Transcript Document
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: 1. 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. 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 What does a conventional radar measure? 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. ct r 2 r = 1 km t = 6.67 ms r = 100 km t = 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. 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) 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). Second Trip Echo: an echo from a pulse that is not the most recent pulse Definitions 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. rmax cTr c 2 2( PRF) Maximum unambiguous range vs. pulse repetition frequency Note weird velocities (characteristic of distant storm) Second trip echoes Why not use a low PRF, insuring a large rmax? 1. Measurements are not made with a single pulse, but rather with 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 PRF How can you eliminate second trip echoes automatically? 1) Change the PRF 2) Use a different PRF every 2-3 pulses, if echo moves, get rid of it! This is the methodology employed by the 88-Ds Other quantities used to describe the transmitted signal: Wavelength (l, cm, mm) and Frequency (ft, Ghz, Mhz) Band designation Frequency range Wavelength range Common Frequency Common Wavelength (Ghz) (cm) (Ghz) (cm) UHF 0.3-1.0 30-100 0.42 71 L 1.0-2.0 15-30 1.3 23 S 2.0-4.0 7.5-15 2.8 10.7 C 4.0-8.0 3.75-7.5 5.5 5.5 X 8.0-12.0 2.5-3.75 9.4 3.2 Ku 12.0-18.0 1.67-2.5 15.5 1.94 K 18.0-27.0 1.11-1.67 24 1.25 Ka 27.0-40.0 0.75-1.11 35 0.86 Millimeter 40-300 0.1-0.75 94 (W band) 0.3 Major wavelength choice issues: 1. Size of equipment 2. Attenuation 3. Size of scatterers relative to wavelength (Rayleigh vs Mie scattering) 4. Peak power (without arcing in waveguide – e.g., 3 MW in unpressurized waveguide for S band, 0.4 MW for K band) S (10 cm) band radar antenna K (0.8 cm) band radar antenna Modulator Stores power Between pulses Magnetron Generates Microwaves when high voltage pulse sent from Modulator Frequency Determined by characteristics of magnetron Duplexer Fast acting Switch that protects sensitive receiver from high energy pulse from magnetron STALO Oscillator Generates a steady frequency COHO Oscillates at lower frequency with same phase as transmitted pulse 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 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! 3,000,000,000 3,000,001,000 Quantities used to describe weather echoes 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! Modulator Stores power Between pulses Klystron Amplifier that creates microwaves at frequency determined by STALO and COHO Duplexer Protects Sensitive Receiver from High energy Pulse from magnetron STALO Oscillator Generates a steady frequency COHO Oscillates at lower frequency with same phase as transmitted pulse A0 A1 cos( d t ) 2 A0 A1 sin(d t ) 2 Amplitude determination: Phase determination: A0 A1 I 2 Q2 2 Q I d tan1 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. Pmax D.R. 10log Pmin 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. PBig close storm D.R. 10 log PLittle cloud at far range WOULD LIKE THESE TO BE THE SAME! Dynamic range of a receiver Pmax D.R. 10log Pmin Dynamic range of precipitation echoes PBig close storm D.R. 10 log PLittle cloud at far range But PBig close storm 105 watts and PLittle cloud at far range 1013 watts DRprecip 10 80 db 8 Linear receivers: Linear receivers: Output voltage is linear with input power Receiver 1: Strong echoes Receiver 2: Weak echoes 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 Other types of receivers: 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 Additional components of radars 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