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 < < 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.

2tcr

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.

r = 1 km t = 6.67 sr = 100 km t = 0.667 ms

Pulse duration (s) and pulse length (h, meters)Pulse repetition period (msec) and pulse repetition frequency (s-1)Duty Cycle (= Tr)

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.

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

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.

Definitions

)(22max PRFccTr r

Maximum unambiguous range vs. pulse repetition frequency

Second trip echoes

Note weird velocities (characteristic of distant storm)

Why not use a low PRF, insuring a large rmax?

1. 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 PRF

1) Change the PRF2) Use a different PRF every 2-3 pulses, if echo moves, get rid of

it!  This is the methodology employed by the 88-Ds

How can you eliminate second trip echoes automatically?

Other quantities used to describe the transmitted signal:

Wavelength (cm, mm) and Frequency (ft, Ghz, Mhz)

Band designation

Frequency range

Wavelength range

Common Frequency

CommonWavelength

(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 equipment2. Attenuation3. Size of scatterers relative to

wavelength (Rayleigh vs Mie scattering)

4. Peak power (without arcing inwaveguide – e.g., 3 MW in unpressurized waveguide for S band, 0.4 MW for K band)

K (0.8 cm) band radar antennaS (10 cm) band radar antenna

ModulatorStores power

Between pulses

MagnetronGenerates

Microwaveswhen high

voltage pulse sent from

Modulator

FrequencyDetermined

by characteristicsof magnetron

DuplexerFast actingSwitch that

protectssensitive

receiver fromhigh energypulse frommagnetron

STALOOscillatorGeneratesa steady

frequency

COHOOscillatesat lower

frequencywith samephase as

transmittedpulse

Quantities used to describe weather echoes

Wavelength ( 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!

3,000,000,000 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 wattsReceived power: 10-9 watts

Receiver must be very sensitive, and must be protected frommain pulse of energy transmitted by the radar!

ModulatorStores power

Between pulses

KlystronAmplifier

that createsmicrowaves at

frequencydeterminedby STALOand COHO

DuplexerProtectsSensitive

Receiver fromHigh energyPulse frommagnetron

STALOOscillatorGeneratesa steady

frequency

COHOOscillatesat lower

frequencywith samephase as

transmittedpulse

)cos(2

10 tAAd

)sin(2

10 tAAd

2210

2QIAA

Amplitude determination:

Phase determination:

IQ

d1tan

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.

min

maxlog10..PPRD

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.

rangefaratcloudLittle

stormcloseBig

PP

RD log10..

Dynamic range of a receiver

min

maxlog10..PPRD

Dynamic range of precipitation echoes

rangefaratcloudLittle

stormcloseBig

PP

RD log10..

WOULD LIKE THESE TO BE THE SAME!

wattsPandwattsPBut rangefaratcloudLittlestormcloseBig135 1010

dbDRprecip 80108

Linear receivers:

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 signal

Receiver 1:Strong echoes

Receiver 2:Weak echoes

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 powerServo amplifiers and/or drive motors: position antennaSelsyns or potentiometers: measure angular coordinates of antennaWaveguides, rotary joints, slip rings: transmit microwaves from

transmitter 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