Radar Basics by radartutorial.eu

Radar Basics

• Radar measurement of range, or distance, is made possible because of the properties of radiated electromagnetic energy.
• The electromagnetic waves are reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction.
• Electromagnetic energy travels through air at a constant speed, at approximately the speed of light. This constant speed allows the determination of the distance between the reflecting objects and the radar site by measuring the running time of the transmitted pulses.
• This energy normally travels through space in a straight line, and will vary only slightly because of atmospheric and weather conditions.
• The electronic principle on which radar operates is very similar to the principle of sound-wave reflection. If you shout in the direction of a sound-reflecting object, you will hear an echo. If you know the speed of sound in air, you can then estimate the distance and general direction of the object. The time required for an echo to return can be roughly converted to distance if the speed of sound is known.
• Radar uses electromagnetic energy pulses in much the same way [as a sound echo]. The radio-frequency energy is transmitted to and reflected from the reflecting object. A small portion of the reflected energy returns to the radar set. This returned energy is called an ECHO, just as it is in sound terminology. Radar sets use the echo to determine the direction and distance of the reflecting object.
• The term RADAR is an acronym made up of the words: Radio Aim Detecting And Ranging.
• Modern radar can extract widely more information from a target's echo signal than its range. But the calculating of the range by measuring the delay time is one of its most important functions.
• The radar antenna illuminates the target with a microwave signal, which is then reflected and picked up by a receiving device. The electrical signal picked up by the receiving antenna is called echo or return. The radar signal is generated by a powerful transmitter and received by a highly sensitive receiver.
• All targets produce a diffuse reflection, i.e. it is reflected in a wide number of directions. The reflected signal is also called scattering. Back-scatter is the term given to reflections in the opposite direction to the incident rays.
• The radar transmitter produces the short duration high-power RF pulses of energy that are shot into space by the antenna.
• The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.
• The receivers amplify and demodulate the received RF-signals. The receiver provides video signals on the output.
• The antenna transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.
• The indicator should present to the observer a continuous, easily understandable, graphic picture of the relative position of radar targets.
• The radar transmits a short radio pulse with very high pulse power. This pulse is focused in one direction only by the directivity of the antenna, and propagates in this given direction with the speed of light.
• The actual range of a target from the radar is known as slant range. Slant range is the line of sight distance between the radar and the object illuminated. While ground range is the horizontal distance between the emitter and its target and its calculation requires knowledge of the target's elevation.
• Since the waves travel to a target and back, the round trip time is divided by two in order to obtain the time the wave took to reach the target.
• Range is the distance from the radar site to the target measured along the line of sight.
• The angular determination of the target is determined by the directivity of the antenna.
• Directivity, sometimes known as the directive gain, is the ability of the antenna to concentrate the transmitted energy in a particular direction. An antenna with high directivity is also called a directive antenna. By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles form the radar to the object or target can be determined.
• The accuracy of angular measurement is determined by the directivity, which is a function of the size of the antenna.
• Radar units usually work with very high frequencies. Reasons for this are:
• quasi-optically propagation of these waves
• High resolution (the smaller the wavelength, the smaller the objects the radar is able to detect)
• High the frequency, smaller the antenna size at the same gain
• The antenna s of most radar systems are designed to radiate energy in a one-directional lobe or beam that can be moved in bearing simply by moving the antenna.
• In actual practice, search radar antennas move continuously; the point of maximum echo, determined by the detection circuitry or visually by the operator, is when the beam points direct at the target.
• It becomes obvious that we cannot send out another pulse until a time windows has passed, in which we expect to see a return echo.
• The maximum range at which a target can be located so as to guarantee that the leading edge of the received back-scatter from the target is received before transmission begins for the next pulse. This range is called maximum unambiguous range or the first range ambiguity.
• The pulse-repetition frequency (PRF) determines this maximum unambiguous range of a given radar before ambiguities start to occur.
• The pulse repetition time (PRT) of the radar is important when determining the maximum range because target return-times that exceed the PRT of the radar system appear at incorrect locations (ranges) on the radar screen. Returns that appear at these incorrect ranges are referred to as ambiguous returns, second-sweep echoes, or second-time around echoes.
• Mono-static pulse radar sets use the same antenna for transmitting and receiving. During the transmitting time the radar cannot receive: the radar is switched off using an electronic switch, called a duplexer.
• The minimal measuring range is the minimum distance which the target must have to be detected. Therein, it is necessary that the transmitting pulse leaves the antenna completely and the radar unit must switch on the receiver.
• The elevation angle is the angle between the horizontal plane and the line of sight, measured in the vertical plane.
• Altitude or height-finding search radars use a very narrow beam in the vertical plane. The beam is mechanically or electronically scanned in elevation to pinpoint targets. If an echo signal is detected in the receiver, then the current elevation angle is equal to the direction of the antenna pattern.
