How Radar Works: Primary vs. Secondary Surveillance Radar

The world’s skies are a busy, constantly moving network. Every day, tens of thousands of aircraft take off, fly, and land across the globe, connecting cities, people, and economies. According to FlightRadar24, between 100,000 and 130,000 flights operate each day worldwide.

Managing such a massive, constantly shifting network is no small feat. Controllers need precise tools to know exactly where every plane is at all times, which is where radar comes in. Radar has been guiding aircraft long before commercial flights filled the skies. The first radar experiments began in the 1930s, primarily for military use to detect enemy aircraft. After World War II, this technology was adapted for civilian aviation, giving air traffic controllers a revolutionary tool to monitor and guide planes.

Today, radar remains at the heart of air traffic control, helping controllers track aircraft. To do this effectively, they rely on two main types: Primary Surveillance Radar (PSR), and Secondary Surveillance Radar (SSR). Together, these systems form the foundation of safe, efficient air travel.

PSR is the original radar technology and is still used today as a reliable safety tool. It works by sending out a pulse of radio waves that travels through the air, bounces off an aircraft, and returns to the radar antenna. By measuring how long it takes for the pulse to come back, controllers can tell how far away the plane is. The radar also shows which direction the aircraft is in, giving controllers a clear picture of its position in the sky.

PSR does have some limitations. It cannot identify the aircraft or provide its altitude. It only shows a “blip” on the screen. Think of PSR as a watchful eye scanning the skies. It’s simple, robust, and doesn’t rely on any equipment on the plane, which makes it essential as a backup if other systems fail.

SSR is a more advanced system that gives controllers more information about each aircraft. It works with a transponder on the plane. The radar sends a signal asking for details, and the transponder responds automatically with the aircraft’s identity, altitude, and position, in addition to its distance and direction.

SSR is like a conversation between the plane and the radar, while PSR just watches. By providing more detailed information, SSR makes it easier for controllers to track multiple planes safely, organize takeoffs and landings, and prevent conflicts in busy airspace.

Feature	PSR	SSR
Detects aircraft without transponder	✅	❌
Provides aircraft identity	❌	✅
Provides altitude	❌	✅
Reliability	High	High
Complexity	Simple	More complex, interactive

While PSR and SSR provide controllers with essential information, growing air traffic and increasingly busy skies demanded smarter, more precise ways to track aircraft. This is where Mode S and ADS-B come in, building on the foundation of traditional radar but offering more detailed, efficient, and sometimes satellite-based tracking.

• Mode S acts like an “intelligent radar.” It can communicate with individual planes and request information such as the aircraft’s ID and altitude. This reduces clutter on the controller’s screen and helps track each plane safely, especially in busy airspace.
  
  
• ADS-B (Automatic Dependent Surveillance–Broadcast) works differently. Each aircraft automatically broadcasts its GPS location, speed, and altitude to controllers and nearby planes. Unlike traditional radar, ADS-B works even in remote areas without radar stations. Services like FlightRadar24 use ADS-B signals to show real-time positions of aircraft around the world.

Radar has come a long way since its early days in the 1930s. From the simplicity of Primary Surveillance Radar to the interactive sophistication of Secondary Surveillance Radar and now the advanced capabilities of Mode S and ADS-B. These technologies form the backbone of modern air traffic management.

As skies grow busier and routes more complex, radar continues to evolve, ensuring that every flight is tracked, every controller is informed, and every passenger reaches their destination safely. The next time you look up and see a plane, remember: there’s an incredible network of technology working behind the scenes to keep it all moving smoothly.

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