308 Flashcards
RADAR
RAdio Detection And Ranging
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.
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
Volume scan
- from site to 240 km
- through 360 degrees
- 0.3 degrees to 65 degrees vertically
- cylinder of data is collected for processing
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
echo tops
a representation of the highest altitude where detectable precipitation is observed
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
Can-USA composite -echo tops
- unreliable
- use with caution
Reasons pulsed radar is used
- only one antenna required
- much less power usage
PRF
Pulse Repetition Frequency- determines length of listening and maximum range.
5 components of the Radar system
- Transmitter
- Receiver
- Antenna
- Processor
- Display
Transmitter
- Produces pulses of appropriate power frequency and duration
- 220v AC power
- synchronizer
- a modulator
- a magnetron
- sent to antenna via a wave guide
Receiver
- Accepts returned energy from antenna
- mixer converts microwave energy to an Intermediate frequency (IF)
- IF amplifier
- detector
- video amplifier
Antenna
- Focuses and directs energy
- receives and concentrates energy
- tower
- paraboloid reflector
- The larger the antenna, the tighter the cone, the better the resolution
Processor
-black box or mini-computer
Display
-Shows representation of echo intensity by precipitation rate of fall, tops of echoes and area of coverage
Wavelength
-the distance from crest to crest or from trough to trough in a wave train-expressed in CM
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
Stupid equation
Wavelength = speed of light / frequency
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
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
Pulse length
- Duration of time RADAR is radiating
- expressed in microseconds
- Canada uses 2 microseconds (600m)
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
Normal propagation
- 4/3 the curvature of the earth
- Strongest refraction is in the lower levels
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.
Factor affecting return power
- number of drops or flakes
- composition, drops or flakes
- Size (most important)
Composition
-snow is 1/5 as reflective as a raindrop of the same water equivalent
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
Reflective capacities list
- large hail
- rain
- small hail
- snow
- clouds
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
Attenuation
- 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
Distortion
-Misrepresentations of the target in any of it’s paramters
2 categories of distortion
- Distortion due to radar design
- beam width
- pulse length
- processor distortion
- Distortion due to propagation characteristics
- Range
- pricipitation attenuation
- fringing
- earth’s curvature
STC
- Sensitivity Time Control
- used to mitigate range distortion
- electronically alters radar strength
Pricipitation distortion
- 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
Fringing distortion
- Areas of weaker precip will go unnoticed in heavier precip
- when receiving reports of broken rain, undetected areas of light precip are also present
Extraneous echoes
- Any echo not derived from clouds or precip
- can be misleading
- coherent when constant
Angel echoes
-echoes of unknown origins
- birds, debris, smoke, bugs etc
- inversions
-cold fronts
-sea breeze fronts
-
Clutter
- Conglomeration of ground based echoes under normal atmospheric conditions.
- trees
- buildings
- sea waves
Second trip echoes
-Echoes that are detected from a previous pulse
Canadian Radar Network
- 31 sites
- limited in west
- C band, 5 cm wavelength
