An Investigation of Systematic Bias in ALOS-2 Multilooked Interferograms

Interferometric synthetic aperture radar (SAR) measurements are affected by systematic bias, referred to as “fading signal,” when explicitly using temporally short-term and spatially multilooked interferograms. Investigations using extensive time-series data from the Sentinel-1 C-band SAR have identified soil moisture and biomass changes as potential causes of the observed biases. Although phase biases are supposed to increase with increasing wavelengths, detailed studies on the biases observed in ALOS-2 L-band SAR have been limited owing to restrictive data distribution policies. We therefore investigated the systematic bias in ALOS-2 multilooked interferograms across various interferometric pairs and land-cover types over 5 years of its observation. With regard to the behavior of the bias for positive and negative variations in soil moisture, our results demonstrated consistency with simulations performed using an interferometric model for soil moisture. The observed bias was >4 mm/yr for an average temporal baseline of 72 days. Our research suggested by using L-band SAR that the observed bias can be attributed to changes in both soil moisture and biomass. Furthermore, we developed a methodology to mitigate the systematic bias in ALOS-2 multilooked interferograms. This methodology utilizes our findings that a noise-filtering technique proposed by Pepe et al. (2015) can overcome these biases through the utilization of medium-term interferograms and redundant triplets, regardless of the land-cover type.