Richards , James A. Scheer , William A. Holm Principles of Modern Radar: Basic Principles is a comprehensive and modern textbook for courses in radar systems and technology at the college senior and graduate student level; a professional training textbook for formal in-house courses for new hires; a reference for ongoing study following a radar short course; and a self-study and professional reference book. While several established books address introductory radar systems, Principles of Modern Radar differs from these in its breadth of coverage, its emphasis on current methods without losing sight of bedrock principles , and its adoption of an appropriate level of quantitative rigor for the intended audience of students and new professional hires. The manuscript for this book was reviewed by over 50 volunteer professionals see Acknowledgments in academia, military, and commercial enterprises. Their extensive comments, corrections, and insights ensure that Principles of Modern Radar will meet the needs of modern radar educators and students around the world.
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In principle, it can pierce fog, darkness, or any atmospheric disturbance all the way to the horizon. Depending on its particular characteristics, a radar unit may show its users clouds, landmasses, or objects such as ships, airplanes, or spacecraft. Radar can measure distance or range to a target object, and aircraft can use radar to determine altitude.
Speed detection is another common application. Radar can be used to land and guide aircraft, probe through ice or soil for geological information, and map the three-dimensional characteristics of oceans and landmasses precisely from orbit.
Military applications include weapons detection e. Unlike water waves, electromagnetic waves do not require a medium to travel through. They can propagate through air, vacuum, and certain materials. Light waves, radio waves, microwaves, and radar waves are all examples of electromagnetic waves.
Just as light reflects off of some surfaces and travels through others, radar waves bounce off some objects and travel through others. Basic radar operation The simplest mode of radar operation is range-finding, performed by time-of-flight calculation. The unit transmits a radar signal, i.
The waves hit the target and are reflected back in the same way that water waves are reflected from the end of a bathtub.
The returning wave is received by the radar unit, and the travel time is registered. Basic physics tells us that distance is equal to rate of travel multiplied by the time of travel. All electromagnetic waves travel at the same speed in a vacuum—the speed of light, which is 3. This speed is reduced by some small amount when the waves are traveling in a medium such as air, but this can be calculated.
A basic radar unit consists of: a frequency generator and timing control unit; a transmitter with a modulator to generate a signal; an antenna with a parabolic reflector to transmit the signal; a duplexer to switch between transmission and reception mode; an antenna to gather the reflected signal; a receiver to detect and amplify this return; and signal processing, data processing , and data display units.
If the transmitter and receiver are connected to the same antenna or to antennas in the same location, the unit is called monostatic. If the transmitter and receiver antennas are in very different locations, the unit is known as bistatic.
In a monostatic system, the unit must switch between sending out a signal and listening for the return reflected from the target; the timing unit controls the duplexer that performs the switching. The transmitter generates a radio signal that is modulated, or varied, to form either a series of pulses or a continuously varying signal. This signal is reflected from the target, gathered by the antenna, and amplified and filtered by the receiver. Finally, the data is presented to the user on the display.
Before target range can be determined, the target must be detected, an operation more complicated than it would seem. Consider radar operation again. A pulse is transmitted in the direction that the antenna is facing. When it encounters a material that is different from the surrounding medium e.
This antenna in turn collects only part of the reflected pulse and sends it to the receiver and the processing units where the most critical operations take place. Because only a small amount of the transmitted pulse is ever detected by the receiving antenna, the signal amplitude is dramatically reduced from its initial value. At the same time, spurious reflections from non-target surfaces or electronic noise from the radar system itself act to clutter up the signal, making it difficult to isolate.
Various filtering and amplification operations help to increase the signal-to-noise ratio SNR , making it easier to lock on to the actual signal. If the noise is too high, the processing parameters incorrect, or the reflected signal amplitude too small, it is difficult for the system to determine whether a target exists or not.
No matter what the source, interference and signal quality are serious concerns for radar system designers and operators. Radar tracking systems Radar systems can send out thousands of pulses per second.
