How Radar Works

Radar is one of the most important technologies in aviation. Without it, air traffic controllers would have no way of knowing exactly where aircraft are, how fast they are moving, or whether they are on a collision course. This lesson explains the basic principles of radar and why it matters so much for aviation safety.

What Does "Radar" Mean?

The word RADAR is an acronym that stands for Radio Detection And Ranging. It was coined during the Second World War, when the technology was first developed for military use. The name tells you exactly what it does:

Radio: It uses radio waves (a form of electromagnetic radiation)
Detection: It finds objects
Ranging: It measures how far away those objects are

The Basic Principle

Radar works on a remarkably simple principle: it sends out a burst of radio energy and listens for the echo.

Here is the process, step by step:

1. The transmitter sends a pulse: A radar antenna transmits a short, powerful burst of radio waves. These waves travel outward from the antenna at the speed of light (approximately 300,000 kilometres per second).

2. The pulse hits an object: When the radio waves encounter a solid object — such as an aircraft — some of the energy is reflected back toward the radar antenna. This reflected energy is called the echo or return.

3. The receiver detects the echo: The radar antenna switches to receive mode and picks up the reflected signal.

4. Distance is calculated: Because the speed of radio waves is known (the speed of light), and the time between sending the pulse and receiving the echo can be measured very precisely, the radar system can calculate the distance to the object. The formula is simple: distance equals speed multiplied by time, divided by two (because the signal travels to the object and back).

5. Direction is determined: Because the radar antenna points in a specific direction when it sends each pulse, the system knows which direction the echo came from. By rotating the antenna through 360 degrees and sending pulses in every direction, the radar builds up a complete picture of everything around it.

This cycle of transmit-receive-calculate repeats many times per second, creating a continuously updated picture of the airspace.

Primary vs Secondary Radar

In aviation, there are two fundamentally different types of radar. Understanding the difference is important because they serve different purposes and provide different information.

#### Primary Surveillance Radar (PSR)

Primary radar works on the basic principle described above — it sends out a pulse and listens for the echo. This is a passive detection method from the aircraft's perspective; the aircraft does not need any special equipment to be detected. As long as the aircraft is big enough and close enough to reflect radio waves, primary radar will find it.

Primary radar can tell you:

Where an aircraft is (distance and direction from the radar)
That something is there (it has detected a target)

Primary radar cannot tell you:

What the aircraft is (it cannot distinguish a Boeing 737 from a flock of birds)
How high it is (basic primary radar measures range and azimuth, not altitude)
Its identity (no callsign, no registration)

#### Secondary Surveillance Radar (SSR)

Secondary radar takes a completely different approach. Instead of relying on reflected energy, it interrogates a device on the aircraft called a transponder. The radar sends out a coded radio signal (the interrogation), and the aircraft's transponder detects this signal and automatically sends back a coded reply.

The reply contains specific information that the transponder has been set to transmit:

A four-digit identification code (called a squawk code)
The aircraft's altitude (if equipped)
In modern systems, much more data including the aircraft's unique address, callsign, and other information

Secondary radar is enormously powerful because it provides identified, information-rich tracks — not just anonymous blips on a screen.

Why Both Types Are Needed

You might wonder: if secondary radar provides so much more information, why bother with primary radar at all?

The answer is redundancy and coverage:

Not all aircraft carry transponders: Some small, older, or military aircraft may not have functioning transponders. Primary radar can still detect them.
Transponders can fail: If a transponder stops working, the aircraft becomes invisible to SSR — but primary radar can still see it.
Security: In an emergency, if a pilot cannot operate the transponder, primary radar provides a basic detection capability.
Military applications: Military aircraft may deliberately turn off their transponders in certain situations.

Most ATC radar installations combine both primary and secondary radar on the same antenna, providing controllers with the best of both worlds: the guaranteed detection of primary radar and the rich information of secondary radar.

Radar in Air Traffic Control

Radar is the foundation of modern air traffic control. Without it, controllers would have to rely on pilots reporting their own positions — a much less precise and slower method.

With radar, controllers can:

See every aircraft in their airspace displayed on a screen, updated every few seconds
Measure separation between aircraft precisely and continuously
Detect potential conflicts early and intervene before aircraft get too close
Guide aircraft through complex airspace, including approaches to airports
Respond to emergencies by identifying the aircraft in trouble and directing other traffic away from it

Radar at Prestwick

Glasgow Prestwick Airport and the surrounding area are covered by radar systems that feed data into both the local approach control and the Prestwick Centre operations room. Controllers managing the Scottish domestic airspace rely on a network of radar stations across Scotland and northern England to maintain their traffic picture.

However, it is worth noting that radar coverage has limits. The further from shore an aircraft flies, the weaker the radar signal becomes, until eventually — somewhere over the ocean — the aircraft flies beyond radar range entirely. This is why the oceanic control function at Prestwick Centre uses different methods, as covered in other lessons.

A Technology Born from War

Radar was developed in secrecy during the 1930s and played a decisive role in the Second World War. The UK's Chain Home radar network gave early warning of incoming German aircraft, contributing significantly to the outcome of the Battle of Britain.

After the war, radar technology was rapidly adapted for civil aviation. The growth of commercial air travel in the 1950s and 1960s drove huge investment in ATC radar systems, and the technology has been refined continuously ever since. Modern radar systems bear little resemblance to their wartime ancestors in terms of capability, but the core principle — transmit, reflect, receive — remains exactly the same.

Key Takeaways

Radar detects objects by sending radio pulses and listening for echoes
The time delay between pulse and echo reveals the distance to the target
Primary radar detects objects by reflection — simple but limited in information
Secondary radar interrogates aircraft transponders — rich in data
Together, they give controllers a comprehensive picture of the airspace
Radar is the backbone of air traffic control, but it has range limits — which is why oceanic airspace needs different solutions