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Radio waves with wavelengths in the centimetre range can be beamed by a reflector, like light in an automobile headlamp, to make up a radar system.
The narrowness of the beam depends on the length of the waves and on the width of… Fundamentals of radar Radar typically involves the radiating of a narrow beam of electromagnetic energy into space from an antenna see the figure. The narrow antenna beam scans a region where targets are expected. When a target is illuminated by the beam, it intercepts some of the radiated energy and reflects a portion back toward the radar system.
Since most radar systems do not transmit and receive at the same time, a single antenna is often used on a time-shared basis for both transmitting and receiving. A receiver attached to the output element of the antenna extracts the desired reflected signals and ideally rejects those that are of no interest.
For example, a signal of interest might be the echo from an aircraft. Signals that are not of interest might be echoes from the ground or rain, which can mask and interfere with the detection of the desired echo from the aircraft.
The radar measures the location of the target in range and angular direction. Range, or distance, is determined by measuring the total time it takes for the radar signal to make the round trip to the target and back see below.
The angular direction of a target is found from the direction in which the antenna points at the time the echo signal is received. Pulse radar The most common type of radar signal consists of a repetitive train of short-duration pulses.
The figure shows a simple representation of a sine-wave pulse that might be generated by the transmitter of a medium-range radar designed for aircraft detection. The sine wave in the figure represents the variation with time of the output voltage of the transmitter.
The numbers given in parentheses in the figure are meant only to be illustrative and are not necessarily those of any particular radar. They are, however, similar to what might be expected for a ground-based radar system with a range of about 50 to 60 nautical miles 90 to kmsuch as the kind used for air traffic control at airports.
It should be noted that the pulse is shown as containing only a few cycles of the sine wave; however, in a radar system having the values indicated, there would be 1, cycles within the pulse.
The power of the pulse, called the peak power, is taken here to be 1 megawatt. Since a pulse radar does not radiate continually, the average power is much less than the peak power. In this example, the average power is 1 kilowatt.
The average power, rather than the peak power, is the measure of the capability of a radar system. Radars have average powers from a few milliwatts to as much as one or more megawatts, depending on the application.
A typical pulse waveform transmitted by radar. In short, the power levels in a radar system can be very large at the transmitter and very small at the receiver. Another example of the extremes encountered in a radar system is the timing.
An air-surveillance radar one that is used to search for aircraft might scan its antenna degrees in azimuth in a few seconds, but the pulse width might be about one microsecond in duration.
Radar waves travel through the atmosphere at roughlykm per second the speed of light. The range to a target is determined by measuring the time that a radar signal takes to travel out to the target and back. From this expression, the round-trip travel of the radar signal through air is at a rate ofkm per second.
For example, if the time that it takes the signal to travel out to the target and back was measured by the radar to be 0.
There are no other devices that can compete with radar in the measurement of range. The range accuracy of a simple pulse radar depends on the width of the pulse: Short pulses, however, require wide bandwidths in the receiver and transmitter since bandwidth is equal to the reciprocal of the pulse width.
A radar with a pulse width of one microsecond can measure the range to an accuracy of a few tens of metres or better. Some special radars can measure to an accuracy of a few centimetres. The ultimate range accuracy of the best radars is limited by the known accuracy of the velocity at which electromagnetic waves travel.
Directive antennas and target direction Almost all radars use a directive antenna—i. The beamwidth of an antenna of fixed size is inversely proportional to the radar frequency.
The direction of a target can be found from the direction in which the antenna is pointing when the received echo is at a maximum.
A typical beamwidth might be about 1 degree. Such a radar system can determine the location of the target in both azimuth angle and elevation angle. A fan beam allows only the measurement of the azimuth angle. Doppler frequency and target velocity Radar can extract the Doppler frequency shift of the echo produced by a moving target by noting how much the frequency of the received signal differs from the frequency of the signal that was transmitted.Report on CNG Cylinders for Automotive Vehicle Applications Product Development, Ashok Leyland Technical Centre, Chennai Page 2 Executive Summary.
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