On a ridge outside almost every major airport in the world stands a machine that has been asking the same question since the 1950s. It is a rotating antenna, turning patiently through day and night, and roughly every five seconds it sweeps its narrow beam across each aircraft in the sky and demands: who are you, and how high? A heartbeat later, a small box behind each cockpit answers. This call-and-response, repeated millions of times a day worldwide, is secondary surveillance radar, and together with its older sibling, primary radar, it is how air traffic control saw every airplane on Earth for half a century before satellites and ADS-B arrived. It still watches them today, because controllers have learned never to trust a single pair of eyes.
Primary radar: Listening for echoes
Primary surveillance radar, PSR in the trade, is radar in the original wartime sense. A transmitter fires an intense pulse of microwave energy, the pulse splashes off anything solid in its path, and a sensitive receiver listens for the faint returning echo. Distance falls out of arithmetic: radio waves travel at the speed of light, about 300 meters every microsecond, so the delay between pulse and echo gives the range, while the direction the antenna was pointing gives the bearing. The technology was born in desperation in the late 1930s, when Britain's Chain Home stations gave fighter squadrons twenty minutes' warning of incoming raids, and it works today on exactly the same principle.
Primary radar has one magnificent virtue: it needs no cooperation. The target carries no equipment, answers no questions, and cannot opt out. A smuggler's aircraft with every switch turned off still returns an echo. That independence is why security and defense radars are primary, and why civil aviation keeps primary radar around airports as a safety net against transponder failures.
Its weaknesses are just as fundamental. An echo is anonymous; the scope shows a blip, not a name. It carries no altitude information unless you add expensive height-finding capability. Echo strength falls off with the fourth power of range, so long-distance coverage demands enormous power. And the world is full of things that echo: rain, flocks of birds, wind turbines, trucks on a highway. Radar engineers spend careers teaching filters to tell a Cessna from a cold front.
Secondary radar: The radar that asks questions
Secondary surveillance radar, SSR, solves the anonymity problem by putting a radio on the aircraft and turning surveillance into a conversation. The ground station transmits an interrogation on 1030 MHz. The aircraft's transponder, a receiver-transmitter whose lineage runs straight back to the wartime Identification Friend or Foe systems, hears the question and replies on 1090 MHz. Because the reply is an active transmission rather than a passive echo, it is strong, clean and packed with data.
What data depends on the interrogation mode. Mode A asks for identity, and the transponder answers with the four-digit squawk code the crew has dialed in, a system with exactly 4,096 possible codes whose most famous members, 7500, 7600 and 7700, get a full treatment in our guide to transponders and emergency squawk codes. Mode C asks for altitude, and the reply carries the aircraft's pressure altitude in 100-foot increments. Modes A and C date from an era of rotary switches and vacuum tubes, and their limits, especially those scarce 4,096 identities, eventually forced a redesign.
That redesign is Mode S, the S standing for select. Instead of shouting a question at every aircraft in the beam, a Mode S interrogator can address one specific aircraft by its unique 24-bit ICAO address and ask detailed questions of it alone. Replies grew from 12 bits of data to 56 or 112, enough to carry callsigns, selected altitudes and intent. Mode S is also the technical foundation on which ADS-B was built, and its addressing scheme, the closest thing aviation has to a digital fingerprint, is unpacked in our companion article on Mode S and ICAO 24-bit addresses.
Did you know?
Secondary radar's 1030 and 1090 MHz frequencies are so busy over central Europe that regulators actively police interrogation rates. A single transponder over Brussels or Frankfurt may be interrogated more than a hundred times per second by overlapping radars, TCAS units and military systems, a congestion problem Eurocontrol has studied for decades.
The choreography of a sweep
Watch a terminal-area radar antenna and you will see it turn about twelve to fifteen times a minute, one sweep roughly every four to five seconds; the big en-route radars that cover airspace between cities turn more slowly, closer to once every ten to twelve seconds. Each time the beam crosses an aircraft, the system fires a string of interrogations, collects the replies, and updates that aircraft's position on the controller's display. Between sweeps, the computer coasts the track forward on its last known heading and speed. This is why classic radar displays update in discrete hops rather than gliding smoothly, and why radar-derived positions on flight tracking sites look jumpier than ADS-B tracks refreshed twice a second. If you have ever wondered why an aircraft icon suddenly leaps a mile sideways, our explainer on why flights vanish and jump on tracking maps walks through the causes.
