# Joint Use of Spaceborne Microwave Sensor Data and CYGNSS Data to Observe Tropical Cyclones
## 5 Discussion
It is worth mentioning here that the wind direction parameter included in the SMAP radiometer data is not added to estimate the center position of TCs at this stage because this parameter has not been provided in the CYGNSS data yet. The errors produced by fusing the SMAP radiometer and CYGNSS data to track TCs are mainly due to the following reasons. (1) Although a complete structure of the TC has been included in the final fusion image, it is still difficult to determine the center location of the TC because of the large high-speed region presented in CYGNSS fusion results, such as in Figure 9(b1). (2) Since the size of a TC eye can vary from a few to tens of kilometers, even up to hundreds of kilometers [40], it is difficult to accurately fix the center of the TC with a point that is the center of black circle and which was used to form the estimated track of the TC in this study. (3) In terms of the shape of the high wind-speed region and the movement speed of the TC, the automatic TC center-fixing algorithm based on visual saliency and the inflow angle model were applied to the SMAP and CYGNSS fusion results. However, we found that this algorithm introduced errors in the estimation of TC's center position because sometimes the extracted skeleton line of the TC cannot be well-matched with the inflow angle model. Besides the aforementioned factors, changes in the spatial-temporal resolution of the observation data can also affect the accuracy of the TC track estimation results, as reported by [PERSON] et al. [41]. On one hand, observation data with a high temporal resolution are very important for the estimation of the TC track during the rapidly evolving stages of a TC's life cycle. As shown in Figure 4, the joint use of SMAP radiometer and CYGNSS data significantly improves the temporal resolution compared to the case in which only the SMAP radiometer data are used. However, when the SMAP radiometer and CYGNSS data were fused to obtain a complete structure of the TC to determine its center position, the temporal resolution was reduced to a large extent. On the other hand, observation data with a high spatial resolution are also very helpful for the estimation of the TC's center position because the fine structure of TCs can be obtained. However, since the grid size of the SMAP radiometer and CYGNSS data used in our study is fixed, more attention was paid to the spatial coverage to obtain a more complete structure of the TCs. As can be seen from Figure 9, the observation data with larger spatial coverage can improve the accuracy of fixing the center position of TCs. When our method was used to measure the maximum wind speeds of the ten hurricanes listed in Table 2, intensity errors were produced mainly due to the following reasons. (1) Sometimes a complete structure of the TC was not included in the CYGNSS fusion results due to the narrow and sparse tracks of the CYGNSS satellite, as well as its limited spatial coverage [42]. As a result, the maximum wind speed area was not always sampled by the CYGNSS constellation, for example, Hurricane Lane. (2) Some of the observed hurricanes, such as Hurricane Harvey, made landfall during the observation period. In other words, the maximum wind speed areas were on land; however,both SMAP radiometer and CYGNSS satellites can only measure the ocean surface wind speed, as mentioned above. (3) Compared with the moored buoy data, the ocean surface wind speed retrieval algorithms adopted for the SMAP radiometer data [31] and also the CYGNSS data [43] can introduce errors, especially for wind speeds above 20 m/s. Besides the above factors, the selection of appropriate spaceborne microwave sensor data can also affect the accuracy of the maximum wind speed measurements. Although various spaceborne microwave sensors have been developed to observe the TCs, however, many of them cannot measure Category 1 hurricanes and more powerful storms except for the SMAP radiometer, the Soil Moisture and Ocean Salinity (SMOS) radiometer [44], the CYGNSS satellite, and some spaceborne SARs [45].