Requirements on the calibration of Hybrid-Compact SAR

Abstract

The state of the art in compact SAR calibration is reconsidered. It is shown that all the existing Compact calibration methods are built on the assumption that the Compact transmits a perfect circular polarization (CP). Unfortunately, the actual technology does not permit the generation of a perfect CP wave. The impact of non circularity of the transmitted polarization on Compact polarization information is assessed. The variations with incidence angles of the helicity and ellipticity of the transmitted polarization are simulated for C-band and L-band Compact SAR using Radarsat-2 and Alos data. This permits quantification of the resulting radiometric error on RH and RV dual-pol measurements. The impact of the non circularity of the transmitted polarization on the degree of polarization (DoP) and Compact polarization synthesis is also quantified. The requirements on the calibration of compact SAR for the extraction of a meaningful polarimetric SAR information from compact, are then setup.

Assessment of RISAT-1 and Radarsat-2 for Sea Ice Observations from a Hybrid-Polarity Perspective

“…At the target, there might be an uncertainty associated with the actual transmitted wave, due to the non-circularity of a HP system in general, which was pointed out in [20] and tested in [14] for RI-1. These uncertainties are associated with, for example, propagation effects, transmitter, and antenna performance [34].…”

“…The HP mode in RI-1 uses right circular transmit and linear horizontal and vertical receive. As pointed out by Touzi et al [20], it is not possible to generate perfect circular polarization using current technology due to the phase errors in combining the horizontal and vertical with 90 • phase difference when transmitting circular waves. Therefore, the transmitted waves in the HP mode of RI-1 will be more elliptical rather than circular, hence the non-circularity property of RI-1 [14].…”

“…The ellipticity angle (χ) of the transmitted wave can be obtained from the axial ratio (AR), which defines the ratio between the major and minor axis from the electric field vector. The AR has been used in, e.g., [14,20] when discussing the non-circularity in general for a HP system. It is well known that an AR of 0 dB indicates perfect circular, while values above 0 dB corresponds to elliptical, and values of infinity is linear polarization [20].…”

“…However, this value was estimated by considering a small range of incidence angles. As highlighted in [20], the non-circularity of the transmitted wave increases with incidence angle [21]. In this study, the simulated complex HP scattering vector is synthesized by considering an AR of 0 dB, which is defined as:…”

Analysis of Circular Polarization Backscattering and Target Decomposition Using Gb-Sar

“…CP radar has several expected benefits compared to LP radar. Touzi discussed that CP is more effective for ship detection at low off-nadir angle because it can directly detect either odd or even bounce back scattering [6]. CP is also known to be insensitive to Faraday rotation effect, where the reference plane of a transmitted microwave rotates as it propagates through the ionosphere of the Earth.…”