Radar

Radar is a cool system that uses radio waves to find out how far away objects are and which way they are going. It helps us see things like aircraft, ships, spacecraft, and weather. The word “radar” comes from “radio detection and ranging,” a name made by the United States Navy in 1940.

A radar system sends out radio waves and waits for them to bounce back from objects. This tells us where those objects are and how fast they are moving. Radar was first made for military use before and during World War II, especially after the invention of the cavity magnetron in the United Kingdom.

Today, radar is used in many ways, such as controlling air traffic, helping ships move, guiding missiles, and even in self-driving cars. It can also watch weather and study the surface of the Earth and other planets. With new technology like digital signal processing and machine learning, radar systems have become even better and more exact.

History

Main article: History of radar

Experimental radar antenna, US Naval Research Laboratory, Anacostia, D. C., from the late 1930s (photo taken in 1945)

Radar started with simple ideas about radio waves. In 1886, a scientist named Heinrich Hertz showed that radio waves could bounce off solid things. In 1895, Alexander Popov made a tool to find lightning using radio waves. He noticed that ships passing by made interference, a first sign that objects could be found this way.

In the early 1900s, inventors like Christian Hülsmeyer used radio waves to see ships in fog. In the 1930s, scientists in places like the United Kingdom, United States, France, and Germany worked to make these ideas better. By 1935, Robert Watson-Watt and his team in Britain showed a system that could find airplanes. This helped make the Chain Home radar network. The network was important during the Battle of Britain in 1940, giving warnings about planes that were coming.

Applications

Commercial marine radar antenna. The rotating antenna radiates a vertical fan-shaped beam.

Radar helps us know where things are by using radio waves. It was first used by the military to find airplanes and ships. Today, radar is used in many places.

In aviation, airplanes use radar to stay safe, see weather, and land even in fog. Ships use radar to stay safe and avoid crashing. Weather experts use radar to watch storms and predict the weather. Police use radar to check how fast cars are going, and cars can use radar to help prevent accidents.

Principles

3D Doppler radar spectrum showing a Barker code of 13

Radar is a system that uses radio waves to find out how far away, in which direction, and how fast objects are moving compared to the radar. It works by sending out radio waves and then listening for the waves that bounce back from objects. This helps detect things like airplanes, ships, and weather.

Radar sends out signals from a transmitter as radio waves. When these waves hit an object, they often reflect or scatter in different directions. Some of these reflected waves come back to the radar receiver, which can then determine details about the object. This works best with objects that have materials that reflect radio waves well, like metals or wet surfaces. The returned signals are usually very weak and need to be strengthened using electronic amplifiers for the radar to process them effectively.

Signal processing

Pulse radar: The round-trip time for the radar pulse to get to the target and return is measured. The distance is proportional to this time.

Radar systems find out how far away and how fast objects are moving. One way is to send out a short burst of radio waves and time how long it takes for them to bounce back. This tells us the distance because radio waves travel at the speed of light.

Another way is to change the frequency of the radio waves over time. By comparing the sent and returned frequencies, we can figure out the distance very accurately. Modern radars often mix these methods to get the best results for both short and long distances. They can also measure speed by looking at how the distance changes over time or by using the Doppler effect, which helps detect movement.

Engineering

Radar systems have several important parts that work together. There is a transmitter that creates radio signals using devices like a magnetron, and a waveguide that carries these signals to an antenna. A duplexer helps switch between sending and receiving signals. The receiver picks up the returned signals, and a display processor shows this information on output devices. An electronic section controls all these parts.

Antennas are very important in radar. Early radars used simple antennas that sent and received signals in all directions. Modern radars often use a steerable parabolic dish to focus the signal into a narrow beam. Some radars use a phased array, which is a group of small antennas that can steer the beam electronically without moving parts. This makes the radar more reliable.

**Radar frequency bands**
Band name	Frequency range	Wavelength range
HF	3–30 MHz	10–100 m
VHF	30–300 MHz	1–10 m
P		> 1 m
UHF	300–1000 MHz	0.3–1 m
L	1–2 GHz	15–30 cm
S	2–4 GHz	7.5–15 cm
C	4–8 GHz	3.75–7.5 cm
X	8–12 GHz	2.5–3.75 cm
K_u	12–18 GHz	1.67–2.5 cm
K	18–24 GHz	1.11–1.67 cm
K_a	24–40 GHz	0.75–1.11 cm
mm	40–300 GHz	1.0–7.5 mm
V	40–75 GHz	4.0–7.5 mm
W	75–110 GHz	2.7–4.0 mm

Regulations

Radar is controlled by international rules made by the International Telecommunication Union's ITU Radio Regulations. It is a system that finds objects by sending out radio signals and measuring how they bounce back. There are two main types of radar: primary radar and secondary radar. These can be used in services like radiolocation service or radiolocation-satellite service.

Configurations

Radar systems can be set up in many ways. They send out and pick up signals to help with jobs like tracking weather or aircraft. Some common types include bistatic radar, where the device sending out signals is in a different place from the one receiving them. Continuous-wave radar sends out a steady signal. Doppler radar can measure how fast something is moving. There are also special radars like synthetic-aperture radar, which creates detailed images from far away.