summer: CH2 P4,6,7,8,9 Flashcards
Antenna:
The device which sends the radar signal into the atmosphere
Types of radars:
- (most common) Directional, meaning that they focus energy into a particular direction. This makes it possible to locate targets in space.
- Isotropic: sends radiation in all directions except bellow
Advantage of radar:
Ability to determine the direction of a target from the radar
Antenna system consists of:
antena and reflector
Antenna system consists of:
Antenna:
- Antenna: (real antenna) the radiating element which transmits the radar signal into the atmosphere toward the reflector
- Most common types:
- Feed horn
- Dipoles
- Most common types:
Antenna system consists of:
reflector
- and reflector: reflect and directs the signals away from the radar
- shape of reflector determines the shape of the antenna beam pattern
- the bigger the reflector the better it is able to direct the signal and the narrower the beam of the antenna
- most common reflector:
- circular parabolic reflectors: reflectors which are parabolic in cross section and circular when viewed from the front and back.
- Beam pattern is:
- Conical
- Narrow
- 1o width for the main lobe of the patters
- Beam pattern is:
- circular parabolic reflectors: reflectors which are parabolic in cross section and circular when viewed from the front and back.
Things you should know about the antenna
- wavelength
- size of reflector
- gain of antenna
- beam width
Things you should know about the antenna
- Wavelength
- Wavelength
- The radar transmitter determines this parameter. The antenna must match the transmitter’s wavelength
Things you should know about the antenna
- Wavelength
- The radar transmitter determines this parameter. The antenna must match the transmitter’s wavelength
- Size of reflector
- Antennas on weather radars (1ft to 30ft)
Things you should know about the antenna
- Wavelength
- The radar transmitter determines this parameter. The antenna must match the transmitter’s wavelength
- Size of reflector
- Antennas on weather radars (1ft to 30ft)
- Gain of antenna
- Gain of antenna is the ration of the power received at a specific point in space on the center of the beam axis with the radar reflector in place to the power that would be received at the same point from an isotropic antenna
g=p1/p2- where p1 is the power of the beam and
- p2 is the power at the same point from an isotropic antenna (p1 and p2 must be measured in the same units)
- antenna gain is measured logarithmically in decibels
Things you should know about the antenna
beam width
- beam width: angular width of the antenna beam measured from the point where the power is exactly half what it is at the same range on the center of the beam axis
- antenna gain and beam width are related
- for circular reflectors the horizontal and vertical beam widths would be equal
g=pi2/O2 - gain is independent of wavelength
- any circular parabolic radar antenna with 1o beamwidth would have the same antenna gain at any frequency
The main lobe and side lobe in real radar antennas
- mainlobe: have a bright spot
- sidelobe: transmit and receive energy off to the sides of the main lobe
- sidelobe exist in all directions away from the mainlobe and are different from one direction to another
Backlobe
Radar energy that go directly behind the antenna
How are antenna beam patterns examined?
Usually in one direction at a time whether
- azimuth or
- elevation
(T/R) switch:
- Special switch added to the radar system to protect the receiver from the high power of the transmitter
Transmitted power:
A few thousand watts to more than 1 MW (1MW = 106watt)
Receiving power:
- 10-14 W or less
What would happen if the transmitter sent its power to the receiver?
- The receiver is very sensitive therefore it will burn up
Duplexer
AKA circulator: an automatic switch that protect the receiver from burning
Duplexer location
- Located between the transmitter and the receiver
How does the duplexer work?
Cons of duplexer:
Need recovery time after transmitter fire, before the receiver is at its full sensitivity
Recovery time depends on:
- The design of the T/R switch
- Power of transmitter
- Typically on the order of 1 to 10 цs
- Correspond to distance 150 m to 1.5 km
- Typically on the order of 1 to 10 цs
Receiver:
- Detect and amplify weak signals received by the antenna
Important characteristic of receivers:
- Must be of very high quality because the signals detected are very weak
Most receivers are:
- Superheterodyne: receive signals are mixed with a reference signal at some frequency which is different from the transmitted frequency
Reference signal:
30 or 60 MHZ above or bellow the transmitted frequency
Mixing process result in:
- Four frequencies
- The original transmitted frequency
- The reference signal frequency
- The sum of these two frequencies
- The difference between these two frequencies
Receivers of modern radars:
- Two receivers
- First one is designed to cover the wide range of reflectivity’s that are present in the real world (up to 8 to 10 orders of magnitude)
- Logarithmic receivers
- used to handle this wide range of reflectivity’s
- Produce output proportional to logarithm of input power
- Logarithmic receivers
- First one is designed to cover the wide range of reflectivity’s that are present in the real world (up to 8 to 10 orders of magnitude)
Can have dynamic range of 80dB or more
Receivers for Doppler velocity measurements:
Usually more sensitive to weak echoes but of much more limited dynamic range
Easiest method to display radar data
- Using oscilloscope(A-scope)
- X-axis: time
- Y-axis: signal strength or intensity
Problem of A-scope display:
Gives no direct information about where the antenna is pointing. We want to know where the target is
PPI:
(Plan Position Indicator):
- Display the radar data in a map like format with the radar at the center.
- Distance given by adding range marks (AKA range rings) around the radar.
- Direction from the radar is shown by the position of the echo relative to the radar
Old displays:
- Showed intensity with varying degrees of brightness (it was only gray)
- Not possible to determine more than a few levels of intensity with these monochrome displays
Modern displays:
Can show several levels of intensity clearly using color coding (15 or more intensity levels)
RHI:
(Range height indicator):
- X-axis: distance from the radar
- Y-axis: height above the radar (it exaggerate the vertical size of the echo)
Research and operational radars are equipped to:
Process the radar data to detect various hazardous weather situations using what are called computer algorithms (or simply algorithms)
What are algorithms?
Specific set of instructions that the computer will execute to see if the storm contains specific features of the characteristic the computer is looking for exist. It will indicate that the event is present
