Seminar on Radar Technology

Contents  Introduction  History  Basic Principle  Locating Target  Distance Determination  Direction Determination  Elevation Angle  Components of Radar system  Antenna  Transmitter and Receiver  Duplexer  Synchronizer  Display unit  Power supply unit  Configurations  Bistatic Radar  Continuous-wave radar  Doppler Radar  Pulse Doppler Radar

Introduction  RADAR Radar is an acronym for “Radio Detection and Ranging." A radar system usually operates in the ultra- high-frequency (UHF) or microwave part of the radio-frequency (RF) spectrum, and is used to detect the position and movement of objects. Radar can track storm systems, because precipitation reflects electromagnetic fields at certain frequencies. Radar can also render precise maps. Radar systems are widely used in air-traffic control, aircraft navigation, and marine navigation.

History  The name radar comes from the acronym RADAR, coined in 1940 by the U.S. Navy for public reference to their highly classified work in Radio Detection And Ranging. Thus, a true radar system must both detect and provide range (distance) information for a target. Before 1934, no single system gave this performance; some systems were Omni-directional and provided ranging information, while others provided rough directional information but not range. A key development was the use of pulses that were timed to provide ranging, which were sent from large antennas that provided accurate directional information. Combining the two allowed for accurate plotting of targets.  The history of radar starts with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects.. However, it was not until the early 20th century that systems able to use these principles were becoming widely available, and it was German engineer Christian Huelsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog Numerous similar systems were developed over the next two decades.

Basic principle  The radar creates an electromagnetic energy pulse which is focused by an antenna and transmitted through the atmosphere. Objects in the path of this electromagnetic pulse, called targets, scatter the electromagnetic energy. Some of that energy is scattered back toward the radar.  The receiving antenna (which is normally also the transmitting antenna) gathers this back-scattered radiation and feeds it to a device called a receiver

Locating target The radar needs 3 pieces of information to determine the location of a target. Distance The first piece of information needed is the distance (D) from radar to target. Direction The second piece of information is the angle of the radar beam with respect to north; called the "azimuth angle". Elevation The second is the angle of the beam with respect to the ground; called the "elevation angle".

Distance Determination  The distance is determined from the running time of the high-frequency transmitted signal and the propagation c 0. 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 dividing by two in order to obtain the time the wave took to reach the target. Therefore the following formula arises for the slant range: R = (c 0 · t )/2 where: c 0 = speed of light = 3·10 8 m / s t = measured running time [s] R = slant range antenna The distances are expressed in kilometers or nautical miles (1 NM = km).

Direction Determination 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 from 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

Elevation Angle The elevation angle is the angle between the horizontal plane and the line of sight, measured in the vertical plane.. The elevation angle is positive above the horizon (0° elevation angle), but negative below the horizon.

Components of a radar system  A practical radar system requires seven basic components as illustrated below: Antenna Transmitter Receiver Duplexer Synchronizer Display Power supply

Antenna The antenna takes the radar pulse from the transmitter and puts it into the air. Furthermore, the antenna must focus the energy into a well- defined beam which increases the power and permits a determination of the direction of the target. The antenna must keep track of its own orientation which can be accomplished by a synchro- transmitter. There are also antenna systems which do not physically move but are steered electronically.

Transmitter and Receiver Transmitter and Receiver The transmitter creates the radio wave to be sent and modulates it to form the pulse train. The transmitter must also amplify the signal to a high power level to provide adequate range. The source of the carrier wave could be a Klystron, Traveling Wave Tube (TWT) or Magnetron. Each has its own characteristics and limitations. The receiver is sensitive to the range of frequencies being transmitted and provides amplification of the returned signal. In order to provide the greatest range, the receiver must be very sensitive without introducing excessive noise. The ability to discern a received signal from background noise depends on the signal-to-noise ratio (S/N).

Duplexer This is a switch which alternately connects the transmitter or receiver to the antenna. Its purpose is to protect the receiver from the high power output of the transmitter. During the transmission of an outgoing pulse, the duplexer will be aligned to the transmitter for the duration of the pulse, PW. After the pulse has been sent, the duplexer will align the antenna to the receiver. When the next pulse is sent, the duplexer will shift back to the transmitter. A duplexer is not required if the transmitted power is low.

Synchronizer The synchronizer coordinates the timing for range determination. It regulates that rate at which pulses are sent and resets the timing clock for range determination for each pulse. Signals from the synchronizer are sent simultaneously to the transmitter, which sends a new pulse, and to the display, which resets the return sweep.

Display The display unit may take a variety of forms but in general is designed to present the received information to an operator. The most basic display type is called an A-scan (amplitude vs. Time delay). The vertical axis is the strength of the return and the horizontal axis is the time delay, or range. The A-scan provides no information about the direction of the target. The most common display is the PPI (plan position indicator). The A-scan information is converted into brightness and then displayed in the same relative direction as the antenna orientation. The result is a top-down view of the situation where range is the distance from the origin. The PPI is perhaps the most natural display for the operator and therefore the most widely used. In both cases, the synchronizer resets the trace for each pulse so that the range

Power supply unit The power supply provides the electrical power for all the components. The largest consumer of power is the transmitter which may require several kW of average power. The actually power transmitted in the pulse may be much greater than 1 kW. The power supply only needs to be able to provide the average amount of power consumed, not the high power level during the actual pulse transmission. Energy can be stored, in a capacitor bank for instance, during the rest time. The stored energy then can be put into the pulse when transmitted, increasing the peak power. The peak power and the average power are related by the quantity called duty cycle, DC. Duty cycle is the fraction of each transmission cycle that the radar is actually transmitting. Referring to the pulse train in Figure 2, the duty cycle can be seen to be: DC = PW / PRF

Configurations  Radar come in a variety of configuration in the emitter, the receiver, the antenna, wavelength, scan strategies, etc. Bistatic radar Continuous-wave radar Doppler radar Pulse-Doppler

Bistatic radar Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a Monostatic radar. Many long-range air-to- air and surface-to-air missile systems use semi-active radar homing which is a form of Bistatic radar

Continuous-wave radar Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Continuous wave (CW) radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving clutter.

Doppler radar A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been altered by the object's motion. This variation gives direct and highly accurate measurements of the radial component of a target's velocity relative to the radar. Doppler radars are used in aviation, sounding satellites, meteorology, police speed guns, radiology, and Bistatic radar (surface to air missile).

Pulse-Doppler Radar Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity (range-rate). It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption. RF energy returning from airborne objects and spacecraft are combined for successive target reflections returning from a dozen or more transmit pulses, and these are integrated using Pulse-Doppler signal processing. Pulse-Doppler reduces microwave power emission and weigh sufficiently for safe and effective use on aircraft

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