Technique and Observations.

The above formulations provide the basis for determining the scattering and propagation parameters from simultaneous H and V transmissions. Given the transmitted values of the covariance quantities, $W_V\vert^{\rm t}$, $(W_H/W_V)\vert^{\rm t}$, $\phi_{{HV}}\vert^{\rm t}$, and $\rho_{HV}\vert^{\rm t}$, the scattering and propagation parameters are determined by measuring the returned values of the same quantities. The transmitted values are readily determined from calibration and monitoring measurements.

Scott (1999) has analyzed the polarization changes geometrically in terms of the Poincaré sphere. The basic results of this study are reported in the next section; one result is that the polarization change produced by $Z_{\rm DR}$and differential attenuation is greatest when the incident value of W_H /W_V is unity, namely when the transmitted signal contains equal Hand V powers. This situation is simulated by radars which alternate between H and V transmissions, but the two signals can also be transmitted simultaneously. The depolarization produced by horizontally aligned particles is independent of the phase difference between the components, so that ${45^\circ}$ linear or circular polarizations are equally effective in determining the scattering parameters. (Scattering by randomly oriented particles, or by particles that are non-horizontally aligned, is different and can be used to identify different particle types, as we later discuss.)

Figure 1 shows the basic block diagram of the simultaneous transmission technique. A power divider replaces the polarization switch and supplies equal powers to the orthomode polarization transducer (OMT) during each transmitted pulse. By changing the relative phases of the power divider outputs the polarization state can be adjusted to a variety of values ( $\pm 45^\circ$ linear, right- or left-hand circular, or any equal-power elliptical state). For the observations reported in this paper, the polarization state was adjusted to be left-hand circular (LHC). The return signals are received in parallel H and V channels and processed to obtain the orthogonal powers W_H and W_V and the complex correlation coefficient $\hat \rho_{HV}= \rho_{HV}e^{j\phi_{{HV}}}$. The radar signals are therefore transmitted in a different polarization basis than they are received. This ensures a good signal-to-noise ratio in the two receiving channels, thus producing a copolar-like signal in the channels.

  

Figure 1: Block diagram of the simultaneous transmission technique.

The New Mexico Tech dual-polarization radar was modified during the spring of 1998 to implement the above technique. The radar transmits about 10 kW peak power at 3.0 cm wavelength and has a 3.7 m diameter Cassegrain antenna of $0.6^\circ$ beamwidth. It had previously been configured as a co- and cross-polar circular (or linear) polarization system and used to study electrically aligned ice crystals in electrified storms (Krehbiel et al., 1996). Initial results of the study by Scott (1999) showed that it would be possible to measure both the electrical alignment directions and the linear polarization parameters if circularly polarized transmissions were received in a linear H-V basis; this led to the simultaneous transmission approach described above. Although independently arrived at, the technique is essentially the same as that originally proposed by Sachidananda and Zrnic (1985) and implemented on the CSU-CHILL radar.