## 1 Introduction  
Conventional polarimetric synthetic aperture radar (SAR) that adopts linearly polarized (LP) antennas on the transmitter and receiver, aptly named as LP-SAR, has already proven its powerful classification ability. Besides, an important performance consideration involves the technical trade-off between the full-linear polarimetric (FLP) and dual-linear polarimetric (DLP) modes. The FLP mode provides complete polarimetric information which enhances the target parameter retrieval and polarimetric discrimination. However, inherent limitations such as reduction of swath width, an increase of system complexity, data rate, and power consumption will be compromised. DLP systems, on the other hand, overcome these limiting factors but they do not afford complete information regarding the full polarization state of the targets. For example, it results in a less accurate alpha angle parameter [1,2] and does not give the possibility to perform three-component(surface, double-bounce, and volume scattering) decomposition. In recent years, to circumvent these drawbacks, the idea to use circular polarization or tilted linear polarization in transmission has emerged, so-called compact polarimetric SAR [3; 4; 5]. Three modes have been discussed, i.e., the \(\pi/4\) mode [4], the dual-circular polarimetric (DCP) mode [5], and the circular transmit while linear receive (CTLR) mode [3]. These different polarimetric SAR modes are categorized as shown in Figure 1.  
More recently, a SAR campaign that utilizes circular polarization in both transmission and reception has been proposed (end-to-end circular polarization) and studied for spaceborne and airborne missions [6]. Although this unique circularly polarized SAR (CP-SAR) system requires almost ideal CP-antennas (i.e., 0 dB of the axial ratio (AR)), which may often be difficult to satisfy, recent advances in antenna technologies can acceptably fulfill this requirement (e.g., AR of 1.1 dB is achieved for RISAT-1) [7]. It is also worth noting that the following advantages of CP-SAR usually outweigh this difficulty in antenna requirement. Circular polarization is already known to be less affected by the Faraday rotation which is a significant problem, especially for lower frequency bands (i.e., L and P) [8]. To be specific, the Faraday rotation effect in the ionosphere does not alter the transmitted polarization which is not the case in linear polarization [9]. Less effect of the interference between direct and reflected signals due to multipath propagation is also expected [6]. In addition, being one of the compact polarimetric modes, the DCP mode of CP-SAR yields more abundant polarimetric information than those of the DLP mode. Furthermore, [PERSON] et al. and [PERSON] et al. highlighted that the DCP mode is the most suitable configuration among compact polarimetric modes to apply the entoropy-alpha (\(H/\bar{\alpha}\)) decomposition when this decomposition is performed on a 2 \(\times\) 2 coherency matrix [10; 11]. Despite its usefulness, however, CP-SAR has not been practically exploited in Earth observation, and many studies on compact polarimetry reconstruct the compact data by converting from LP-SAR data (i.e., simulated circular polarization data). Among polarimetric SAR practices, rice monitoring has been one of the important applications since rice is a staple food for almost half the world's population. A less-explored topic in this field is the rice phenology retrieval from the time-series SAR data. This information is of great importance in planning of cultivation practices, yielding estimation and water management. Various analyses of rice monitoring from polarimetric SAR data have been addressed for this purpose [12; 13; 14; 15; 16]. Although the classical LP-SAR is the most commonly utilized tool for this task, current studies have been focusing on the compact SAR modes because of their advantages mentioned above [12; 14]. Figure 1: Categorization of various polarimetric synthetic aperture radar (SAR) modes.