Radar : Acronym for Ra dio D etection a nd R anging

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# Radar : Acronym for Ra dio D etection a nd R anging - PowerPoint PPT Presentation

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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Presentation Transcript

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)
Why microwaves?

Microwaves can penetrate haze, fog and snow readily, and rain and hail less readily, so radar can “see through” these conditions.

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.

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.

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).

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.

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

• Change the PRF
• 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)

Major wavelength choice issues:
• Size of equipment
• Attenuation
• Size of scatterers relative to
• wavelength (Rayleigh vs Mie
• scattering)
• 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

Duplexer

Fast acting

Switch that

protects

sensitive

high energy

pulse from

magnetron

Modulator

Stores power

Between pulses

Magnetron

Generates

Microwaves

when high

voltage pulse

sent from

Modulator

Frequency

Determined

by characteristics

of magnetron

STALO

Oscillator

Generates

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

Compare the received power with the transmitted power:

Peak transmitted power: 106 watts

Receiver must be very sensitive, and must be protected from

main pulse of energy transmitted by the radar!

Modulator

Stores power

Between pulses

Duplexer

Protects

Sensitive

High energy

Pulse from

magnetron

Klystron

Amplifier

that creates

microwaves at

frequency

determined

by STALO

and COHO

STALO

Oscillator

Generates

frequency

COHO

Oscillates

at lower

frequency

with same

phase as

transmitted

pulse

Amplitude determination:

Phase determination:

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.

WOULD LIKE THESE TO BE THE SAME!

Dynamic range of precipitation echoes

Linear receivers: Output voltage is linear with input power

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

Strong echoes

Weak echoes

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