###### Abstract  
The Global Navigation Satellite System (GNSS) has made important progress in Earth observation and applications. With the successful design of the BeiDou Navigation Satellite System (BDS), four global navigation satellite systems are available worldwide, together with Galileo, GLONASS, and GPS. These systems have been widely employed in positioning, navigation, and timing (PNT). Furthermore, GNSS refraction, reflection, and scattering signals can remotely sense the Earth's surface and atmosphere with powerful implications for environmental remote sensing. In this paper, the recent developments and new application progress of multi-GNSS in Earth observation are presented and reviewed, including the methods of BDS/GNSS for Earth observations, GNSS navigation and positioning performance (e.g., GNSS-PPP and GNSS-NRTK), GNSS ionospheric modelling and space weather monitoring, GNSS meteorology, and GNSS-reflectometry and its applications. For instance, the static Precise Point Positioning (PPP) precision of most MGEX stations was improved by 35.1%, 18.7%, and 8.7% in the east, north, and upward directions, respectively, with PPP ambiguity resolution (AR) based on factor graph optimization. A two-layer ionospheric model was constructed using IGS station data through three-dimensional ionospheric model constraints and TEC accuracy was increased by about 20-27% with the GIM model. Ten-minute water level change with centimeter-level accuracy was estimated with ground-based multiple GNSS-R data based on a weighted iterative least-squares method. Furthermore, a cyclone and its positions were detected by utilizing the GNSS-reflectometry from the space-borne Cyclone GNSS (CYGNSS) mission. Over the years, GNSS has become a dominant technology among Earth observation with powerful applications, not only for conventional positioning, navigation and timing techniques, but also for integrated remote sensing solutions, such as monitoring typhoons, river water level changes, geological geohazard warnings, low-altitude UAV navigation, etc., due to its high performance, low cost, all time and all weather.  
GNSS; geohazards; GNSS meteorology; GNSS ionosphere; GNSS-reflectometry 25 October 20243 December 20243 December 20243 December 20243 December 20243 December 20243 December 20243 December 202433 December 202433333334  
that major emerging applications such as those arising from, for example, GNSS-R Remote Sensing with SmallSats (BuFeng-1, CYGNSS, Fengyun-3C, FormoSat-7/COSMIC-2 (FS7/COSMIC2), FSSCat, HydroGNSS, PRETTY, and the Spire CubeSats series) might be of interest for the new applications, considering that remote sensing capabilities relative to the Earth are being tested and applied by these new satellites through the use of advanced payload technologies. The ECOSTRESS and SMAP satellites, for instance, were mainly designed to acquire information on soil moisture in areas covered by dense forests, representing a significant advancement in this developing field [3].  
In this paper, the most recent results and the state of the multi-GNSS art for Earth observation are presented and reviewed, including but not limiting to, the following topics: tropospheric and ionospheric observations; modelling and assimilation from terrestrial and space-based GNSS observations; multi-GNSS NRTK, PPP, and PPP-RTK theories and methods; analysis of errors; systematic effects in the new GNSS solutions; and GNSS reflected signals and radio occultation, as well as the new applications for these technologies. Section 2 shows the technique and methods of BDS/GNSS for Earth observations, recent developments and new progress are presented in Section 3, and finally conclusions and future prospects are given in Section 4.