The Maturation of GNSS Reflectometry to Provide Measurements of Essential Climate and Weather Variables

Global Navigation Satellite System Reflectometry (GNSS-R) is a form of bistatic radar that utilizes the constellation of GNSS navigation satellites (GPS, Galileo, ...) as its transmitters and deploys its own customized GNSS receivers to measure their signals after they are scattered by the Earth surface. Modifications to the strength, waveform and propagation time of the navigation signals by the scattering process contain information about the surface’s properties, and these are detected by the receiver. A wide range of surface geophysical parameters can be determined from GNSS-R measurements, including wind speed over the ocean, soil moisture over unfrozen land, freeze/thaw state over land in general, vegetation biomass, and the extent of inland water bodies (e.g. wetlands and flood inundation). Beginning in the late 1990s, GNSS-R sensors have been deployed on towers, high altitude balloons, aircraft and spacecraft, and their use continues to grow, as does the technological sophistication and measurement capabilities of new generations of sensors.

Two key advantages of spaceborne GNSS-R measurements of surface geophysical parameters are noteworthy. One is their relative insensitivity to clouds, precipitation and light to moderate vegetation canopies. This is a result of the low microwave frequencies at which GNSS systems operate. The other is the potential for very frequent global revisit times (of order hours) when constellations of satellites are deployed. This is a result of the low size, mass, power, and cost associated with GNSS-R receivers, which allows for large numbers of small satellites at a reasonable cost. There have been at least 3 spaceborne GNSS-R constellations (CYGNSS with 8 spacecraft, Spire with 5, and Tianmu with 22), and others are in development. There are also disadvantages with the GNSS-R approach, the most notable being: 1) the transmitting signal is not controlled by the remote sensing system so any changes in it must be tracked and accounted for as part of the calibration process; and 2) the location of surface measurements is determined by the relative positions of the transmitting and receiving satellites and so, while predictable, is also not entirely controlled. Many science data products have been developed for spaceborne GNSS-R constellations over the past ~10 years, and considerable effort has been expended to accentuate their advantages and mitigate their disadvantages.

This presentation will provide a summary of the state of the art in spaceborne GNSS-R science data products, focusing on their short term accuracy and precision, their long term stability, their spatial and temporal sampling properties, and their suitability to provide measurements of essential climate and weather variables.