Using a rapid sequence of pulses, a radar system can not only determine the range of a target, but it can also track target motion. Ranging can be performed with an omnidirectional antenna, but target location and tracking require a more sophisticated system with knowledge of the antenna elevation vertical angle and azimuthal horizontal angle with respect to some fixed coordinate system.
Land-based systems generally define true north as the azimuthal reference and the local horizontal as the elevation reference. The azimuthal reference for air and sea systems is the bow of the ship, but elevation reference varies depending on the pitch and roll stabilization of the ship or plane.
When you are driving a car down the street, you might characterize other cars as to your left, to your right, or behind you; you define the location of the cars in terms of your own coordinate system. Similarly, when a radar system receives the reflection from a target, it checks the orientation of the receiving antenna with respect to the coordinate axes to determine the object location.
Moreover, just as you can use a road-map to determine the absolute location of an object, so a radar system can be used to locate a target in terms of longitude and latitude. Multiple pulses are required to track the motion of a target. Air Traffic Control uses radar to track and direct the courses of the many planes in civilian airspace. An Air Traffic Control interrogator system sends a signal to the transponder that prompts it to reply with identification and altitude information. In this way, air traffic controllers can monitor the courses of planes in their region.
A military version of the beacon, known as identification, Friend or Foe IFF uses coded signals to identify aircraft. Doppler radar A specialized type of radar uses the Doppler effect to detect the speed of a target. You have probably observed the Doppler effect hundreds of times without realizing it.
The change in pitch as a vehicle approaches, then drives past you is an example of the Doppler frequency shift. The sound waves shift to a higher frequency as the vehicle comes toward you, raising the pitch, then as the vehicle pulls away the frequency of the sound is lowered, dropping the pitch.
Doppler frequency shift is the difference between the frequency of the pulse transmitted to the target and the frequency of the return pulse. If this can be measured, then the radial speed, or speed along the line-of-sight can be determined. Note, however, that target velocity at right angles to the radar system line-of-sight does not cause Doppler shift. In such a case, the speed detector would register a target speed of zero. Similarly, if a target is moving at some angle to the direct line-of-sight, the system would only detect the radial component of its velocity.
A cosine term can be added to the basic equation to account for non-radial motion. More sophisticated radar systems include this compensation, but typical law enforcement speed detectors do not, with the result that the measured velocity of the target is somewhat lower than the actual velocity. A Doppler radar system consists of a continuously transmitting source, a mixer, and data and signal processing elements.
The signal is sent out to the target continuously. Duplexer— In a monostatic system, the device that switches system operation between transmit and receive mode. Modulation— Variation, as in modulation of an electrical signal.
Monostatic— A radar system in which a single antenna both transmits and receives; a system in which transmitting and receiving antennas are at the same location.
Transponder— A beacon. In the case of an Air Traffic Control radar beacon system, a device that is capable of transmitting certain information when queried. The Doppler shift is averaged over several samples and processed to yield target speed. Effective operating range of a radar system is limited by antenna efficiency, transmitted power, the sensitivity of the detector, and the size of the target or energy it reflects.
Reflection of electromagnetic waves from surfaces is fundamental to radar. All objects do not reflect radar waves equally well—the strength of the wave reflection depends on the size, shape, and composition of the object.
Metal objects are the best reflectors, while wood and plastic produce weaker reflections. So-called stealth airplanes are based on this concept and are built from materials that produce a minimal reflection. In recent years laser radar systems have been developed.
Laser radar systems operate on essentially the same principle as conventional radar, but the significantly shorter wavelengths of visible light allow much higher resolution. Laser radar systems can be used for imaging and for measurement of reflectivity. They are used for vibration detection in automotive manufacturing and for mapping power lines. Because they are more difficult to detect than conventional radar systems, laser radar speed guns are increasingly being adopted by law enforcement agencies.
Radar has undergone considerable development since its introduction in the s. As of , exploratory scientific radars were in orbit around Saturn and Mars. Richards, Mark. Radar Signal Processing. New York : McGraw-Hill,
Radar: Principles, Technology, Applications
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Radar: Principles, Technology, Applications
Radar : Principles Technology Applications
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