Measurement of Ionospheric Faraday Rotation in Simulated and Real Spaceborne SAR Data

The influence of the atmosphere on a frequency-modulated electromagnetic wave traversing the ionosphere is becoming increasingly important for recent and upcoming low-frequency and wide-bandwidth spaceborne synthetic aperture radar (SAR) systems. The ionized ionosphere induces Faraday rotation (FR) at these frequencies that affects radar polarimetry and causes signal path delays resulting in a reduced range resolution. The work at hand introduces a simulation model of SAR signals passing through the atmosphere, including both frequency-dependent FR and path delays. Based on simulation results from this model [proven with real Advanced Land Observing Satellite Phased Array L-band Synthetic Aperture Radar (PALSAR) data], estimation of FR in quad-polarized SAR data using the given approach is shown for raw, range-compressed, and focused radar images. Path delays and signal chirp bandwidth effects are considered. Investigations discuss the suitability of raw and compressed data versus combination of total electron content maps with the Earth's magnetic field for FR estimation and deduced from a large number of analyzed PALSAR data sets.

I. Introduction

With the successful launch of the Advanced Land Observing Satellite (ALOS) and the on-board Phased Array L-band Synthetic Aperture Radar (PALSAR) instrument, spaceborne synthetic aperture radar (SAR) data at L-band have become available with a relatively wide bandwidth. Depending on the acquisition mode, the sensor's range chirp bandwidth can be as high as 28 MHz. However, in high solar conditions, ionospheric path delays and Faraday rotation (FR) become significant for wide-bandwidth SAR applications [1]; the use of large chirp bandwidths is susceptible to signal degradation that can result in a suboptimal resolution, and FR may distort or even destroy important information otherwise available from polarimetric SAR data.