Mitigation of Ionospheric Noise in Azimuth Offset Based on the Split-Spectrum Method

Abstract

The pixel-offset method has been utilized as a powerful tool to measure large ground movements. However, L-band spaceborne synthetic aperture radar (SAR) data are often affected by the ionosphere, which produces serious noises in the azimuth component of the pixel offset field, called as azimuth streaks. Here, we propose a new method to mitigate azimuth streaks based on physical modeling. Azimuth streaks cannot be removed by simply combining the known relationship between ionospheric azimuth offset and the ionospheric phase delay with the phase delay obtained by the split-spectrum method. Thus, taking into account that image matching (coregistration) affects the measurement of azimuth offsets, we formulate a theoretical correction formula of azimuth streaks by subtracting the coregistration-induced effects approximated by a polynomial function from ionospheric azimuth offsets modeled using the split-spectrum method. Applying the method to two pairs of Advanced Land Observing Satellite-2 (ALOS-2)/Phased Array type L-band Synthetic Aperture Radar-2 (PALSAR-2) stripmap images which are severely affected by the ionosphere, we demonstrate effective mitigation of azimuth streaks. In the application to the case that no significant ground movement is detected by Global Navigation Satellite System (GNSS), while the standard deviation of azimuth pixel offset before the correction is 87.2 cm, the value after the correction is 29.2 cm, which is comparable to the theoretical measurement accuracy of the azimuth pixel offset. In the application to the 2016 Kumamoto earthquake, we substantially reduce the azimuth streaks and successfully extract the ground movements from the azimuth offset fields within an accuracy of about 20 cm. The result suggests the proposed method enables more accurate and operational estimation of the 3-D ground displacement.

Three-dimensional deformation and its uncertainty derived by integrating multiple SAR data analysis methods

“…Another significant noise source in azimuth SBI is ionospheric azimuth shift, which may reach several meters (Wegmüller et al 2006;Kim 2013). While mitigation of the azimuth shift is possible to some extent using ionospheric phases in the range estimated from the SSM, it is difficult to completely correct short-wavelength streaks (Liang and Fielding 2017;Yamashita et al 2021).…”

“…The most common approach is to set the reference area outside the deformation zone. Another point that needs to be carefully considered is the long-wavelength noise caused by orbit or coregistration errors, which can be approximated by a polynomial ramp (Hanssen 2001;Fattahi and Amelung 2014;Yamashita et al 2021). This ramp is often estimated and removed by flattening the area without deformation.…”

“…In particular, the ascending orbit (observed around midnight in local time) in summer often contains considerable azimuth streaks because medium-scale traveling ionospheric disturbances (MSTIDs) are highest during summer nights around Japan (Tsugawa et al 2007;Morishita 2020). The azimuth ionospheric noise correction based on SSM (Yamashita et al 2021) was, therefore, applied to reduce the noise to some extent, although significant noise (especially short-wavelength components) remained in some cases (Additional file 1: Fig. S2).…”

Challenges and Prospects to Time Series Burst Overlap Interferometry (BOI): Some Insights From a New BOI Algorithm Test Over the Chaman Fault

“…Currently, the correction of azimuth ionospheric errors is mainly based on split spectrum methods and filtering methods (e.g., directional filtering and frequency domain filtering), which are commonly used for along-track monitoring of ALOS-1/2 SAR operating in the low-frequency band of ~300 Hz [70][71][72][73][74], but the ionospheric errors of BOI along-track monitoring based on Sentinel-1A/B SAR data which operates in high-frequency up to ~5000 Hz is hard to handle with. In this context, Lazecky and Hooper proposed to use IRI2016 global model to mitigate ionospheric errors for each pixel in burst overlap regions [24].…”