# Basic RADAR Principles Prof. Sandra Cruz-Pol, Ph.D. Electrical and Computer Engineering UPRM.

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Basic RADAR Principles Prof. Sandra Cruz-Pol, Ph.D. Electrical and Computer Engineering UPRM

Microwaves = have air wavelengths in the mm – cm scales with frequencies in the Giga Hertz [GHz] range Light is an electromagnetic wave.

Electromagnetic Spectrum

Microwave Remote Sensing Percentage transmission thought the earth’s atmosphere, along the vertical direction, under clear sky conditions.

Cloud and Precipitation Remote Sensing Atmospheric gases absorption spectrum at the ground in various humidity conditions indicated by the specific humidity values.

Types of Doppler Radar Continuous Wave (CW) – Simple – No range information Frequency Modulated CW, (FMCW) – Fine range resolution – Artifacts from target motion Pulse Doppler – Range and Doppler – No artifacts (except when pulse compression used)

We will see that Radars work by… Transmitting microwave pulses…. and measuring the … Time delay (range) Amplitude Polarization Frequency … of the microwave echo in each range gate

hello Compare to: Acoustic Echo-location

hello Acoustic Echo-location

hello distance Acoustic Echo-location

Hi !! time t = 2 x range / speed of sound Example: range = 150 m Speed of sound ≈ 340 meters/second t = 2 X 150 / 340 ≈ 1 second

Target Range time t = 2 x range / speed of light measure t, then determine Range Example: t =.001 sec Speed of light = c = 3x10 8 meters/second Range =.001 x 3x10 8 / 2 = 150,000 m = 150 km Tx Rx

We will see that Radars work by… Transmitting microwave pulses…. and measuring the … Time delay (range) Amplitude Polarization Frequency … of the microwave echo in each range gate

Range Resolution Top View: 2D 

DCAS advantages Elimination of multiple echoes False Echo Radar 2 (Area B) Radar 1 (Area A) A&B Unambiguous range of 1 radar Example illustrates DCAS method for identifying real targets and false targets that are aliased in range

DCAS advantages (cont) Resolution optimization Radar 1 Has signal Radar 2 Has signal x’x’ y’y’ New resolution: RR XX Target Range gate Beam

We will see that Radars work by… Transmitting microwave pulses…. and measuring the … Time delay (range) Amplitude Polarization Frequency … of the microwave echo in each range gate

Polarization Describes the way the electric field of the wave moves through space as seen from behind along time. V = vertical lineal H = Horizontal lineal

Sizes for cloud and rain drops

Raindrops symmetry Differential Reflectivity Z dr

Polarimetric radars, also called dual-pol radars, transmit radio wave pulses that have both H and V orientations. [NOAA] TropiNET radars are the first Polarimatric Doppler radars in PR.

28 Reflectivity Factor, Z Is defined as so that And expressed in dBZ to cover a wider dynamic range of weather conditions.

WHAT VARIABLES ARE MEASURED w/ Dual-pol radar? Differential Reflectivity –ratio of the reflected H & V power returns. Indicator of drop shape & good estimate of average drop size. Linear Depolarization Ratio –ratio of a V power return from a H pulse or a H from V. indicator of regions where mixtures of precipitation types occur. Specific Differential Phase –returned phase difference between the H V pulses caused by the difference in the number of wave cycles (or wavelengths) along the propagation path for horizontal and vertically polarized waves. It’s a "propagation effect.” very good estimator of rain rate.

Benefits of polarimetric radars Meteorologists: can significantly improve the accuracy of the estimates of amounts of precipitation can tell the difference between very heavy rain and hail, which will improve flash flood watches and warnings can identify types of precipitation in winter weather forecasts, improving forecasts of liquid water equivalent or snow depth is more accurate than conventional radar, saving the forecasters the step of having to verify radar data can contribute to increased lead time in flash flood and winter weather hazard warnings. Hydrologists: provides critical rainfall estimation information for stream flow forecasts and river flooding

Raindrop shapes

Average within sample volume

We will see that Radars work by… Transmitting microwave pulses…. and measuring the … Time delay (range) Amplitude Polarization Frequency … of the microwave echo in each range gate

Doppler Effect

Target Radial Velocity Frequency f t Frequency f t + f d In Weather radars, the Doppler frequency shift, is caused by the motion of the cloud and precipitation particles

Target Radial Velocity Frequency f t Frequency f t + f d

Zero Velocity for “Crossing Targets” Frequency f t Frequency f t + f d Doppler Frequency

0.1 mm/hr 1 mm/hr 15 mm/hr 100 mm/hr >150 mm/hr QPE – Quantitative Precipitation Estimation

Doppler effect: shows vortex

Cloud and Precipitation Remote Sensing Type of data collected by the millimeter-wave radar. Observations were made through the melting region of a stratiform cloud previously named “ bright band ” because of a systematic maximum of echo intensity observed just below the 0º isotherm. Source: Dr. Steve Sekelsky 2004

Melting Layer at Mayaguez-Jun2011 (data from Doppler Pol radar at CID UPRM)

43 Radar equation for Meteorology For weather applications for a volume 

44 Radar Equation For power distribution in the main lobe assumed to be Gaussian function.

45 Radar Equation R c dB =radar constant (including atmospheric attenuation) For calibrated target

References The COMET project [ http://www.comet.ucar.edu/] NASA TRMM NCAR (National Center for Atmospheric Research) - University Corporation for Atmospheric Research (UCAR) NOAA http://www.nssl.noaa.gov/research/radar/dualpol.php NOAA Educational Page [http://www.nssl.noaa.gov/edu/ideas/radar.html] Dave McLaughlin Basics of Radars presentation NWS [http://www.crh.noaa.gov/fsd/soo/doppler/doppler.htm] http://www.radartutorial.eu/07.waves/wa04.en.html