The five-step loop
1. The transmitter emits a microsecond-long pulse of radio waves through a directional antenna. 2. The pulse travels outward at the speed of light. 3. When it strikes something with a different electrical conductivity than air — an aircraft, a raindrop, a hillside — a tiny fraction reflects back. 4. The receiver, sharing the same antenna, listens for the echo. 5. The system measures the round-trip time and computes range as (time × speed of light) / 2.
A pulse 1 microsecond long produces a range resolution of about 150 metres. Shorter pulses give finer resolution but less energy, so radars trade pulse width against range using pulse compression — a chirped pulse that is long on transmit and short on receive.
Pulse Repetition Frequency (PRF)
A radar sends pulses many times per second. The PRF sets the maximum unambiguous range: if a pulse comes back after the next one has gone out, the system cannot tell which echo belongs to which pulse. Low PRF (a few hundred Hz) gives long range — useful for air-search radars. High PRF (tens of kilohertz) gives accurate Doppler velocity — useful for fighter intercept radars.
Direction comes from the antenna
Range comes from time. Bearing comes from where the antenna is pointed when the echo arrives. A rotating dish gives a full 360° sweep every few seconds. A phased array steers the beam electronically in microseconds — the entire array stays still and the beam moves through interference patterns between hundreds of small radiators.
Why this shows up in games
Signal//Lock's radar uses exactly this idea. A directional sweep ticks across the screen. When it crosses a signal, the signal becomes 'lockable' for a brief window — the same way a real PPI scope only shows a contact when the antenna is pointed at it. Holding the lock while the sweep moves away is the game's core challenge, and it maps directly onto the real problem an air-defence operator faced in 1942.
The Duplexer: The High-Speed Switch
In a pulsed radar system, the transmitter and receiver usually share a single antenna to save space and weight. However, the transmitter generates thousands of watts of power, while the receiver is sensitive enough to detect nanowatts of reflected energy. Without protection, the outgoing pulse would instantly incinerate the receiver's delicate circuitry. This problem is solved by the duplexer, a high-speed switching device that acts as a one-way valve. During transmission, it physically disconnects the receiver and directs the energy to the antenna; in the microsecond after the pulse ends, it reconnects the receiver to catch the faint return echo.
The timing requirements for the duplexer are extreme. It must switch state in nanoseconds, frequently using a gas-discharge tube that ionizes under high voltage to create a short circuit across the receiver's path. This physical limitation creates the 'minimum range' or blind zone. Because the receiver cannot listen while the transmitter is firing or while the duplexer is switching, any object closer than a few hundred meters—such as the ground directly beneath an aircraft—cannot be detected. Modern solid-state limiters have reduced this recovery time, but the fundamental physics of the transmit-receive transition remains a core design constraint.
Echo Strength and the Inverse Fourth Power Law
Detection is not just about timing; it is about energy. As a radar pulse travels outward, its power density spreads over the surface of an expanding sphere, decreasing with the square of the distance. When the pulse hits a target, only a fraction is reflected toward the radar. This reflected signal then spreads out again, losing power at another inverse square rate. Consequently, the power returning to the radar decreases according to the inverse fourth power of the range. Doubling the distance to a target doesn't require twice the power; it requires sixteen times the power to maintain the same signal strength.
To compensate for this massive energy loss, radar engineers focus on the Radar Cross Section (RCS). This is a measure of how 'visible' an object is to radio waves, determined by its shape, material, and size. A large airliner might have an RCS of 100 square meters, while a stealth fighter is designed to have an RCS smaller than a marble. By using radar-absorbent materials and angled surfaces that deflect pulses away from the source, stealth aircraft exploit the inverse fourth power law, ensuring that the return signal remains below the receiver's noise floor until the aircraft is dangerously close.
The Pulse Width and Range Resolution Trade-off
Pulse width determines the radar's ability to distinguish between two close-proximity targets, a metric known as range resolution. A pulse lasting one microsecond physically occupies about 300 meters of space as it travels. If two aircraft are flying 100 meters apart along the same line of sight, a 300-meter pulse will hit both simultaneously, and their echoes will merge into a single return on the display. To separate them, the radar must use a shorter pulse, but shorter pulses carry less total energy, which significantly reduces the maximum detection range of the system.
Modern systems solve this via pulse compression, often using a technique called 'chirping.' Instead of a simple burst of a single frequency, the transmitter sends a longer pulse that sweeps across a range of frequencies. Upon return, a matched filter in the receiver 'crushes' this long signal back into a narrow spike. This provides the high energy of a long pulse for distance alongside the sharp resolution of a short pulse for identification. It is the primary reason modern air traffic control can distinguish between individual aircraft in a dense flight corridor while maintaining long-range surveillance.
Range Ambiguity and the 'Ghost' Echo
Every pulsed radar faces a physical limit defined by the speed of light: the pulse must return before the next one is sent. If an echo from a distant target returns after the radar has already transmitted its second pulse, the timing logic incorrectly assumes the echo belongs to the most recent pulse. This creates a 'second-time-around' echo, a ghost target that appears much closer to the radar than it actually is. For example, at a PRF of 1,000 pulses per second, any target beyond 150 kilometers will be misreported as being within the 0 to 150-kilometer window.
To mitigate this, engineers employ 'staggered PRF,' where the interval between pulses is varyied in a predictable pattern. Real targets will show consistent range calculations regardless of the pulse interval, whereas ghost echoes will jump around the display because their perceived range changes with each timing shift. This technique was critical during the Cold War for Early Warning systems, preventing distant atmospheric anomalies or high-altitude bombers from being misinterpreted by automated tracking computers as immediate, close-range threats.