# Significant Wave Height Estimation from Space-Borne Cyclone-GNSS Reflectometry
## 2 Observations and Data Processing  
### CYGNSS Data  
This paper uses CYGNSS GNS-R data to estimate the SWH. The CYGNSS mission was funded by the National Aeronautics and Space Administration (NASA) and launched in 2016 with a constellation of eight tiny low-orbit CYGNSS satellites for the inner wall and eye wall of tropical cyclones, typhoons and hurricanes throughout their life cycle. The nearby ocean surface winds are frequently measured to obtain more data for predicting typhoon intensity. The CYGNSS was fully operational in 2019.  
The Cyclone-GNSS consists of 8 micro-satellites with each weight of about 28.9 kg, which are running at orbit with inclination of 35\({}^{\circ}\) and height of about 475 km. Each CYGNSS satellite is equipped with four Delay Doppler Surveyors (named DDMI), including: three low-noise amplifiers (LNAs) and two left-handed circularly polarized (LHCP) L-band antennas and a right-hand circularly polarized (RHCP) L-band antenna; a delay mapping receiver (DMR) consisting of three RF front-ends and a digital processing unit. The LHCP antenna points to the Earth to receive GPS signals from the specular reflections and scattering points scattered on the ocean surface. The RHCP antenna points to the zenith direction to allow the satellite to directly receive GPS signals to determine its position and velocity. This technique produces data even if the target area is obscured by clouds and storms.  
Due to the roughness of the land or sea surface, the signal received by the receiver includes the energy reflected from the scattering point. The scattered points locate in the area called glistening zone, and its size is related to the sea surface roughness and the height and elevation angle of the receiver. In the glistening zone, since the positions of the GNSS satellites and receivers are constantly changing, each scattering point corresponds to a different Doppler shift and delay, as shown in Figure 1, respectively connecting the same delay point and the same Doppler point forms the iso-Delay line (near ellipse) and the iso-Doppler line (near hyperbola). A 2-D DDM can be obtained by mapping each point of the glistening zone to the Doppler and delay space. The original DDM size of CYGNSS on the star is 128 \(\times\) 20, of which 128 is the time extension and 20 is the Doppler length. In order to compress the transmitted data, the size is converted to 17 \(\times\) 11, as shown in Figure 2, the delay and Doppler resolution are 0.25 chips (1 chip=1/1023000 s) and 500 Hz, respectively. The default Doppler and delay of specular point is 0 chip and 0 Hz, as the reddest grid represents the specular point which has the maximum power, and the less powerful blue area is considered the noise floor.  
Every CYGNSS satellite has four DDMIs to observe four different specular points, so the CYGNSS system can obtain 32 DDM every second ideally. The development of the CYGNSS gives us the first opportunity to estimate the global SWH based on DDM obtained by CYGNSS.  
Figure 1: Bistatic radar scattering map.  
Figure 2: Cyclone-Global Navigation Satellite System (CYGNSS) delayed Doppler map (DDM).