Radar Technology
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INTRODUCTION
1.1 OVERVIEW
The following figure shows the operating principle of a primary radar set. 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.
WHAT IS RADAR?
The word “Radar “is an acronym derived from the words Radio Detection And Ranging. It refers to the technique of using radio waves to detect the presence of objects in the atmosphere. Radar was designed shortly before World War II. Its primary purpose was to detect the presence of aircraft. Today, radar is used for a wide array of applications, but primarily to detect precipitation and other meteorological events.
Radar is an object-detection system that uses electromagnetic waves - specifically radio waves - to identify the range, altitude, direction, or speed of both moving and fixed objects.
If you want to walk at night, you can shine a torch in front to see where you're going. The light beam travels out from the torch, reflects off objects in front of you, and bounces into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don't trip over things. Radar works in much the same way.
The basic idea behind radar is very simple: a signal is transmitted, it bounces off an object and it is later received by some type of receiver. This is like the type of thing that happens when sound echo's off a wall. (Check out the image on the left) However radars don't use sound as a signal. Instead they use certain kinds of electromagnetic waves called radio waves and microwaves. This is where the name RADAR comes from (RAdio Detection And Ranging). Sound is used as a signal to detect objects in devices called SONAR (SOund NAvigation Ranging). Another type of signal used that is relatively new is laser light that is used in devices called LIDAR (you guessed it...LIght Detection And Ranging).
HISTORY OF RADAR
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. The name radar comes from the acronym RADAR, coined in 1940 by the U.S. Navy .Several inventors, scientists, and engineers contributed to the development of radar .The first to use radio waves to detect "the presence of distant metallic objects" was Christian Hülsmeyer. Before the Second World War developments by the British, the Germans, the French, the Soviets and the Americans led to the modern version of radar.
World War II saw more rapid developments in radar technology. Both the British and the Germans were engaged in a race to produce larger and more sophisticated radars. However, the Germans were not able to fully harness it. It was the British that were able to utilize it more effectively.
TARGET DETECTION
• Radars create an electromagnetic (EM) pulse that is focused by an antenna, and then transmitted through the atmosphere (Figure A).
• Objects in the path of the transmitted EM pulse, called "targets" or "echoes," scatter most of the energy, but some will be reflected back toward the radar (Figure B).
• The receiving antenna (normally also the transmitting antenna) gathers back-scattered radiation and feeds it to a "receiver."
• An EM pulse encountering a target is scattered in all directions. The larger the target, the stronger the scattered signal (Figure C).
• Also, the more targets, the stronger the return signal, that is, the targets combines to produce a stronger signal (Figure D).
• The radar measures the returned signal, generally called the "reflectivity."
• Reflectivity magnitude is related to the number and size of the targets encountered.
TARGET VELOCITY
• Doppler radars, like NEXRAD*(next radar generation), can also measure "radial.
• velocity," the component of target velocity moving toward or away from the radar.
• For example, at "time interval 1" (T1), an EM pulse transmitted by the radar is intercepted by a target at distance "D1".
• At "time interval 2" (T2), another pulse returns a target distance "D2." Doppler radars measure the change in "D" from T1 to T2. These changes, the radar's wavelength, and the time interval between T1 and T2, are used to compute target velocity.