# Joint Use of Spaceborne Microwave Sensor Data and CYGNSS Data to Observe Tropical Cyclones
## 3 Data Processing  
In this section, we take Hurricane Florence as an example to demonstrate how to process the aforementioned observation data to fix the center of a TC and to estimate its intensity. Hurricane Florence originated from a large tropical wave that departed off the west coast of Africa on 30 August, 2018 and reached tropical storm status on 1 September 2018. Its wind speed increased to 33.3 m/s on September 9 and weakened to 30.8 m/s on September 15 [33]. To examine the evolving stage of Hurricane Florence, the SMAP radiometer data and the CYGNSS data that were acquired from 8 to 12 September 2018 were employed for TC observations. The best track of Hurricane Florence was provided by the National Hurricane Center (NHC) [33] and this was used for validation, as shown in Figure 3, and its observation region was chosen as 44\({}^{\circ}\) to 80\({}^{\circ}\) W longitude and 20\({}^{\circ}\) to 32\({}^{\circ}\) N latitude. The time-series results for Hurricane Florence, which were obtained over the selected observation region on 10 September 2018 using the SMAP satellite and CYGNSS constellation, are shown in Figure 4. The SMAP radiometer can only acquire two samples over the selected observation area, which correspond to its ascending and descending passes, and the time interval between these two samples is about 12 h. On the other hand, although the SMAP radiometer has a wide swath, it cannot always acquire the complete structure of a TC. For example, as shown in Figure 4b, Hurricane Florence was not captured by the SMAP radiometer. Another example is shown in Figure 4m, where only a fraction of Hurricane Florence was captured by the SMAP radiometer. Since the CYGNSS constellation consists of eight low earth-orbiting microsatellites, the median value of the revisit time is 2.8 h and the mean revisit time is 7.2 h. Furthermore, it can produce 32 specular reflections per second, leading to 32 separate tracks which are 25 km wide and typically hundreds of kilometers long [29]. Thus, compared to the SMAP radiometer, the CYGNSS constellation is able to take more samples (thirteen samples in Figure 4) of a TC over the same region to significantly shorten the observation interval and it can also measure the ocean surface wind speed over a larger observation area. Nevertheless, as can be seen from Figure 4, it is difficult to accurately fix the center of the TC with any CYGNSS sample  
Figure 2: Wind speeds retrieved from version 4.3 of Soil Moisture Active and Passive (SMAP)radiometer Level 2 data products on 12 September 2018.  
because of the sparse tracks of CYGNSS satellite. Therefore, a data fusion method was used to obtain a complete structure of the TC. A flowchart of the data processing is shown in Figure 5, and each step is described below. Figure 4: The time-series results for Hurricane Florence acquired by the SMAP satellite and CYGNSS constellation on 10 September 2018. Subfigures (**a**–**o**) were obtained by the SMAP radiometer or CYGNSS satellites at specific moments. Figure 3: The National Hurricane Center’s (NHC) best track of Hurricane Florence recorded from 31 August to 17 September 2018 [33]. The first step of data processing is to analyze the daily time-series results for TCs obtained during the observation period. In this step, the following three cases were considered:  
* Case 1. If a result is obtained by the SMAP radiometer and it contains a complete structure of a TC or none at all, it is reserved with no further processing, similar to the two examples shown in Figure 6. This is because the observation results obtained by the SMAP radiometer provide a better visualization to analyze the TC position than those acquired by the CYGNSS constellation. * Case 2. If a result is obtained by the SMAP radiometer and contains only a fraction of the TC, it will be fused with several CYGNSS observation results to obtain the complete structure of the TC. Here, the CYGNSS data used for data fusion were acquired around the time when the aforementioned SMAP radiometer results were obtained.