Pulse Compression Orthogonal Coding for Polarization Diversity Technique with Millimeter Spaceborne Doppler Radars

The WIVERN satellite concept is designed for measuring global in-cloud wind. Wind observations from this millimeter (94 GHz) spaceborne Doppler radar should improve forecast models and provide better understanding of the global cloud and precipitation system. The high frequency is needed to achieve a 1 km vertical resolution when the radar’s 500 m pulses sample the medium at angles around . However, at this frequency, the main challenges are the range velocity dilemma and signal short decorrelation time. The polarization diversity technique has been proposed to overcome these constraints.

In the polarization diversity technique, pulses at two horizontal (H) and vertical (V) polarizations are transmitted with a short separation time. In detail, two pairs of HV and VH pulses are transmitted alternatively. For WIVERN, the separation time is in order of few microseconds to provide large unambiguous velocity () and to maintain a high degree of correlation between the co-polar signals. There are two crucial considerations of this configuration that need to be addressed: 1) the effect of cross talk interference between the two H and V signals due to the de-polarizing surface and hydrometeors; and 2) the significant increase in Doppler accuracy at low signal to noise ratios (SNR) and small . Cross-polar interference signals do not bias Doppler estimates since they come from different range gates. However, they act like additive color noise and cause a reduction of the SNR. The surface and strongly de-polarizing targets (e.g. the melting layer) are primary sources of cross-polarization interference. Strong cross-talk signals can make the Doppler retrievals impossible and create “blind zones” near the surface.

In this work, a novel pulse compression orthogonal coding for the polarization technique is presented. By employing pulse compression with “nearly” orthogonal waveforms at the two polarization channels, cross-polarization signals will be suppressed; hence, the effect of the cross-talk problem will be minimized. In addition, the use of the pulse compression technique improves range resolution and sensitivity of co-polar signals. The proposed technique will be illustrated by simulation with inputs from measurements by the National Research Council Canada (NRC) Convair 580 W-band airborne radar system.