Toward Sustained L-Band Soil Moisture and Wetland Monitoring with the Growing Spire GNSS-R Nanosatellite Constellation

Abstract:

Spire Global operates the world’s largest and rapidly growing constellation of CubeSats (nanosatellites) performing GNSS based science and Earth observation. The Spire constellation, performs a variety of GNSS science, including radio occultation (GNSS-RO), ionosphere and space weather measurements, and precise orbit determination. In December 2019, Spire launched two new satellites to perform GNSS reflectometry (GNSS-R) primarily for soil moisture remote sensing, with plans to launch two more follow-on GNSS-R satellites in 2020. GNSS-R is a relatively new technique based on a passive bistatic radar system that observes L-band GNSS signals (e.g., GPS) reflecting from the Earth surface. The potential of space-borne GNSS-R observations for ocean and land applications has been previously demonstrated by the NASA Cyclone Global Navigation Satellite System (CYGNSS) and the UK’s Technology Demonstration Satellite, TechDemoSat (TDS-1). Although it was primarily designed for ocean remote sensing, CYGNSS has demonstrated remarkable soil moisture and land surface water remote sensing capabilities, and Spire has plans to operationalize GNSS-R observations for long-term, sustained Earth surface monitoring.

We will present the GNSS-R satellite missions and initial results from the first two Spire GNSS-R satellites that were launched serendipitously to overlap CYGNSS observations and are primarily focused on retrieving soil moisture (but can also estimate other Earth surface properties, such as land surface water and wetlands mapping and ocean wind speeds). Prior to the launch of Spire’s GNSS-R satellites and in preparation for operational data production, we developed algorithms and processing chains for a CYGNSS-based soil moisture product which is now being extended with measurements from the Spire GNSS-R satellites. Additionally, work has begun on surface water mapping products derived from GNSS-R data since L-band forward scattering is capable of penetrating vegetation canopies overlying some wetlands. We will conclude with plans to continue growing the Spire GNSS-R constellation to provide a sustained source of data for monitoring the hydrological cycle.