The Accuracy of Faraday Rotation Estimation in Satellite Synthetic Aperture Radar Images

Abstract

Spaceborne linearly polarimetric synthetic aperture radar (SAR) provides information about surface properties and scatterer types by measuring the covariances between images formed using orthogonally polarized radio waves. However, ionospheric Faraday rotation (FR) and system calibration errors alter the balance in the polarimetric channels and distort the covariance matrix. Since FR angles are greater at lower radio frequencies, this paper focuses on the Biomass satellite, which is the European Space Agency's 7th Earth Explorer mission and will carry a P-band (435 MHz) polarimetric SAR. Its primary objective is to measure forest biomass density by combining estimates derived from the polarimetric covariance matrix with estimates based on measuring forest height with polarimetric interferometry and exploiting height-biomass allometric relations. The accuracy of four methods for estimating FR from polarimetric SAR is assessed using simulated images of forest with a range of biomass densities and system errors (H/V channel imbalances, antenna crosstalk, and noise) and over a large spread of FR angles. All methods have biases dependent on the FR, the relative phases of the crosstalk components, and the channel imbalance phase. The best performing method estimates FR to better than 5 • under worst case system errors at all FR angles, so the accuracy of biomass density estimates should not be significantly affected. However, crosstalk phases are predicted to vary greatly across the Biomass antenna beam, and this could potentially cause the FR bias to vary by several degrees across the swath. These effects may occur even for the small values of FR met in L-band data.

The Impact of System Effects on Estimates of Faraday Rotation From Synthetic Aperture Radar Measurements

“…For the published estimators, the best overall performance against a range of metrics (see [13] and [14]) is given by the Bickel and Bates algorithm [5]. In this paper, we therefore exclusively deal with this algorithm.…”

Ionospheric Reconstructions Using Faraday Rotation in Spaceborne Polarimetric SAR Data

“…Assuming δ 1 = δ 2 = δ 3 = δ 4 = δ for convenience [2,4,32], the scattering matrix of Equation (2) [2,5]. We can see that in area of Changbai, Ω F performs the best and can be applied in CIT reconstructions.…”

“…The effects of channel phase imbalance, i.e., f = 1 × exp(iφ), on the FR estimators are first evaluated under typical values [2,5]. By using the full-pol data sets of Changbai and Qingdao, the results are shown in Tables 4-6.…”

“…For a full-pol SAR system at L-band or lower, all the linearly polarized waves in each channel will encounter the ionospheric effects. The measured scattering matrix can then be written as Rogers and Quegan [5]:…”

Co-polarization channel imbalance determination by the use of bare soil

“…The FR can be eliminated by external means, such as a CR and the dual-frequency global positioning system (GPS) measurement total electron content (TEC) (Rogers and Shaun, 2014), and the instrument noise can also be estimated when the PolSAR sensor investigates calm water or shadow. In this research, we mainly focus on the distortion matrices, and the FR/noise is omitted.…”