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1

RADAR

RAdio Detection And Ranging

2

radar basics

- electromagnetic is propagated through space at ~ the speed of light
-Microwave energy is pulsed at about 2 microseconds or longer
-transmits a directional beam (conical)
-Targets absorb and reflect energy back to antenna
-listens for returns during time in between pusles
-returns are amplified and displayed
-the larger the energy return, the larger the target.
-distance is calculated by elapsed time
-antenna may be rotated for azimuth or elevated for height.

3

PPI

Plan position indicator- a display of radar echoes set at lowest elevation angle , superimposed over a grid of geography or other features. viewed from overhead
-good for low level stratiform cloud

4

Volume scan

-from site to 240 km
-through 360 degrees
-0.3 degrees to 65 degrees vertically
-cylinder of data is collected for processing

5

CAPPI

Constant Altitude Plan Position Indicator-
representation of of echoes observed at a constant altitude above sea level
-radar is the centre of the display
-only effective to 130 km, then becomes a PPI

6

echo tops

a representation of the highest altitude where detectable precipitation is observed

7

Can-USA composite - low level

-Canadian echoes are CAPPI
-US echoes are PPI
-resolution is 4.2 nm per pixel
-always set on rain, snow may not show

8

Can-USA composite -echo tops

-unreliable
-use with caution

9

Reasons pulsed radar is used

-only one antenna required
-much less power usage

10

PRF

Pulse Repetition Frequency- determines length of listening and maximum range.

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5 components of the Radar system

-Transmitter
-Receiver
-Antenna
-Processor
-Display

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Transmitter

-Produces pulses of appropriate power frequency and duration
-220v AC power
-synchronizer
-a modulator
-a magnetron
-sent to antenna via a wave guide

13

Receiver

-Accepts returned energy from antenna
-mixer converts microwave energy to an Intermediate frequency (IF)
-IF amplifier
-detector
-video amplifier

14

Antenna

-Focuses and directs energy
-receives and concentrates energy
-tower
-paraboloid reflector
-The larger the antenna, the tighter the cone, the better the resolution

15

Processor

-black box or mini-computer

16

Display

-Shows representation of echo intensity by precipitation rate of fall, tops of echoes and area of coverage

17

Wavelength

-the distance from crest to crest or from trough to trough in a wave train-expressed in CM

18

Cycle/Frequency

-The path that a wave travels in one wavelength is called a cycle
-The number of cycles in a given unit of time is the frequency
-Expressed in hertz, one hertz = one cycle

19

Stupid equation

Wavelength = speed of light / frequency

20

Wavelength and Precipitation Detection

-Short wavelengths detect light precip well, but attenuates very easily by that precip.
-Longer wavelengths detect heavy precip well, but they do not see light precip well
-3 cm works for snow, but has poor coverage and penetration
10 cm good for SH and TS, misses light precip
-American - primary 10 cm
- secondary 5 cm

21

Wavelength range

-.86 cm - 23 cm / 30000 MHz - 1300 MHz
most common 3cm, 5cm, 10 cm (5cm is commonly used in Canada {C})
-beam width is narrower, giving better resolution

22

Pulse length

-Duration of time RADAR is radiating
-expressed in microseconds
-Canada uses 2 microseconds (600m)

23

Radar Beam dimensions

As the beam gets further away, it gets larger
-Typical 6 degree beam is 600m in width at 50 km and 2000m is width at 200km
-3.7 m parabolic antenna produces a conical beam width of 1 degree

24

Normal propagation

-4/3 the curvature of the earth
-Strongest refraction is in the lower levels

25

Anomalous Propagation

-more or less bending of a normal beam due to temperature or humidity variations in the atmosphere
-Super-refraction - curve becomes greater than normal
-from inversion
-Sub-refraction- beam bends upwards giving a reduction in range
- occurs when there is a decrease in temperature and increase of water vapor with height.

26

Factor affecting return power

-number of drops or flakes
-composition, drops or flakes
-Size (most important)

27

Composition

-snow is 1/5 as reflective as a raindrop of the same water equivalent

28

Size of drop or flake

-if a drop or flake doubles in size it will have 64 times the return power.
-Rain drops are 5-7 mm in diameter

29

Reflective capacities list

-large hail
-rain
-small hail
-snow
-clouds

30

3 types of attenuation

-Range
-Absorbtion - happens within the first 160 km of the radar
-Scattering - Backscatter is the energy returned to the antennaradar for processing