Modern installations usually mount the two radars together: a large curved primary reflector with a narrow secondary array riding on top like a spoiler. The processing computers fuse both channels, tagging each secondary reply with identity and altitude and correlating any primary-only blips, the uncooperative strangers, for the controller's attention. In busy airspace, data from many overlapping radar heads merges into a single mosaic, so a flight from Atlanta to Boston is handed seamlessly from one antenna's coverage to the next without anyone noticing a seam.
| Characteristic | Primary radar (PSR) | Secondary radar (SSR) |
|---|---|---|
| Principle | Passive echo off the airframe | Interrogation at 1030 MHz, transponder reply at 1090 MHz |
| Needs equipment on aircraft | No | Yes, a transponder |
| Identity of target | Unknown | Squawk code or 24-bit address, callsign with Mode S |
| Altitude | Not directly | Yes, from Mode C or Mode S |
| Range for airliners | Tens of nautical miles typical for terminal PSR | 200 to 250 nautical miles for en-route SSR |
| Weakness | Clutter, no identity, high power demand | Blind to aircraft without working transponders |
Why controllers keep both
It would be tidy to say radar is obsolete now that aircraft report their own GPS positions. Air navigation providers emphatically disagree. ADS-B is dependent surveillance: it relies on the aircraft's own navigation system and its own honesty. If GPS is jammed, degraded or spoofed, ADS-B degrades with it. Secondary radar is independent of the aircraft's navigation, measuring position by beam angle and timing, and primary radar is independent of the aircraft entirely. The FAA's surveillance modernization strategy and Eurocontrol's surveillance services both describe a layered future in which ADS-B carries the routine load while radar remains the cross-check and the backstop. GPS interference incidents in several regions since 2022 have only strengthened that argument.
Primary radar trusts nothing but physics; secondary radar trusts the aircraft to answer; ADS-B trusts the aircraft to tell the truth. Controllers sleep better with all three.
There is also plain economics in the mix. A radar head costs millions to buy and a fortune to maintain, which is why thinly trafficked countries are decommissioning some sites in favor of ADS-B and multilateration. But dense, high-stakes airspace, the London and New York terminal areas, the approaches to hub airports, keeps its radar redundancy. Where radar is retired, networks of simple listening stations can reconstruct positions from transponder signals alone using time-difference mathematics, the technique explained in our article on multilateration and how it finds silent aircraft.
What this means on your tracking map
Public flight trackers cannot receive primary radar echoes, and civil radar data is rarely published in real time, so what you see on a tracking site is mostly ADS-B, filled in with multilateration and, on some services, aggregated government feeds. Understanding radar still sharpens your reading of the map. Aircraft labeled with only a squawk code and altitude are often Mode A/C or Mode S targets without ADS-B position. Trails that update in slow, angular steps betray a non-broadcast source. And the ghostly absence of military traffic reminds you that a whole parallel world of primary-only surveillance exists that no public map will ever show. To see how trackers weave these different sources into one picture, read our breakdown of where flight tracking data comes from, then open the live map and see if you can spot the seams.
Key takeaways
- Primary radar detects passive echoes off the airframe; it needs no cooperation but gives no identity and, in basic form, no altitude.
- Secondary radar interrogates on 1030 MHz and receives transponder replies on 1090 MHz containing identity and altitude.
- Mode A carries one of 4,096 squawk codes, Mode C carries pressure altitude, and Mode S addresses each aircraft individually via its 24-bit ICAO address.
- Terminal radars sweep roughly every 4 to 5 seconds and en-route radars every 10 to 12, which is why radar tracks update in hops.
- Regulators keep radar as an independent cross-check on ADS-B, especially against GPS jamming and spoofing.
- Public tracking maps show ADS-B and multilateration, not radar echoes, so uncooperative aircraft remain invisible to hobbyists.
Frequently asked questions
Can air traffic control see a plane with its transponder off?
Often, yes, but only as an anonymous primary radar blip, and only within primary coverage, which is concentrated around airports and defense installations. Over oceans and much of the remote en-route structure there is no primary radar, which is why a non-transmitting aircraft can effectively disappear.
Why does secondary radar use two different frequencies?
Separating the uplink at 1030 MHz from the downlink at 1090 MHz prevents the ground station's own powerful interrogation from drowning out the aircraft's reply, and lets every radar and every transponder in the world share one common pair of channels.
Is military stealth about defeating primary or secondary radar?
Primary. Stealth shaping and coatings reduce the echo an airframe returns to a primary radar. Secondary radar is irrelevant to stealth because it depends on the aircraft choosing to reply; a military aircraft that wants to be invisible simply does not respond, or transmits nothing at all.
Do radars still use rotating antennas?
Mostly, in civil aviation. The familiar rotating heads remain standard because they are proven and economical. Some modern defense and weather systems use electronically scanned arrays that steer beams without moving parts, and a few civil surveillance sites are trialing them, but the turning antenna will be with us for years.
Half a century of blips and replies built the safest transportation system in history. The rotating antenna on the ridge is no longer the only witness to your flight, but it is still turning, still asking, and the sky is still answering.
