# Assessment of CYGNSS Wind Speed Retrievals in Tropical Cyclones
## 2 Datasets and Methodology  
### CYGNSS Wind Products  
The CYGNSS mission has been providing ocean surface wind speeds since March 2017. It uses a constellation of eight small satellites with four receivers each. The physical principle of operation is based on GNSS-reflectometry, where the signals from different transmitters within the existing GNSS network from the US Global Positioning System (GPS) are reflected by the wind-roughened ocean surface to the CYGNSS receivers [3; 4]. The narrow ground tracks of the satellites provide daily coverage of the globe between 38S to 38N without major gaps. These noncontiguous ground tracks create a complicated spatial/temporal sampling pattern over the course of a day, which is illustrated in Figure 1a for a sample day. More details on the CYGNSS mission and the science data products are provided in [16]. Wind speed retrievals from the eight CYGNSS small satellites are processed at the SOC and are available in L2 (swath) and L3 (gridded) daily NetCDF files archived on PO.DAAC ([[https://podaac.jpl.nasa.gov/CYGNSS?sections=data](https://podaac.jpl.nasa.gov/CYGNSS?sections=data)]([https://podaac.jpl.nasa.gov/CYGNSS?sections=data](https://podaac.jpl.nasa.gov/CYGNSS?sections=data)) (accessed on 17 September 2021). Figure 1: (**a**) Time of observation (hour of the day, UTC) for the CYGNSS tracks over the full 24 h within a sample day (SDR V3.0 dataset), 13 September 2018. (**b,c**) Observation time from the SMAP satellite for ascending (**b**) and descending (**c**) passes over 24 h, for the same sample day. The figure illustrates the challenges in colocating CYGNSS observations with a sample radiometer, within a short time window of 1–4 h.  
Among the currently available public datasets, some early data records (SDR V2.1 [27] and CDR V1.0 [28]) and the NOAA V1.1 [29] start in spring 2017, and other recent ones (SDR V3.0 [30], CDR V1.1 [31]) only include data from August 2018 until present. The daily Level 2 files include the CYGNSS 10 m winds organized as time-tagged and geolocated satellite tracks' samples corresponding to a 25 km resolution in addition to other important parameters associated with each sample, such as the GPS space vehicle number, the CYGNSS spacecraft and receiver numbers, the viewing geometry, and quality flags. As mentioned in the introduction, two main wind products are included in the SDR files processed at the CYGNSS SOC: the FDS and YSLF winds, crafted for different applications focused on low-to-moderate winds or high winds, respectively. The algorithms are described in detail in [32] and can be summarized as follows: the DDMs for radar cross section and scattering area from the L1B files are first combined to determine two observables, the average DDM (DDMA) and the leading edge slope (LES). The FDS GMF was developed by matching these observables to the MERRA-2 reanalysis 10 m winds. The YSLF GMF was instead developed by matching the CYGNSS measurements to ocean surface wind speeds produced by the NOAA/NCEP Hurricane Weather Research and Forecasting model (HWRF) at a reference height of 10 m. The retrieval of both FDS and YSLF wind products involves the inversion of each respective GMF, given the two observables. The SDR L2 are produced with a maximum latency of 6 days, most recently within 24 h of the observations. The wind retrievals processed by the NOAA team involve additional steps to correct for calibration biases, which are described in detail in [13]. First, the overlapping radar cross-section \(\sigma_{0}\) measurements are averaged over a 25 km grid to homogenize the sampling rate over each track. The NOAA semi-empirical GMF was developed by matching the CYGNSS observed \(\sigma_{0}\) to 10 m winds from the ECMWF model and includes an additional parameter which takes into account the effects of sea state on the ocean surface scattering properties: the significant wave height (SWH) from the Ifremer Wavewatch 3 model [33].