Based on a comparison of ground-based radiometer measurements with microwave radiative transfer calculations, it is shown that raindrops with an oblate shape and a preferred horizontal orientation have a significant effect on microwave polarization signals when compared with spherical particle shape. Measurements with a dual-polarized 19-GHz radiometer reveal a polarization difference of as much as Ϫ18 K in the downwelling microwave radiation at 30Њ elevation angle. Averaging all rain observations within 19 months leads to a signal of Ϫ6 K. Model calculations covering roughly the same range of weather conditions as that inferred from the meteorological data recorded with the radiometer measurements were carried out with spherical raindrop shape and an oblate particle shape with a fixed horizontal alignment. From the model results, positive polarization difference is expected for spherical particles. This signal was never observed in the recorded data. For oblate drops, the averaged model results lead to a polarization difference of Ϫ8 K, which is in reasonable agreement with the long-term averaged observations. Case studies that compare isolated rain events usually lead to a better match of model and observations. However, there are some major discrepancies in some cases. Possible reasons for the remaining differences are the short-term variations in the cloud microphysics for which the model does not correctly account, such as variations in the melting layer, drop oscillations, or variations in the drop size distribution or angular distribution of the drop alignment. Three-dimensional effects are also important when observing small-scale heavy precipitation. Despite remaining small uncertainties, the comparison presents strong evidence that the oblate raindrop shape, with fixed horizontal alignment, is by far the better choice for accurate radiative transfer calculations than is the spherical shape. The omission of this shape effect can cause significant errors when developing remote sensing algorithms based on model results.

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Abstract. Measurements of upwelling and downwelling rain brightness temperatures show significant differences between horizontal and vertical polarization, which depend on wavelength and rain rate. These measurements are related to the first and second Stokes parameter. Also, the third Stokes parameter of atmospheric microwave emission will have measurable values under certain conditions. A polarimetric model for the description of the microwave emission and scattering by hydrometeors, including all four Stokes parameters, is presented. This model is applied to the scenario of plane layers of ice crystals and rain above a rough surface. The magnitudes of the third Stokes parameter for microwave emission from oriented nonspherical rain drops and for scattering by ice crystals are estimated. It is shown that the third Stokes parameter for rain is dominated by the first azimuthal harmonic of the emission vector. Finally, an algorithm for the computation of polarized brightness temperatures emanating from cylindrical rain cells above a surface is developed.

Digital beamforming (DBF) has been studied to obtain automatic beam steering towards desired signals and simultaneous elimination of multipath and jamming signals at GNSS receivers, which is made possible by spatial and temporal digital signal processing. In this paper, the limitations of conventional multipath and jamming suppression techniques, which have been proven and widely used in GPS, are investigated. Different DBF algorithms suitable for GNSS applications are investigated theoretically. New ideas for future development of DBF are presented. The implementation of digital beamforming in FPGA/ DSP for practical application environments is also discussed.

A compact navigation receiver comprising a decoupled and matched four-element L1-band antenna array with an inter-element separation of a quarter of the free-space wavelength is presented in this paper. We investigate the impact of the decoupling and matching network on the robustness of the navigation receiver. It is observed that in order to achieve high robustness with a compact antenna array, it is necessary to employ a decoupling and matching network, particularly in case of three spatially separated interferers. Furthermore, we study the influence of the polarization impurity of the compact planar antenna array on the equivalent carrier-to-interference-plus-noise ratio (CINR) of the receiver when impinged with different numbers of diametrically polarized interference signals. It is shown that the higher-order modes possess strong polarization impurity, which may halve the available degrees-of-freedom for nulling in the presence of linear-polarized interferers, using a conventional null-steering algorithm. We verify the robustness of the designed compact receiver by means of a complete global-navigation-satellite-system demonstrator. It is shown that the maximum jammer power that is allowed us to maintain the CINR above 38 dBHz with three interferers can be improved by more than 10 dB if a decoupling and matching network is employed.

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This paper presents a new approach to interference mitigation. We propose to equip a Global Navigation Satellite System (GNSS) receiver with a diversely polarized antenna array in order to combine signal processing in the spatial and polarization domains in a novel way. The new algorithm is evaluated using measurement data, and results show significant improvement in receiver robustness against interferences when the dual-polarization approach is used.Two specific benefits come to light. First, the carrier-to-noise-density ratios of the line-of-sight components are improved since the receiver can make use of the power present on the left-hand circularly polarized (LHCP) channels because of non-ideal polarization purity of the antenna, resulting in better receiver computed position, velocity, and time (PVT) solutions. Second, interference mitigation becomes more effective because of the possibility of filtering in the polarization domain and the additional number of available degrees of freedom, increasing receiver robustness and enabling tracking and PVT in severe interference scenarios.

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