Error Evaluation of L-Band InSAR Precipitable Water Vapor Measurements by Comparison with GNSS Observations in Japan

Keita Matsuzawa,Yohei Kinoshita

doi:10.3390/rs13234866

Abstract

Interferometric synthetic aperture radar (InSAR) enables us to obtain precipitable water vapor (PWV) maps with high spatial resolution through the phase difference caused by refraction in the atmosphere. Although previous studies have evaluated the error level of InSARPWV observations, they validated it only with C-band InSARPWV observations. Since ionospheric disturbance seriously contaminates the InSAR phase in the case of the lower-frequency SAR system, it is necessary for a PWV error level evaluation correcting the ionospheric effect appropriately if we use lower-frequency SAR systems, such as the Advanced Land Observing Satellite-2 (ALOS-2). In this paper, we evaluated the error level of the L-band InSARPWV observation obtained from ALOS-2 data covering four areas in Japan. We compared the InSAR observations with global navigation satellite system (GNSS) atmospheric observations and estimated the L-band InSARPWV error value by utilizing the error propagation theory. As a result, the L-band InSARPWV absolute error reached 2.83 mm, which was comparable to traditional PWV observations. Moreover, we investigated the impacts of the seasonality, the interferometric coherence, and the height dependence on the PWV observation accuracy in InSAR.

Highlights

Water vapor in the atmosphere plays an important role in weather and climatic processes [1,2]
The estimated standard deviation of residuals was 7.36 mm, which was used to estimate the observation error of precipitable water vapor (PWV) derived from Interferometric synthetic aperture radar (InSAR) data, which was found to be
We investigated the dependences of the observation season and altitude and found that the standard deviation of the residual increased in the wet season, which may be due to the high variability and a larger amount of water vapor in this season

Summary

Introduction

Water vapor in the atmosphere plays an important role in weather and climatic processes [1,2]. Interferometric synthetic aperture radar (InSAR) has been used to measure precipitable water vapor (PWV) maps with high spatial resolution (a few tens of meters) (e.g., [8,9,10,11,12]). When microwaves emitted from a satellite travel through the neutral atmosphere, the phase velocity of the microwave is changed, and the travel path is bended due to having a different refractive index to the vacuum. This effect is called the neutral atmospheric propagation delay effect. In the case of InSAR, water vapor plays a dominant role in the neutral atmospheric propagation delay effect due to its high variability in both time and space. In the case of the standard strip map observation mode, the effect of the dry atmosphere is smaller than that of the wet atmosphere [13]

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Conclusion