• The height of a target over the earth's surface is called height or altitude.
• If high-frequency energy is emitted by an isotropic radiator, than the energy propagates uniformly in all directions. Areas with the same power density form spheres around the radiator. The same amount of energy spread out on an incremented spherical surface at an incremented spherical radius. That means: the power density on the surface of a sphere is inversely proportional to the square of the radius of the sphere.
• [Antenna] gain is obtained by directional radiation of the power.
• The target detection isn't only dependent on the power density at the target position, but also on how much power is reflected in the direction of the radar. In order to determine the useful reflected power, it is necessary to know the radar cross section. This quantity depends on several factors, but it is true to say that a bigger area reflects more power than a smaller area.
• In order to double the range, the transmitted power would have to be increased by 16-fold!
• For every receiver there is a certain receiving power as of which the receiver can work at all. This smallest workable receive power is frequently often called MDS--Minimum Discernible Signal-- in radar technology.
• If one quadruples the antenna gain, it will double the maximum range.
• Frequency diversity typically uses two transmitters operating in tandem to illuminate the target with two separate frequencies.
• An important advantage of the multiple frequency procedure is the high jamming immunity of the procedure.
• The linear addition of the signals of different frequency components increases the probability of detection of the target. However, this brings disadvantages with regard to the jamming immunity like a radar with a single TX-frequency only.
• Accuracy is the degree of conformance between the estimated or measured position and/or the velocity of a platform at a given time and its true position or velocity.
• Accuracy should not be confused with resolution.
• The target resolution of a radar is its ability to distinguish between targets that are very close in either range or bearing.
• Weapons-control radar, which requires great precision, should be able to distinguish between targets that are only yards apart.
• Search-radar is usually less precise and only distinguishes between targets that are hundreds of yards or even miles apart.
• Resolution is usually divided into two categories: range resolution and bearing resolution.
• Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing but at different ranges. The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator. Pulse width is the primary factor in range resolution.
• If the spacing between two aircraft is too small, then the radar "see" only one target.
• Angular resolution is the minimum angular separation at which two equal targets can be separated when at the same range.
• The smaller the beam width, the higher the directivity of the radar antenna.
• The range and angular resolutions lead to the resolution cell. The meaning of the cell is very clear: unless one can rely on eventual different Doppler shifts it is impossible to distinguish two targets which are located inside the same resolution cell.
• The broader the spectrum of the transmitted pulse and the narrower the aperture angle are, the smaller the resolution cell is and the higher the interference immunity of the radar station is.
• A radar is not designed to detect aircraft directly above the radar antenna. This gap is known as the cone of silence.
• Despite a large number of radars organized in a radar network radar, a space will always remain in extremely low altitude at which an aircraft can fly below the radar. In practice, however, this is for the pilot not as easy as it seems as he must know exactly where to fly to remain as far away from each radar.
• Most processes in pulsed radar are time-dependent.
• The time that an antenna beam spend on a target is called dwell time.
• The value of hits per scan says how many echo signals per single target during every antenna rotation are received.The hit number stands for the number of the received echo pulses of a single target per antenna turn.
• The higher the frequency of a radar system, the more it is affected by weather conditions such as rain or clouds. But the higher the transmitted frequency, the better is the accuracy of the radar system.
• The higher the frequency, the higher is the atmospheric absorption and attenuation of the waves.
• An imaging Radar forms a picture of the observed object or area.
• A pulse radar is a radar device that emits short and powerful pulses and in the silent period receives the echo signals. In contrast to the continuous wave radar the transmitter is turned off before the measurement is finished.
• Primary Surveillance Radar (PSR) works with passive echoes. The transmitted high-frequency impulses are reflected by the target and then received by the same radar unit.
• Secondary Surveillance Radar (SSR) work with active answer signals. The secondary radar units transmits and also receives high-frequency impulses. The signals aren't simply reflected, but received by the target by means of a transponder which receives and processes the pulse. After this the target answers with another frequency.
• A Continuous Wave (CW) radar transmits a high-frequency signal continuously. The echo signal is received and processed permanently. On has to resolve two problems with this principle:
• prevent a direct connection of the transmitted energy into the receiver
• assign the received echoes to a time system to be able to run time measurements
• LPI radar (Low Probability of Intercept) is a class of radar systems that possess certain performance characteristics that make them nearly undetectable by today's intercept receivers. LPI features prevent the radar from tripping off alarm systems or passive radar-detection equipment in a target.
• Synthetic Aperture Radar (SAR) is a coherent mostly airborne or space-borne side-looking radar system which utilizes the flight path of the platform to simulate an extremely large antennas or aperture electronically, and that generates high-resolution remote sensing imagery.