Beyond SMAP: Can GNSS‐R Platforms Ensure Continuity of Global Soil Moisture Observations?

Abstract

The absence of a definitive follow‐on strategy for the Soil Moisture Active Passive (SMAP) mission, which has already surpassed its planned mission lifetime, presents a significant challenge for the hydrological science community. GNSS‐Reflectometry (GNSS‐R), which exploits reflected satellite navigation signals, has emerged as a rapidly growing alternative for soil moisture retrieval, increasingly positioned as a low‐cost path to global observation. The two approaches, however, differ fundamentally in how they sense the surface, and the hydrological science community should weigh these differences carefully before decisions are made that could compromise the continuity of this essential climate variable's data record. In this study, we compared soil moisture products from these two systems across seven perspectives: (a) observation characteristics, (b) spatial resolution, (c) coverage and frequency of observations, (d) vulnerability to radio frequency interference, (e) complementarity potential, (f) performance, and (g) cost considerations. Our assessment suggests that GNSS‐R is better understood as a complement to SMAP than as a replacement. A GNSS‐R constellation can deliver sub‐daily observations, and lower upfront costs, capabilities that genuinely extend what single‐platform missions can offer. At the same time, GNSS‐R retrievals exhibit variable spatial sampling and canopy attenuation, and the highest‐performing products are typically trained, calibrated, or benchmarked against SMAP (an implicit cost that the low‐cost framing of GNSS‐R does not fully capture). Taken together, these findings underscore the need for sustained investment in both SMAP‐class L‐band missions and the expanding GNSS‐R constellation, each contributing complementarity strengths to a resilient global soil moisture observing system.