two, four thousand.”
Ground Track Versus Heading
One more consideration when receiving traffic advisories: ATC’s clock positions are generally based on your ground track, not your heading.
If you’re correcting for a strong crosswind with a 20-degree crab to the left, traffic called at your eleven o’clock may actually be closer to twelve o’clock out the windshield.
When searching for traffic, account for the difference between where your aircraft is pointed and where it is moving over the ground.
The Takeaway
ATC’s traffic advisories are only as good as the information available to the controller. Understanding what ATC knows about the traffic—and how that information relates to what you see out the windshield—will help you make better use of those advisories.