Radar Principles

Introduction This paper will discuss many of the theoretical and physical principles that underlie the operation of radar systems. The word radar is derived from an acronym for radio detection and ranging. Radar is an electronic and electromagnetic system that uses radio waves to detect and locate objects. Radar operates by transmitting a particular kind of radio frequency waveform and detecting the nature of the reflected echo. When radio waves strike an object, some portion is reflected, and some of this reflected energy is returned to the radar set, where it is detected. The location and other information about these reflective objects, targets, can be determined by the reflected energy. Monostatic radars have transmitters and receivers that are co-located, whereas bistatic radars have transmitters and receivers that are physically separated. This paper will focus on monostatic radar, although many principles developed for monostatic radar can readily be transferred to bistatic radar. Radio waves have many properties including speed, frequency, and power; they display many phenomena such as refraction, attenuation, and reflection. The known speed and reradiation properties of radio waves are fundamental to the theory of radar operation, since radar systems are, at their core, transmitters and receivers of radio waves. Electromagnetic waves are composed of both electric and magnetic fields, both of which oscillate perpendicular to each other and the direction of propagation. These transverse waves can be identified by certain characteristics, such as wavelength and frequency. Wavelength is the distance between successive peaks, whereas frequency is the number of peaks passing a point in a period of time. Radio waves occupy a portion of the electromagnetic spectrum from frequencies of a few kilohertz to a few gigahertz, less than one-billionth of the total electromagnetic spectrum. In a vacuum, radio waves travel in straight lines. The speed of radio wave propagation in a vacuum (310 8 m/s) is a universal constant, c. The speed of wave propagation differs from c if the medium of propagation is matter. When electromagnetic waves travel in nonconducting materials, such as air, the wave speed v, is slower than c and is given by SYRACUSE RESEARCH CORPORATION Page 2 of 2 v = 1 εµ (1) where ε and µ are the permittivity and permeability of the material, respectively. This velocity difference is not significant in air, but can have large effects for radars such as ground penetrating radar. A measure derived from this …