_{Arthur F. Thurnau Professor of Electrical Engineering and Computer Science, University of Michigan in Ann Arbor. Ph.D. - University of Texas at Austin.}

_{Professor of Systems Modeling at Uppsala University, Ph.D. - Polytechnic Institute of Bucharest, Romania.}

Radar (Radio Detection and Ranging) is a technology that uses radio waves to detect and locate objects. It is based on the principle of transmitting a radio frequency (RF) signal and measuring the reflection of that signal from an object. The reflected signal, also known as the echo, is then used to determine the range, velocity, and other characteristics of the object.

Radar systems typically consist of a transmitter, a receiver, and an antenna. The transmitter generates the RF signal and sends it through the antenna, which then transmits the signal into the environment. The receiver, also connected to the antenna, captures the reflected signal, known as the echo. The receiver then processes the echo to extract the information about the object.

One of the key characteristics of radar is its ability to determine the range, or distance, to an object. This is done by measuring the time delay between the transmission of the RF signal and the reception of the echo. The range to the object is then calculated using the speed of light and the time delay.

Another important characteristic of radar is its ability to determine the velocity of an object. This is done by measuring the Doppler shift of the echo signal. The Doppler shift is a change in the frequency of the signal caused by the motion of the object. By measuring the Doppler shift, the radar can determine the velocity of the object along the line of sight.

Radar systems can also be used for direction finding and beamforming. Direction finding involves determining the direction of the object by measuring the phase of the echo signal at multiple antenna elements. Beamforming involves forming a directional beam by combining the signals from multiple antenna elements. This allows the radar to focus its energy in a specific direction and increase its sensitivity in that direction.

Radar systems can be classified into different categories based on their operating frequency and applications. One of the main categories is the microwave radar, which operates in the frequency range of 1 GHz to 100 GHz. Microwave radars are widely used in applications such as weather forecasting, air traffic control, and naval navigation.

Another category is the millimeter-wave radar, which operates in the frequency range of 30 GHz to 300 GHz. Millimeter-wave radars are used in applications such as imaging, object detection, and industrial automation. Due to the high frequency and small wavelength of millimeter-wave radar, it can provide high resolution images and high penetration capability.

Another category is the terahertz radar, which operates in the frequency range of 0.3 THz to 3 THz. Terahertz radars have the potential to be used in applications such as medical imaging, security scanning, and spectroscopy. Due to the unique properties of terahertz waves, such as high penetration through non-metallic materials and strong absorption by specific molecules, terahertz radars have the potential to be used in a wide range of applications.

Finally, another category is the active radar, which transmits a signal and receives the echo, and passive radar, which receives the signal but do not transmit any signal. Passive radar systems are used in applications such as surveillance, navigation, and missile guidance, where the radar needs to operate covertly.

In conclusion, radar is a powerful technology that uses radio waves to detect and locate objects. It has a wide range of applications in fields such as transportation, defense, meteorology and many others. The development of new technologies and techniques has led to advances in radar systems, such as increased sensitivity, improved resolution, and increased operating frequency. The use of radar will continue to be important in the fields of engineering, transportation and military.