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Mod3 > 308 > Flashcards

308 Flashcards

1
Q

RADAR

A

RAdio Detection And Ranging

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2
Q

radar basics

A
  • 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.
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3
Q

PPI

A

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

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4
Q

Volume scan

A
  • from site to 240 km
  • through 360 degrees
  • 0.3 degrees to 65 degrees vertically
  • cylinder of data is collected for processing
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5
Q

CAPPI

A

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

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6
Q

echo tops

A

a representation of the highest altitude where detectable precipitation is observed

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7
Q

Can-USA composite - low level

A
  • Canadian echoes are CAPPI
  • US echoes are PPI
  • resolution is 4.2 nm per pixel
  • always set on rain, snow may not show
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8
Q

Can-USA composite -echo tops

A
  • unreliable

- use with caution

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9
Q

Reasons pulsed radar is used

A
  • only one antenna required

- much less power usage

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10
Q

PRF

A

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

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11
Q

5 components of the Radar system

A
  • Transmitter
  • Receiver
  • Antenna
  • Processor
  • Display
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12
Q

Transmitter

A
  • Produces pulses of appropriate power frequency and duration
  • 220v AC power
  • synchronizer
  • a modulator
  • a magnetron
  • sent to antenna via a wave guide
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13
Q

Receiver

A
  • Accepts returned energy from antenna
  • mixer converts microwave energy to an Intermediate frequency (IF)
  • IF amplifier
  • detector
  • video amplifier
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14
Q

Antenna

A
  • Focuses and directs energy
  • receives and concentrates energy
  • tower
  • paraboloid reflector
  • The larger the antenna, the tighter the cone, the better the resolution
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15
Q

Processor

A

-black box or mini-computer

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16
Q

Display

A

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

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17
Q

Wavelength

A

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

18
Q

Cycle/Frequency

A
  • 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
Q

Stupid equation

A

Wavelength = speed of light / frequency

20
Q

Wavelength and Precipitation Detection

A

-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
Q

Wavelength range

A

-.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
Q

Pulse length

A
  • Duration of time RADAR is radiating
  • expressed in microseconds
  • Canada uses 2 microseconds (600m)
23
Q

Radar Beam dimensions

A

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
Q

Normal propagation

A
  • 4/3 the curvature of the earth

- Strongest refraction is in the lower levels

25
Q

Anomalous Propagation

A
  • 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
Q

Factor affecting return power

A
  • number of drops or flakes
  • composition, drops or flakes
  • Size (most important)
27
Q

Composition

A

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

28
Q

Size of drop or flake

A
  • 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
Q

Reflective capacities list

A
  • large hail
  • rain
  • small hail
  • snow
  • clouds
30
Q

3 types of attenuation

A
  • Range
  • Absorbtion - happens within the first 160 km of the radar
  • Scattering - Backscatter is the energy returned to the antennaradar for processing
31
Q

Attenuation

A
  • The lessening of the concentration of energy per unit volume of the beam
  • may cause targets to not be displayed with their true relative intensity or not at all
32
Q

Distortion

A

-Misrepresentations of the target in any of it’s paramters

33
Q

2 categories of distortion

A
  • Distortion due to radar design
    • beam width
    • pulse length
    • processor distortion
  • Distortion due to propagation characteristics
    • Range
    • pricipitation attenuation
    • fringing
      - earth’s curvature
34
Q

STC

A
  • Sensitivity Time Control
    • used to mitigate range distortion
    • electronically alters radar strength
35
Q

Pricipitation distortion

A
  • rapidly attenuates radar energy
  • trailing edges may not be well defined
  • systems seem to intensify as they move closer to the radar
  • trailing edge will seem to expand as definition is achieved
36
Q

Fringing distortion

A
  • Areas of weaker precip will go unnoticed in heavier precip

- when receiving reports of broken rain, undetected areas of light precip are also present

37
Q

Extraneous echoes

A
  • Any echo not derived from clouds or precip
    • can be misleading
    • coherent when constant
38
Q

Angel echoes

A

-echoes of unknown origins
- birds, debris, smoke, bugs etc
- inversions
-cold fronts
-sea breeze fronts
-

39
Q

Clutter

A
  • Conglomeration of ground based echoes under normal atmospheric conditions.
  • trees
  • buildings
  • sea waves
40
Q

Second trip echoes

A

-Echoes that are detected from a previous pulse

41
Q

Canadian Radar Network

A
  • 31 sites
  • limited in west
  • C band, 5 cm wavelength

Mod3 (9 decks)

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