Toward Operational Compensation of Ionospheric Effects in SAR Interferograms: The Split-Spectrum Method

The differential ionospheric path delay is a major error source in L-band interferograms. It is superimposed to topography and ground deformation signals, hindering the measurement of geophysical processes. In this paper, we proceed toward the realization of an operational processor to compensate the ionospheric effects in interferograms. The processor should be robust and accurate to meet the scientific requirements for the measurement of geophysical processes, and it should be applicable on a global scale. An implementation of the split-spectrum method, which will be one element of the processor, is presented in detail, and its performance is analyzed. The method is based on the dispersive nature of the ionosphere and separates the ionospheric component of the interferometric phase from the nondispersive component related to topography, ground motion, and tropospheric path delay. We tested the method using various Advanced Land Observing Satellite Phased-Array type L-band synthetic aperture radar interferometric pairs with different characteristics: high to low coherence, moving and nonmoving terrains, with and without topography, and different ionosphere states. Ionospheric errors of almost 1 m have been corrected to a centimeter or a millimeter level. The results show how the method is able to systematically compensate the ionospheric phase in interferograms, with the expected accuracy, and can therefore be a valid element of the operational processor.

I. Introduction

Ionospheric propagation delay is one of the most relevant error sources in low-frequency spaceborne synthetic aperture radar (SAR) interferograms. SAR interferometry is a successful technique used to measure the Earth's topography and to study geophysical processes such as earthquakes, volcanoes, landslides, and glacier movements. Unfortunately, the accuracy of these measurements is limited by distortions that the ionosphere causes in the propagation of microwaves. In SAR interferograms, topography and ground deformation signals are mixed with ionospheric disturbances [1]. In order to avoid that the ionospheric propagation delay is confused with ground signals, it has to be estimated and removed [2]. The ionospheric distortions on the propagation of microwaves cause an additional group delay and a phase advance on SAR images, whose magnitude is inversely proportional to the frequency of the system. For this reason, L-band SAR systems such as the Advanced Land Observing Satellite (ALOS) Phased-Array type L-band SAR (PALSAR), its follow-up, i.e., ALOS-2, or the future Tandem-L, due to the lower frequency, experience more severe ionospheric distortions compared with C-band or X-band systems.