GNSS-Reflectometry for Ocean Remote Sensing

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative and rapidly developing approach to Earth Observation that makes use of signals of opportunity from Global Navigation Satellite Systems (GNSS), which have been reflected off the Earth’s surface. GNSS-R is particularly promising as it does not require a dedicated transmitter, thereby reducing mass and power requirements of the instrument, which gives an opportunity to build a constellation of low-cost sensors providing short revisit times and unprecedented sampling capabilities.

Ocean data has been collected regularly from GNSS-R instruments since 2014 from technology demonstration missions, e.g. TechDemoSat-1 (TDS-1, 2014 – 2018) and DoT-1 (2019 – Present), as well as operational missions, e.g. CyGNSS (2016 – Present). These missions have had different aims and setups, providing different perspectives on the capabilities of GNSS-R. TDS-1 and DoT-1 are polar orbiting, allowing the additional collection of data over sea-ice, where GNSS-R has shown strong signal sensitivity in coherent scattering conditions. The CyGNSS mission is the first GNSS-R constellation and consists of eight small satellites in an orbit that provides revisit times of only a few hours between the latitudes of ±35º, achieving a much higher sampling rate and faster revisit than TDS-1. Additionally, DoT-1 demonstrates onboard processing of signals originating from both GPS and Galileo satellites. The strong signal sensitivity to geophysical parameters such as ocean wind speed, when using signals from the Galileo system, shows the potential for further improvement in sampling when using signals from multiple Global Navigation Satellite Systems.

Both the CYGNSS and TDS-1 missions have shown consistent performance in the retrieval of ocean wind speed once instrument calibration steps have been taken to mitigate some of the challenges inherent to GNSS-R technology. However, a number of geophysical variables theorised to be impacting the GNSS-R observables were investigated over a range of ocean wind speeds. The major dependencies affecting wind speed retrieval appear to be significant wave height and precipitation, which have their greatest impact at low wind speeds, with sea surface temperature having a weaker impact but across all wind speeds. These geophysical dependencies, additional to wind speed, can have a significant impact on retrievals and may need to be isolated for accurate retrievals.  

GNSS-R has shown strong capabilities for ocean remote sensing of multiple variables, using platforms and instruments that are still advancing to most effectively utilise the technology. A number of such advancements will be employed in the forthcoming ESA HydroGNSS mission, where the National Oceanography Centre (NOC) are leading the development of ground segment processors for Level-1 signal calibration and Level-2 ocean surface wind speed and sea-ice extent products.