# Hybrid Dual-Polarization Synthetic Aperture Radar
## 2 Provenance
### Historical Overview  
Within the discipline of polarimetric SAR applications, there were several influential early papers on compact polarimetry. The high-level objective of that line of inquiry was to exploit coherent dual-polarized radar data to realize many of the benefits of a quadrature-polarized system without its attendant disadvantages (see Appendix A). Compact polarimetry was proposed as a reasonable strategy when system resources (e.g., power, mass, data volume, and cost) or user demands (e.g., coverage, wide swath, or ScanSAR [10]) precluded full polarimetry. Potential users liked the idea from the outset, but many professionals in the technical radar remote sensing community were quick to find fault.  
One of the first compact polarimetric concepts to appear is the \(\pi\)/4 mode [11], which posits radiating a linearly polarized field at 45\({}^{\circ}\) (with respect to the customary H or V orientation), then receiving the resulting H and V backscatter components coherently. Meteorological polarimetric radarsenjoy a multi-decade heritage with this mode [12; 13], a discipline in which the \(\pi\)/4 mode is known as the slant mode. All modern weather radars routinely employ compact polarimetry.  
The \(\pi\)/4 mode idea subsequently was extended to include circularly polarized transmission [14], with the suggestion that the two types of transmitted polarization should lead to equivalent results. It is now accepted that these two transmit polarizations lead to different results [15; 16]. The most obvious reason for that difference is that slant-polarized polarization remains a single linear polarization. Yes, backscatter can be collected in the H- and V-polarized orientations, but at only half power. That loss cannot be recovered. Like-wise, the backscatter polarization constituent, which is orthogonal to the transmitted linear polarization is not excited.  
Another form of compact polarimetry is to transmit a circularly-polarized field while receiving coherent dual-circular polarizations in response [17]. End-to-end circular polarization has an extensive heritage in Earth-based radar astronomy [18; 19; 20] from which very nice results have been obtained, including extensive precedent for backscatter classifications based on the Stokes parameters [20; 21; 22]. In principle, a space-borne SAR circularly polarized on transmit and dual-circularly polarized on receive would be a viable approach to compact polarimetry. However, such an approach implies additional electronics and hardware, and more stringent constraints on the system to facilitate calibration. Space-based implementation is better served by a simpler approach, having less demanding requirements.  
The hybrid dual-polarized architecture form of compact polarimetry featured in this review satisfies both science and hardware requirements on a spacecraft embodiment. In this hybrid form, circular polarization is transmitted, while orthogonal linear polarizations are received. That architecture avoids the principal undesirable science features of the slant polarization, while also avoiding the hardware implications of an end-to-end circularly polarized radar.