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Dual Polarization Radar

Table of contents
« Polarimetric Radar »
  1. Block Diagram
  2. Operating Modes with Dual Polarization
  3. Polarimetric Radar Products

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

Figure 2: A falling raindrop displaces the air. On its sides the flow lines are compressed and a pulling force is created (similar to the aerodynamics on an airplane wing). This pulls the drop into a wider width. The larger the drop, the stronger this compression and the stronger this force.

Figure 2: A falling raindrop displaces the air. On its sides the flow lines are compressed and a pulling force is created (similar to the aerodynamics on an airplane wing). This pulls the drop into a wider width. The larger the drop, the stronger this compression and the stronger this force.

Dual Polarization Radar

The use of dual polarization is one way to distinguish between hail and raindrops. The radar transmits and receives linearly polarized RF signals and switches rapidly between horizontal and vertical polarization, either alternating between individual transmit pulses or between groups of pulses. Modern dual polarized radars usually transmit both polarization directions simultaneously.

Block Diagram of Dual Polarized Radar

In this example of a dual polarized radar, the transmitted energy is divided into two parts by a −3dB coupler. Both power parts are then transmitted simultaneously with different polarization via a bipolar horn radiator.

Figure 4: Simplified Block Diagram of a Polarimetric Radar

Figure 4: Simplified Block Diagram of a Polarimetric Radar

Figure 4: Simplified Block Diagram of a Polarimetric Radar

Using a switch, it is also possible to transmit in only one polarization plane (but then with double the power) for reflectivity measurements. In this case, both echo signals are also received and evaluated in the signal processor.

This procedure is useful because the polarization orientation of the electromagnetic waves can also shift when penetrating a rain area and subsequent reflection. It may give additional information about the size and shape of the hydrometeors.

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 power divider.

Switch dual/single polarization

The part of transmitter's 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 feed horns 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 amplify and demodulate the receives RF-signals. The receiver provides video signals 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.

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:

In practice, however, simultaneous transmission and reception have become generally accepted because the hardware requirements (double transmission power and two independent receivers) are easy to realize and, in contrast, the time saving by simultaneous processing is advantageous. Sometimes the manufacturers of the radar sets name the simultaneous modes

Alternating use of different polarizations has become less meaningless due to the multiplied time needed. Nevertheless, some manufacturers use a particular marketing term for it: Quad-Pol stands for the ability of a radar to perform all four modes of dual polarization.

Figure 3: The larger the raindrops, the flatter their shape, the larger the ZDR

Figure 3: The larger the raindrops, the flatter their shape, the larger the ZDR

Polarimetric Radar Products
Differential Reflectivity

The two receiving signals are called ZH and ZV and from these, the differential reflectivity ZDR is calculated. In a medium to heavy rain the raindrops are large and flatten during their free fall, forming flattened spheroids. This, in turn, is the cause of a stronger echo when the polarization is horizontal.

Formel für die differentielle Reflektivität

The dielectric constant of solid ice is only about 20% of that of water and the particle shape, therefore, has a much smaller effect in hail than in rain. Hail particles tumble when falling, so ZDR will be small. So hail is identified by a high ZH and low ZDR value. If there are even linearZDR values smaller than one (or with a negative decibel value), this is a typical sign for hailstones. (Only these can finally fall down “upright”!)

With a polarimetric radar, the size of the water droplets can be measured to a certain degree. When falling, the hydrometeors flatten out a little. The ratio of height and width of hydrometeors is slightly dependent on their size. But more important is the size of the reflectivity. Above a certain amount of water per cubic meter, the water drops must have a certain size. If the differential reflectivity matches this, the result is meaningful.

Linear depolarization ratio

In the case of radiating in horizontal polarization, nevertheless, the receiver of the vertically polarized channel works. It receives the part of the horizontally polarized radiation turned into the vertical plane (“depolarized”). The logarithmic ratio of reflectivity for horizontal radiation and vertically polarized reception to that for purely horizontally polarized transmission and reception is called Linear Depolarization Ratio (LDR).

For spherical particles, LDR will theoretically approach to negative infinity, but in practice will only reach values down to −40 dB.

Unfortunately, no dual polarized radar known to me can directly measure a hail particle size. It can only determine that it is hail from the differential reflectivity ZDR and the measurable phase differences between the horizontally and vertically polarized received signal. From the comparison of the reflectivities measured so far with exactly this radar and the experiences made so far over then real precipitation types have fallen, the radar can determine whether it concerns hail or not! Whether it is then many small graupels or few large hailstones, the radar cannot measure.

Differential Phase (φDP)

The differential phase φDP is a measure of the difference in 2-way attenuation for the horizontally and vertically polarized pulses in a pulse volume. It is defined as:

φDP = φHH − φVV

(2)

Where: φHH, φVV are the cumulative differential phase shift for the total round trip between radar and resolution volume.

When the horizontally and vertically polarized radar pulses pass a given target, both pulses are not only attenuated but even slowed down, changing the phase shift of each pulse. Within nonspherical hydrometeors, the path for the horizontally polarized wave is slightly longer than for the vertically polarized wave. The differential phase φDP shows the difference in phase between the horizontally and vertically polarized pulses. This provides information about the shape and density of radar targets.

The differential phase φDP is used to distinguish between non-meteorological and meteorological echoes. For this purpose, we calculate the root of the mean square deviation of the differential phase. Large deviations are then not volume targets but point targets.

Specific Differential Phase (KDP)

However, the values of φDP are accumulated along each radial, i.e., within each pulse period, which makes interpretation difficult. Therefore, a specific differential phase is calculated, which describes the change of φDP along the propagation path. KDP can also be described as a gradient of phase:

(3)

Like ZDR, KDP also depends on the particle size of hydrometeors and their shape and orientation. It is thus another valuable quantity to distinguish raindrops from hail and sleet.

Co-polar Cross-correlation Coefficient (ρHV)

The co-polar cross-correlation coefficient ρHV is a statistical correlation between the horizontally and vertically polarized echo signals. It is a measure of the diversity of hydrometeor forms in a given resolution cell and can be a good indicator of areas where there is a mixture of precipitation types, such as from rain and snow.

The mean values of the scattering amplitudes (SHH, SVV measured for the resolution cell from two or more pulse periods are used for the calculation.
ρHV is calculated as follows:

(4)