*2.1. Data Products*

CYGNSS is within the NASA's Earth System Science Pathfinder (ESSP) program. The original goal of CYGNSS was to further advance extreme weather predictions with a focus on TCs inner core process studies. CYGNSS was designed to resolve the lack of accuracy with current TCs intensity forecasts, which lie in inadequate measurements and modeling of the inner core. The inaccurate measurements result from two main aspects:


CYGNSS was specifically designed to address these two limitations by combining the use of GNSS-R with the sampling properties of a constellation of eight microsatellites [15–19]. CYGNSS provides an unprecedented spatiotemporal resolution of the Earth's surface thanks to the multi-static nature of GNSS-R and the use of such a dense constellation of microsatellites. CYGNSS is the first operational GNSS-R mission launched into space, in 2016.

Since CYGNSS's launch in December 2016, various science data products (Figure 2) have been created, such as an expansion of its wind speed measurements, ocean surface heat flux measurements, and land observations [20]. These scientific data products expand CYGNSS's core mission beyond TCs observations and bolster its reach and influence within the Earth science community. The results presented here focus on some of these products that have been developed and published since launch. All CYGNSS data products available to the public are distributed by the Physical Oceanography Distributed Active Archive Center (PODAAC).

Session 1: CYGNSS Mission Overview and Data Products I

The current v2.1 CYGNSS mission science data products assume that the GPS transmit power is constant. The Climate Data Record (CDR) v1.0 was released in May 2020 to improve product performance. This is the most stable, and most accurate product currently available. Current v3.0 products incorporate real time monitoring of GPS power. At present, all eight CYGNSS spacecrafts are healthy and operating nominally. There was the open call "A.27 CYGNSS Competed Science Team" within NASA Research Opportunities in Space and Earth Science (ROSES) 2020 with the following research topics: Atmospheric River Generation and Development, Wetland Methane Emissions, Dynamic Inland Water Mask Development, Process, Coupling and Feedback Studies, and Weather and Storm Surge Data Assimilation Studies. This ROSES seeks to support the continued use of both the ocean and land data products through scientific investigations and end-user applications.

Several significant advances in ocean research activities were presented by the CYGNSS Science Team. By comparing the CYGNSS measured Mean-Square Slope (MSS) and modified Wave-Watch 3 (WW3) modeled MSS, it was shown that the mean MSS ratio has a dependence on both GPS Pseudo-Random Noise codes (PRNs) and CYGNSS Flight Models (FMs) (starboard and port antennas). A new end-to-end CYGNSS Level 1 calibration approach was proposed in [21] to: (1) improve the data quality of the CYGNSS Level 1 calibration and the Level 2 wind speed and MSS products; and (2) improve our understanding of the impacts of wind-wave and swell-wave on the GNSS-R ocean observations. The detailed performance assessments will be reported in the future. Sensitivity in CYGNSS data to wind direction was demonstrated by computing the kurtosis over areas in the glistening zone of different size and symmetry. CYGNSS raw Intermediate Frequency (IF) data processing at Institute of Space Sciences (ICE)—Institute of Space Studies of Catalonia (IEEC) demonstrated potential applications over land, inland water bodies, and in extreme events (e.g., hurricanes and floods) and regions of great geophysical interest (e.g., Himalayan glaciers). Finally, variational wind speed retrievals of uncalibrated CYGNSS data demonstrated a better response to high winds than the Advanced SCATterometer (ASCAT) radars and similar response as the Soil Moisture Ocean Salinity (SMOS) mission (Figure 3).

**Figure 2.** Data processing flow of the CYGNSS data products [19].

**Figure 3.** Sentinel-1A and -1B wind retrievals for Typhoon "Trami", acquired on Sep 28 9:35 (lower) and Sep 29 9:27 (upper) 2018, approximately 24 h apart (from https://cyclobs.ifremer.fr, 3 April 2021). The colored tracks are CYGNSS passes ~8 h before the first Synthetic Aperture Radar (SAR) wind field image (Sep 28, between 00:50 and 01:26) when the eye was further South. The CYGNSS wind retrievals for each of the tracks, estimated by A. Rius et al. [22], are shown in black dots ("Est" legend) in the panels surrounding the images. The red dots are for European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-5 wind speeds interpolated to the CYGNSS tracks, and Advanced SCATterometer ASCAT-A/B are shown in blue circles when co-located.

Session 2: Data Products II

A CYGNSS L1 land product was developed by the University Corporation for Atmospheric Research (UCAR). This L1 land product [23] contains variables also found in the L1 ocean product, such as position, velocity, time, attitude, and antenna gain, while eliminates variables that were only applicable to ocean applications. The sandbox version of this product is available to all the Science Team members and includes almost all land bodies observed within CYGNSS coverage and a 50 km "skirt" of the ocean; however, very high-altitude regions, such as the Tibetan Plateau are not available due to on-board open loop tracker limitations.

In order to better assist with the CYGNSS wind speed product, several institutions including the National Oceanic and Atmospheric Administration (NOAA) have developed CYGNSS L2 wind speed products, which have been released through the PODAAC. As compared to the original CYGNSS L2 wind speed product, the NOAA version is gridded into 25 × 25 km grid cells, and it uses a custom Geophysical Model Function (GMF) with a track-wise debiasing algorithm. At the time of this meeting, the presenters highlighted some of the minor changes going into v1.1 of the product, such as, e.g., resolving an issue with the star tracker flag, which led to an 18% increase in daily data on some dates. Most importantly, v1.1 of the CYGNSS-NOAA winds have led to improvements in wind speed retrievals within the cores of TCs. This product (Figure 4) was released through the PODAAC during the last fall, with data availability from March 2017 through the present [24].

**Figure 4.** Full day of wind speed observations (m/s) on 30 August 2020 from the NOAA CYGNSS L2 Science Wind Speed 25-km v1.1 product.

An ocean surface heat flux product was released in August 2019, which combines CYGNSS L2 winds with reanalysis data from Modern-Era Retrospective analysis for Research and Applications (MERRA-2) in order to estimate the latent and sensible heat fluxes at each CYGNSS specular point [25]. An update of L2 CDR v1.0 of the CYGNSS ocean surface heat flux product was released in October 2020. As compared to the original version (Science Data Record (SDR) v1.0), the latest version of this product uses the CDR v1.0 L2 winds from CYGNSS, leading to a slightly improved performance as compared to buoy data and improved data availability for the Fully Developed Seas (FDS) versions of these products. Additionally, MERRA-2 variables are now matched in time and space to CYGNSS specular points using a tri-linear interpolation method rather than nearest neighbor, which has removed some biases and fixed issues near land. Similar to the CYGNSS-NOAA product, this product has been released through the PODAAC (Figure 5), with data availability from March 2017 through the present (with a 1-month data release latency) [26].

**Figure 5.** Full day of latent heat flux (W/m2) observations on 30 August 2020 from the CYGNSS L2 Ocean Surface Heat Flux CDR v1.0 product.

NASA JPL serves as the primary data lead for CYGNSS within PODAAC, and continually provides updates to the CYGNSS Science Team regarding data publication. At the time of the meeting, the L1 ocean product remained the most popular CYGNSS product being downloaded, and the CYGNSS usage from October 2019 to May 2020 rebounded to pre-FTP retirement levels. It was also noted that by early 2021, cloud services for select datasets would become operational. Around the time of the meeting, PODAAC's web portal went through a revitalization to improve the user experience. This includes listing

all of the CYGNSS's data products on a single organized page, allowing users to have a better access to the CYGNSS's scientific data products.
