Towards Sub-daily GNSS-IR Soil Moisture Estimation

Sub-daily soil moisture dynamics are critical for understanding land-atmosphere coupling, however GNSS Interferometric Reflectometry (GNSS-IR) for soil moisture estimation has traditionally been limited to daily temporal resolution. We improve the resolution of GNSS-IR soil moisture estimates using a rolling average (boxcar filter) aggregated at hourly time steps with window lengths of up to 12 hours. This approach produces an apparent diurnal soil moisture signal, however further investigation reveals the dominating presence of a systematic error we term the "sidereal drift artifact."

This artifact arises from the mismatch between the solar day (24 h) and the GPS orbital repeat period, the sidereal day (~23 h 56 m). Each satellite track drifts approximately 4 minutes earlier in local solar time per day, completing a full cycle through all 24 hours in just under a year. Each track samples a distinct spatial footprint characterised by different vegetation density, soil properties, and topography, resulting in systematic inter-track measurement biases. As the subset of tracks contributing to any given time window rotates throughout the year, these spatial biases become aliased into the temporal domain. This behaviour can be observed when processing existing stations worldwide and is additionally shown through the simulation of a synthetic GPS measurement constellation with track specific biases.

We evaluate the performance of our initial methods for mitigating inter-track biases, including pairwise track comparisons and an existing vegetation correction. These approaches show partial success in removing or attenuating the artifact, particularly at Plate Boundary Observatory (PBO) site Marshall (MFLE) in the western United States, where the corrected signal has peak timing estimates consistent with in-situ sensors. We conclude with a discussion of the requirements of sub-daily GNSS-IR soil moisture retrievals and site characteristics that determine vulnerability to sidereal aliasing.