Presentation on theme: "Radar: Acronym for Radio Detection and Ranging"— Presentation transcript:
1 Radar: Acronym for Radio Detection and Ranging Radar is a remote sensing technique: Capable of gathering information about objects located at remote distances from the sensing device.Two distinguishing characteristics:Employs EM waves that fall into the microwave portion of the electromagnetic spectrum(1 mm < l < 75 cm)2. Active technique: radiation is emitted by radar – radiation scattered by objects is detected by radar.
2 Why microwaves?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
3 What does a conventional radar measure? 1. Distance to an object or collection of objectsDetermined by the time it takes energy to travel to the objects and return at the speed of light.r = km Dt = 6.67 msr = 100 km Dt = ms2. 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.
4 Pulse duration (t, ms) and pulse length (h, meters) Meteorological radars send out pulses of energy with relatively long periods of “listening” between pulses. Pulses are required, rather than continuous waves, to determine the distance to the target.Pulse duration (t, ms) and pulse length (h, meters)Pulse repetition period (msec) and pulse repetition frequency (s-1)Duty Cycle (= t/Tr)
5 Resolution along the direction of the beam: 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).
6 Second Trip Echo: an echo from a pulse that is not the most recent pulse
7 DefinitionsPulse 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.
8 Maximum unambiguous range vs. pulse repetition frequency
9 Note weird velocities (characteristic of distant storm) Second trip echoes
10 Why not use a low PRF, insuring a large rmax? Measurements are not made with a single pulse, but ratherwith the average of many pulses – since the antenna is rotating,dwell time (observing the same location) is an issue.2. Measurement of Doppler velocities require a high PRFHow can you eliminate second trip echoes automatically?Change the PRFUse a different PRF every 2-3 pulses, if echo moves, get rid of it! This is the methodology employed by the 88-Ds
11 Other quantities used to describe the transmitted signal: Wavelength (l, cm, mm) and Frequency (ft, Ghz, Mhz)Band designationFrequency rangeWavelength rangeCommon FrequencyCommonWavelength(Ghz)(cm)UHF30-1000.4271L15-301.323S7.5-152.810.7C5.5X9.43.2Ku15.51.94K241.25Ka350.86Millimeter40-30094 (W band)0.3
12 Major wavelength choice issues: Size of equipmentAttenuationSize of scatterers relative towavelength (Rayleigh vs Miescattering)Peak power (without arcing inwaveguide – e.g., 3 MW inunpressurized waveguide forS band, 0.4 MW for K band)S (10 cm) band radar antennaK (0.8 cm) band radar antenna
14 Quantities used to describe weather echoes Wavelength (l l, cm, mm) and Frequency (ft fD) Ghz, Mhz)fD is the Doppler shift, the change in frequency that occursbecause scatterers are moving toward or away from the radar.Doppler shift is typically no more than a few kilohertz, whileThe transmitted frequency is typically gigahertz!3,000,000,0003,000,001,000
15 Quantities used to describe weather echoes Received Power: typical value: nanowattsCompare the received power with the transmitted power:Peak transmitted power: 106 wattsReceived power: wattsReceiver must be very sensitive, and must be protected frommain pulse of energy transmitted by the radar!
18 Dynamic range of a receiver 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 echoesRatio (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.
19 WOULD LIKE THESE TO BE THE SAME! Dynamic range of a receiverDynamic range of precipitation echoes
20 Linear receivers: Output voltage is linear with input power Single linear receiversTypically 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 signalReceiver 1:Strong echoesReceiver 2:Weak echoes
21 Other types of receivers: Logarithmic and Square law receivers:Output voltage is non-linear with input powerHave worse resolution than linear receivers but cover full dynamic range of weather echoes
22 Additional components of radars Power supplies: provide powerServo amplifiers and/or drive motors: position antennaSelsyns or potentiometers: measure angular coordinates of antennaWaveguides, rotary joints, slip rings: transmit microwaves fromtransmitter to antenna while antenna is rotatingDirectional couplers: allow sampling of transmitted signal or to inject test signals into receiverRadomes: to protect antenna from weather and windOther electronic components