CYGNSS Observations of Ocean Winds and Waves and Its Potential Utility to NOAA Applications

Near real-time measurements of satellite ocean surface winds, and waves are being widely used in critical operational NOAA forecasting and warning activities. In the last three decades much has been learned about the importance and utility of satellite wind and wave data in operational weather forecasting and warning by exploiting microwave radiometer and scatterometer wind data and altimeter wave data in near real-time. With oceans comprising over 70 percent of the earth’s surface, the impacts of these data have been tremendous in serving society’s needs for weather and water information and in supporting the nation’s commerce with information for safe, efficient, and environmentally sound transportation and coastal preparedness.

The CYclone Global Navigation Satellite System (CYGNSS), launched on December 15, 2016, represents the first dedicated GNSS-R satellite mission specifically designed to retrieve ocean surface wind speeds in the Tropical Cyclone (TC) environment. CYGNSS uses a constellation of eight microsatellite observatories that can receive both the direct and reflected signals from GNSS. To support its CYGNSS calibration/validation responsibilities, the NOAA/NESDIS/Center for Satellite Applications and Research Ocean Surface Winds Science Team developed a comprehensive collocation data set containing remotely-sensed wind and wave products and analysis fields from numerical weather and ocean models. The satellite wind data utilized included products from the ASCAT-A/B scatterometers, and the SSMI-F16/17, GMI, AMSR-2, WindSat and SMAP radiometers. Winds are also obtained from the ECWMF and GDAS numerical weather models. Remotely-sensed significant wave height measurements are obtained from the Jason-2/3, Altika, CryoSat and Sentinel 3 altimeters, while model wave parameters are obtained from the French Research Institute for Exploitation of the Sea (IFREMER) and MeteoFrance implementations of the WaveWatch3 (WW3) models. To monitor the health of the CYGNSS constellation, daily monitoring of satellite parameters as well as measurement observables and retrievals have been established and are posted on the Ocean Surface Winds Science Team web page http://manati.star.nesdis.noaa.gov

Pre-launch, CYGNSS studies have utilized simulated data from the CYTNSS End to End Simulator (E2ES), which assumed the surface slope variances and correlation are completely locally wind-driven, and thus were calculated solely based on the local wind speed and wind direction. Analysis of the actual CYGNSS measurements during the course of the calibration and validation process indicates that this assumption is not valid over a large portion of the measurements range. Specifically, CYGNSS has demonstrated an ability to detect large swell waves in low wind regimes. While the primary objective of the CYGNSS mission is measuring ocean winds in tropical cyclones, examination of collected GNSS-R measurements have shown that the signal is a function of both winds and waves. This paper discusses the CYGNSS capability of measuring both winds and waves over the ocean and its possible impact on NOAA applications.

As noted above, the CYGNSS constellation consists of 8 observatories with two downward looking and one zenith antenna. Our analysis assessed the performance and consistency between all 16 downward pointed antennas. We also validated the CYGNSS performance with respect to the different GPS satellites. The operational GPS satellite constellation commitment is to have 24 operational satellites being available 95% of the time. To accomplish this the U.S. Air Force is currently flying 31 operational GPS satellites. There are 3 distinct satellite series or blocks of GPS satellites (IIR, IIR-M, and IIF). Each of these blocks are different, but as long as accurate knowledge of the transmitter characteristics are known an accurate measurement can be determined. The CYGNSS observatories are capable of collecting four simultaneous reflections each, thus providing high temporal-resolution measurements of ocean surface. The unique sampling capabilities distinguish CYGNSS measurements from current polar orbiting satellites regularly utilized in NOAA operations. The complementary information provided by the CYGNSS observational strategy relative to current operational wind products have been investigated and will be presented in this paper.