###### Abstract
[18] studied the stratospheric GW activities over the globe and the properties of GW activity during the major sudden stratospheric warming (SSW) events. [PERSON] et al. [19, 20] used the temperature profiles from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) RO mission to investigate the spatial and temporal variations of GW potential energy (Ep) over the northern hemisphere (NH) during the winter seasons and analyzed the interaction of GW Ep with wind and convection. Using the COSMIC RO temperatures, [PERSON] et al. [2] investigated the GW activities above the Southern Andes mountains and Antarctic peninsula during 2006 to 2012. Some other studies concerned the practicability and the methods for deriving the GW momentum flux (MF) from RO data [21, 23, 25]. In these previous GW-related studies based on RO data, the length of the time period is generally less than ten years and the trends in the GW activities were not focused on. On the other hand, using ground-based observations, the long-term (e.g., decadal) variations in GW activities and the possible related impact factors have been studied. [PERSON] et al. [26] studied the relationship between the variations of GWs and zonal wind trends using MF radar wind data at juliusruh (55\({}^{\circ}\) N, 13\({}^{\circ}\) E) during the twenty years from 1990 to 2010. Using the MF radar winds at Hawaii (22\({}^{\circ}\) N, 160\({}^{\circ}\) W), [PERSON] et al. [27] investigated the GW activities during the ten years from 1990 to 2000 and analyzed the influence of strong El Nino events on the variations of GW activities. The correlations between GW activities and the solar cycles have also been investigated using long-term ground-based observations [28, 29, 30, 31]. In these studies, only specific regions are focused on due to the limited geographic coverage of the ground-based observations. For a long time, studying the trends of GWs over the globe was challenging due to the limited temporal extension of the satellite observation data. [PERSON] et al. [9], for the first time, studied the trends in GW activities over the globe during 2002 to 2015, and the responses of GW activities to solar activity, quasi-biennial oscillation (QBO), and El Nino-Southern Oscillation (ENSO) based on the temperature data provided by the satellite mission of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). According to the characteristics of the SABER data, the GW activities in the altitude range from 30 km to 100 km are highlighted in their work. Until now, the trends of the GW activities over the globe at a height lower than 30 km have not yet been investigated. The observations from multi-GNSS RO missions, which are of longer than one decade duration now, can be used for the study of the long-term variabilities of GW activities at the lower and middle stratosphere over the globe. The present work is aimed to derive the trends of the global GW activities at the altitude range of 20-35 km and their responses to solar activity, QBO, and ENSO using GNSS RO data. The remainder of this paper is organized as follows. Section 2 introduces the data and methods, including the GNSS RO missions used for our study, the method for deriving the GW Ep from RO temperature profiles, the quality control scheme in the data processing, and the multivariate linear regression (MLR) method applied for the trend analysis of GW activities. The results and analyses are presented in Section 3, including the spatial and temporal variations of GW activities at 20-35 km, the distributions of the trends of GW activities, and the responses of GW activities to solar activity, QBO, and ENSO. Our discussion on the responses of GW activities at 20-35 km to solar activity, QBO, and ENSO is given in Section 4. Section 5 presents the conclusions.