ATC radar & flight following

ATC can see you on radar and warn you about traffic you'd never spot on your own — but only if you ask for the service, and only as far as their workload allows.

What radar gives you

ATC radar comes in two flavors, and the difference matters:

• Primary radar bounces a pulse off your airframe — a raw "skin paint." It shows a blip with no identity and no altitude.
• Secondary radar interrogates your transponder, which replies with a code and, with Mode C, your pressure altitude. That's far more useful, which is why ATC leans on secondary returns.

With basic radar service, a controller can give you traffic advisories and safety alerts for terrain or traffic conflicts. But radar isn't omniscient: it's line-of-sight like everything else, so coverage thins at low altitude and behind terrain. Down in a valley you may be below radar coverage entirely, and the controller simply can't see you.

VFR flight following

Flight following is VFR traffic advisories provided on a workload-permitting basis. You request it, get a discrete squawk code, and ATC watches your data block and calls traffic to you.

When traffic is called, it comes as a clock position relative to your ground track (not your nose), with distance and altitude: "traffic, two o'clock, three miles, northbound, altitude indicates three thousand five hundred." Translate that into where to look — and remember it's a snapshot; by the time you've processed it, the traffic has moved, so look in a wider arc.

Two things to keep straight:

• The service is workload-permitting — a busy controller may say "unable" or drop you when the sector saturates.
• See-and-avoid is still entirely yours. Advisories supplement your eyes; they don't replace them, and a missed call is not the controller flying your airplane.

Three things flight following is not, and students reliably mix them up. It is not a clearance — you still need an explicit clearance to enter Class B or C, and you remain responsible for staying clear of any airspace you're not cleared into. It is not a VFR flight plan — that's a separate search-and-rescue tool you file with Flight Service and open and close yourself. And it does not relieve you of any VFR responsibility for weather minimums, terrain, or fuel. It's an advisory service layered on top of all of that — useful, but not a net that catches everything.

The transponder and ADS-B

• §91.215 requires a Mode C transponder in Class A, B, and C airspace, within the 30-NM Mode C veil around a Class B primary airport, and at or above 10,000 ft MSL (excluding airspace below 2,500 ft AGL). Squawk 1200 for VFR unless ATC assigns a code.
• §91.225 requires ADS-B Out in essentially that same airspace. ADS-B Out continuously broadcasts your GPS position and altitude, which is what makes modern cockpit and controller traffic displays work.
• If you also have ADS-B In, you receive TIS-B (traffic) and FIS-B (subscription-free weather and NOTAMs) in the cockpit — a genuine aid, but its traffic picture is only complete where coverage and equipage allow, so it supplements the controller and your eyes rather than replacing either.

The full airspace breakdown — exactly where each rule applies — lives in I.E (Airspace); here the point is simply what the equipment does and why ATC wants it.