Thanks to the beam flexibility and the polarization capability of the polarimetric phased array radar (PPAR), it has attracted a lot of attention in weather observation, air traffic control, and air surveillance [1
]. However, as one of the major obstacles, the requirement of high accuracy for polarimetric measurement still slow the use of the PPAR technology [3
]. Specifically, the significant cross-polarization component is produced for the PPAR, which is caused by beam steering off the principal plane [1
]. Hence, the polarization measurement bias is generated, while resulting in the bias of the polarization variables including the differential reflectivity (
), the linear depolarization ratio (
), and the differential phase
. Therefore, it is crucial to mitigate the bias resulting from the cross-polar or the cross-polar itself, thereby realizing the potential of the PPAR.
The sequential enthusiasm and efforts are poured into the bias correction methods. The bias correction technique for polarization measurement in PPAR was firstly suggested in [1
], simulated in [5
], and further updated in [9
], and demonstrated in [2
] using the large-scale testbeds. To be specific, the bias correction is done either to the measured polarization scattering matrix (PSM) or to polarimetric variables [1
]. Nonetheless, most of the bias correction methods focus on the narrow-band issue. An iterative frequency division method is proposed for the wideband PPAR in [11
], which expands the projection matrix correction method in [1
]. Moreover, the correction matrix based on the theoretical pattern is not necessarily feasible due to the distortion of the radiation pattern, which results from mutual coupling and the edge diffraction between antenna elements. To reduce the effect of non-ideality, a reconstruction technique for the antenna pattern is proposed to correct the polarization measurement bias [12
]. However, the source of bias still exists, and the bias increases with the larger of the beam scanning angle deviation.
To address the problem, the polarization state configuration (PSC) technique is an alternative strategy to reduce the cross-polarization level. The polarization state can be described with the Poincaré sphere [14
]. Each point on the surface of a unit sphere represents each polarization state. The magnitude ratio and phase difference of two orthogonal components of electromagnetic field vectors are mapped to the sphere’s surface. In the practical application, the agile beam of the PPAR will be scanned to a wide steering direction. The polarization state with lower cross-polarization is superior for a given direction, so the characteristics of amplitude and phase in the dual-polarization antenna should be modulated appropriately. Moreover, the requirement of the cross-polarization level for the accurate polarimetric measurement is strict for the Alternate Transmission and Simultaneous Reception (ATSR) mode (
dB). In addition, the parameter requirement is
dB for the Simultaneous Transmission and Simultaneous Reception (STSR) mode [10
]. Therefore, the polarization state is necessary to be modulated optimally to reduce the cross-polarization level, especially for the polarization measurement applications.
To suppress the cross-polar component, a PSC method is proposed to obtain the desired polarization state. The motivation is to reduce the cross-polarization level dramatically and make the co-polarization at an acceptable level. Each beam direction corresponds to a type of configured polarization state. The modulation operation depends on the beam direction and is equivalent to finding the optimal point on the Poincaré sphere. Therefore, the problem is formulated as a minimum problem under some parameter constraints, while achieving the lower cross-polarization. The magnitude and phase of two polarized ports are modulated, thereby denoting that the radiated wave is not the linearly polarized wave but the elliptically polarized wave, including the right-hand elliptical polarization (RHEP) and left-hand elliptical polarization (LHEP). We assume that the RHEP is the co-polarization component, and the LHEP is the cross-polarization component. This pair of orthogonal vectors are antipodal points on the surface of a unit sphere. The structured polarization states are achieved according to the CPI requirement. The simulated results and comparisons illustrate the priority of PSC with respect to the fixed linear polarization (LP) and circular polarization (CP). Furthermore, the polarization variables are used to assess the performance of the PSC method on the polarization measurement application. The results reveal that the value of and could meet the accuracy requirement for the polarization measurement.
In Section 2
, the radiation model with elliptical polarization (EP) wave is developed. The corresponding radiation model is deduced based on the crossed dipoles antenna, and the constrained nonlinear multivariable function is formulated. In Section 3
, the comparisons between radiation patterns of the linear, circular, and elliptical polarization waves are illustrated. The performance and priority of the elliptical polarization are analyzed based on the radiation pattern and polarization variables. Conclusions and discussions are presented in Section 4
In this paper, an effective polarization state configuration method has been proposed to reduce the cross-polarization level and enhance the CPI in a wide beam scan range of the PPAR antenna. The amplitude and phase of the dual-polarization elements are modulated for each beam direction according to the CPI requirement. In contrast to the LP and CP state, the radiated polarization wave is EP state, and it is received by using a set of elliptical polarization basis. For the defined co-polarization and cross-polarization components, the PSC technique is formulated as a minimum problem of CPI, thereby achieving the available amplitude ratio and phase difference. This method is beneficial for reducing the polarization measurement bias even in a wide scan angle. Moreover, we take a dual-polarization dipole antenna, for example, to test the priority of the proposed method. The biases of the polarization variables and are less than dB and dB, respectively, in most of the beam directions within . Numerical simulations reveal that the obtained CPI could meet the requirement of the polarization measurement for PPAR.