The Potential of Broadband L-Band SAR Systems for Small Scale Ionospheric TEC Mapping

Abstract

Ionospheric effects have a significant impact on broadband L-band SAR systems. Range delay, range defocussing and Faraday rotation are the most prominent. All effects are a function of the number of free electrons in the ionosphere, counted along the the ray-path of the SAR signal. Methods based on 2-frequency GPS observations allow to measure the number of free electrons in the ionosphere with a high temporal resolution. The spatial resolution however is coarse due to the low density of GPS ground stations. In this paper we investigate the potential of broadband L-band SAR systems for ionospheric mapping. As a basis, ionospheric effects on traversing broadband microwave radiation are theoretically derived and analyzed. Starting from that, we estimate the effects imposed on SAR images for a variety of carrier frequencies and systems bandwidths. As the ionosphere especially affects low frequency systems, our main emphasis is put on the analysis of upcoming spaceborne L-band SAR systems. Additionally, we emphasize the idea to make use of the high sensitivity of L-band SAR systems with respect to the ionosphere for estimating its electron content from SAR data. In particular, we propose two different algorithms: i) Using a geolocation procedure to exploit the differential range shift between observations with different ionospheric conditions; ii) Analyzing the spread of the impulse response function in range direction, in order to measure the absolute electron density in the ionosphere.