Sensing the tropospheric water vapor from grazing-angle Spaceborne GNSS-R

In this work, we investigate a novel method to estimate the tropospheric wet delay, and further the vertically integrated water vapor (VIWV) and its horizontal gradients, using the coherent-reflection GNSS signals received by a CubeSat in the low-Earth orbit (LEO).  It can complement existing observation approaches over some polar and ocean regions where GNSS signals can be coherently reflected.

The precise altimetry using coherent-reflection GNSS signal carrier phase measurements has gained popularity over the past couple of decades for the observation of ocean, sea ice, lake, and river surfaces.  The troposphere delay error is found to be a major error source for GNSS-R phase altimetry, especially at a low elevation angle.  However, if we can model the reflection surface elevation relatively well, then the estimated residual phase from GNSS-R signal can be dominantly contributed by the mis-modeled tropospheric wet delay, and GNSS-R signal can become a new data source for tropospheric water vapor sensing.  The GNSS-R approach can observe the horizontal gradients of VIWV along the specular point (SP) track, as the SP moves at high speeds of ~5 km/s, and the spatial resolution is 10s of km by ~1 km.

In the presentation, we will provide examples using Spire Global’s grazing-angle GNSS-R data and comparisons with the ECMWF reanalysis VIWV data.  We will also discuss the applicable regions, performance, and error mitigations of the proposed method in estimating tropospheric wet delay and issues to be addressed in the further retrieval of VIWV.