The Potential of Broadband L-Band SAR Systems for Small Scale Ionospheric TEC Mapping
Franz Meyer(1) , Richard Bamler(1)
, Norbert Jakowski(1)
, and Thomas Fritz(1)
German Aerospace Center (DLR),
Ionospheric effects have a significant impact on broadband L-band SAR systems. Range delay, range defocussing and Faraday rotation are most prominent. All effects are a function of the number of free electrons 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. Small scale variations, e.g. scintillation effects, can not be covered by standard algorithms. Methods for studying those small scale turbulences based on GPS measurements exist, but detecting scintillations in GPS data is a binary statistical decision problem which is not easy to solve and suffers from false alarms. In addition, strong amplitude or phase scintillations may cause errors in the GPS data due to loss-of-lock or cycle slips.
In this paper we investigate the potential of broadband L-band SAR systems for ionospheric mapping. As a basis we analyze the effects a broadband microwave signal experiences when traversing through the ionosphere. Starting from that, we estimate the effects that have to be expected in SAR images of varying carrier frequency and bandwidth. As especially low frequency systems are affected by ionospheric effects we quantify them for upcoming space-based L-band SAR systems. This analysis will mainly include the L-band system concepts of TerraSAR-L, ALOS, and HABITAT. Additionally, we propose methods for estimating the electronic content of the ionosphere from SAR data by exploiting the ionospheric effects observed in SAR images. 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 for measuring the absolute electron density in the ionosphere. Estimating the electron density of the ionosphere with high spatial resolution from SAR data will not only contribute to the probing of the atmospheric chemistry, but will also improve the imaging process of L-band SAR systems and the interpretability of L-band SAR interferograms. By estimating the current ionospheric state for every SAR acquisition its influence on SAR imaging and SAR interferometry can be significantly reduced or even eliminated.