Radar : Acronym for Ra dio D etection a nd R anging. Radar is a remote sensing technique : Capable of gathering information about objects located at remote distances from the sensing device. Two distinguishing characteristics:.
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Radar is a remote sensing technique: Capable of gathering information about objects located at remote distances from the sensing device.
Two distinguishing characteristics:
Microwaves can penetrate haze, fog and snow readily, and rain and hail less readily, so radar can “see through” these conditions.
An elementary radar system
1. Distance to an object or collection of objects
Determined by the time it takes energy to travel to the objects and return at the speed of light.
r = 1 km Dt = 6.67 ms
r = 100 km Dt = 0.667 ms
2. Azimuth and elevation angle to the object(s)
Determined by the pointing angles of the antenna.
3. Physical properties of the object(s)
Determined by the magnitude of the backscattered power.
Pulse duration (t, ms) and pulse length (h, meters)
Pulse repetition period (msec) and pulse repetition frequency (s-1)
Duty Cycle (= t/Tr)
half the pulse length (h)
The back of the pulse at “a” will arrive at “b” at the same time that radiation scattered from objects at the front end of the pulse at “c” will arrive back at “b”.
When energy arrives back at the radar, an instantaneous sample will include all radiation scattered between locations b and c: the sample volume is half the pulse length (h/2).
Pulse repetition frequency (PRF): The frequency that pulses are transmitted, measured in hertz (s-1)
Pulse repetition period (Tr): The time between pulses (typical value 1 ms)
Maximum Unambiguous Range (rmax): The maximum distance that an object can be located such that a pulse arriving at the object can return to the radar before another pulse is emitted.
Second trip echoes
How can you eliminate second trip echoes automatically?
Wavelength (l, cm, mm) and Frequency (ft, Ghz, Mhz)
S (10 cm) band radar antenna
K (0.8 cm) band radar antenna
Wavelength (l l, cm, mm) and Frequency (ft fD) Ghz, Mhz)
fD is the Doppler shift, the change in frequency that occurs
because scatterers are moving toward or away from the radar.
Doppler shift is typically no more than a few kilohertz, while
The transmitted frequency is typically gigahertz!
Received Power: typical value: nanowatts
Compare the received power with the transmitted power:
Peak transmitted power: 106 watts
Received power: 10-9 watts
Receiver must be very sensitive, and must be protected from
main pulse of energy transmitted by the radar!
Ratio (db) of input power that causes the video output to reach its maximum (saturation) level, to the lowest power that produces a detectable input.
Dynamic range of precipitation echoes
Ratio (db) of maximum echo power received from a very intense storm close to the radar to the minimum power received from the weakest cloud that can be detected at the greatest range of interest.
Dynamic range of a receiver
Dynamic range of precipitation echoes
Linear receivers: Output voltage is linear with input power
Single linear receivers
Typically have only half of the dynamic range of precipitation echoes, so two receivers are often used in tandem with automatic switching depending on the magnitude of the returned signal
Logarithmic and Square law receivers:
Output voltage is non-linear with input power
Have worse resolution than linear receivers but cover full dynamic range of weather echoes
Power supplies: provide power
Servo amplifiers and/or drive motors: position antenna
Selsyns or potentiometers: measure angular coordinates of antenna
Waveguides, rotary joints, slip rings: transmit microwaves from
transmitter to antenna while antenna is rotating
Directional couplers: allow sampling of transmitted signal or to inject test signals into receiver
Radomes: to protect antenna from weather and wind
Other electronic components