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Dual Polarization Radar

Figure 1: Stylized measurement processes of polarimetric radars

Figure 1: Stylized measurement processes of polarimetric radars

Stylized measurement processes of polarimetric radars, 
© 2015 Christian Wolff www.radartutorial.eu

Figure 1: Stylized measurement processes of polarimetric radars

Dual Polarization Radar

Another method for hail detection uses dual polarization. The radar transmits and receives linear polarised signals and switches rapidly between horizontal and vertical polarization, either between individual pulses or groups of pulses.

More modern polarimetric radar sets, like the METEOR 1500 of Gematronik, send even both polarization directions simultaneously. The two returns are referred to as ZH and ZV and from these the differential reflectivity ZDR is calculated. In moderate to heavy rain the rain drops are large and as they fall they flatten to become oblate spheroids, giving a stronger echo for horizontal polarization.

Formel für die differentielle Reflektivität

The dielectric constant of solid ice is about 20% of that of water and therefore particle shape has a much smaller effect in hail than in rain. Also hail particles tumble as they fall so ZDR will be small. Hail is identified by high ZH and low ZDR. If even ZDR results less than one should appear (or a negative Decibel- value), this is a typical sign for hailstones. Only these can fall down in vertical orientation finally!

Dual Polarization Modes

Transmission and reception with different polarization can be carried out either simultaneously or alternately from pulse period to pulse period. This depends on whether you want to split the transmitting power into two channels, and whether two independent receiving channels are available. Four modes of dual polarization are possible:

Sometimes the manufacturers of the radar sets refer to the simultaneous modes as

in summary.

Block Diagram

At this example of a polarimetric radar the transmitting energy is divided by a −3dB-coupler into two equal parts. These two parts are transmitted by a dual polarization feed horn with a different polarization simultaneously.

Figure 3: Simplified Block Diagram of a Polarimetric Radar

Figure 3: Simplified Block Diagram of a Polarimetric Radar

Figure 3: Simplified Block Diagram of a Polarimetric Radar

By a switch this radar can transmit in a single polarization for measurements of the reflectivity only (just with this one then with the double power, however). But in this case however the radar also receive in the two polarization levels. Both signals are evaluated in the signal processor.

This is very expedient since the orientation of the electric field always changes itself at a reflection.

Transmitter

The radar transmitter produces the short duration high-power rf pulses of energy.

Power divider (−3dB Hybrid #1)

This power divider divides the transmitters power into two equal parts. One part of power will be radiated into space by the vertical polarized feedhorn, the other one by the horizontal polarized feedhorn.

Power divider (−3dB Hybrid #2)

If single polarization is switched, then the divided parts of energy are added again into this powerdivider.

Switch dual/single polarization

The part of transmitters power for vertically polarization would be conduct either to the feedhorn for vertically radiation (then the radar ist dual polarized), or back to the main horizontally feedhorn (then the radar ist single polarized).

 

Detour line for the phase coincidence

The detour line provide phase coincidence of the both parts of energy.

Parabolic reflector

The directivity of the feedhorns is added to that of the parabolic reflector. The resulting pattern is a very narrow and concentrated beam with dual polarization.

Dual Feedhorn

A dual feedhorn is used to feed the transmitters energy into the parabolic reflector with the necessary polarizations.

Ferrite circulator (Duplexer)

The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.

Receiver

The receivers amlify and demodulate the receives RF-signals. The receiver provides videosignals on the output.

Signal Processor

The signal processor is that part of the system which separates weather information from clutter on the basis of Doppler content and amplitude characteristics.

CAPPI - Scope

The indicator should present to the observer a continuous, easily understandable, graphic picture of weather information.