specular reflector quasi-specular reflector

quasi-Lambert reflector Lambert reflector

Limiting Forms of Reflection and Scatter from a Surface

Doppler Radar

From Josh Wurman

NCAR S-POL DOPPLER RADAR

Doppler Shift: A frequency shift that occurs in electromagneticwaves due to the motion of scatterers toward or away from the observer.

Doppler radar: A radar that can determine the frequency shift through measurement of the phase change that occurs in electromagnetic waves during a series of pulses.

Analogy: The Doppler shift for sound waves is the frequency shift that occurs as race cars approach and then recede from a stationary observer

00 2cos tfEtE ttThe electric field of a transmitted wave

The returned electric field at some later time back at the radar 11 2cos ttfEtE tt

The time it took to travelcrt 2

Substituting:

11

22cos crtfEtE tt

The received frequency can be determined by taking the time derivative if the quantity in parentheses and dividing by 2

dtrt

tt

ttr ffcvff

dtdr

cff

crtf

dtdf

222221

1

Sign conventions

The Doppler frequency is negative (lower frequency, red shift) for objects receding from the radar

The Doppler frequency is positive (higher frequency, blue shift) for objects approaching the radar

These “color” shift conventions are typically also used on radar displays of Doppler velocity

Blue: Toward radar

Red: Receding from radar

Note that Doppler radars are only sensitive to the radial motion of objects

Air motion is a three dimensional vector: A Doppler radar can only measure oneof these three components – the motion along the beam toward or away from the radar

Magnitude of the Doppler Shift

Transmitted Frequency

X band C band S band

9.37 GHz 5.62 GHz 3.0 GHzRadial velocity

1 m/s

10 m/s

50 m/s

62.5 Hz 37.5 Hz 20.0 Hz

625 Hz 375 Hz 200 Hz

3125 Hz 1876 Hz 1000 Hz

These frequency shifts are very small: for this reason, Doppler radars must employ very stable transmitters and receivers

Clear-Air Radar

VHFUHF10 cm

1/3

4

VHFUHF10 cm

1/3

4

6 6

1

2N

i ii

n a

2nC

Clear-Air Turbulence

Clear-Air Wind Profilers

Wind Profiler SpecificationsFrequency (MHz)

Wavelength (m)

Maximum Altitude (km)

Antenna Size (m)

Target Band Designation

50 6 20 100 x 100 Clear Air VHF

449 0.75 15 15 x 15 Clear Air and Heavy

Precipitation

UHF

915 ~0.3 5-6 5 x 5 Clear Air and

Precipitation

UHF

1036 ~0.3 5.5-6 5 x 5 Clear Air and

Precipitation

UHF

915 MHz

50 MHz

Radial VelocityPositive TowardSurface

Signal Power

Bragg Diffraction (Scattering)

NWS-NOAA Profiler Network

SODAR

SODARParameter Performance

Horizontal wind speed components Range 0-20 m/s, accuracy 0.2 m/s

Horizontal wind speed vectors Range 0 - 25 m/s

Vertical wind speed components Range 0 - 10 m/sec, accuracy 0.1 m/s

Horizontal wind Direction 0 - 359 degrees

Resolution of reading 0.1 m/s

Sampling Height 50 m - 900 m AGL

Environmental Conditions -10C to + 40C, 0 - 100% humidity

Acoustic Frequency 1525 - 2225 Hz (selectable)

Performances of ARPL SODAR

Radio-Acoustic Sounding (RASS)

Beam Geometry

Pulsed Radar

Unambiguous Range

Coherent Integration