This reprint of the Special Issue "BDS/GNSS for Earth Observation” highlights and discusses major aspects of Earth monitoring. Among the most noteworthy works are articles presenting variations in ...the plasmaspheric total electron content (TEC) and correlations between the seismo-ionospheric anomalies of GNSS-TEC and earthquake energy. Those addressing the analysis of the Earth's ionosphere employ two new methods developed for determining the optimal thin layer ionospheric height in the polar regions and for estimating the multi-GNSS differential code bias without using the ionospheric function model. In addition, the troposphere is investigated using a modified interpolation method of multi-reference station tropospheric delay. There are two papers addressing precise orbit determination, employing the Haiyang-2b altimetry satellite and GRACE-FO antenna phase center modeling. Works on GNSS signals are also published, with variations in multi-channel differential code biases from new BDS-3 signal observations, and the modelling and assessment of a new triple-frequency IF1213 PPP with BDS/GPS. Furthermore, GNSS precipitable water vapor (PWV) and its applications are discussed with regard to individual station meteorological data, while three other works examine improvements in iGNSS-R ocean altimetric precision, maritime multiple moving target detection using multiple-BDS radar, and a regional groundwater storage anomaly by combining GNSSs and surface mass load data. Lastly, a review addressing the application of multi-GNSS for Earth observation and its emerging applications is presented.
In the domain of electronic navigation, satellite navigation (GNSS) is one of the most important complex modern systems. GNSS is a key aspect of infrastructure which supports the development and ...improvement of power grid systems, banking operations, global transportation systems, and global communication systems. Today, GNSS requires the use of several positioning networks and sensors, such as radio networks and MEMS. The Earth’s atmosphere, particularly the ionosphere and troposphere, can be seen as a huge laboratory where multiple processes and phenomena directly affecting the propagation of EM waves occur. Like all complex systems, GNSS technology has also gone through certain evolutionary stages. Factors affecting the future evolution of GNSS technology include the appearance of new signals and frequencies, complementary technologies in use, etc., but in the domain of GNSS technologies, it is essential to study the impact of space weather on GNSS systems. A key part of research related to GNSS technologies is the vertical TEC distribution and anomalies related to earthquakes and volcanic eruptions on Earth. There are many challenges that need to be addressed because they affect reliability, accuracy, and all other essential parameters of GNSS systems. It addresses some of these issues by publishing manuscripts which study GNSS risk assessment, different effects of space weather disturbances on the operation of GNSS systems, environmental impacts on the operation of GNSS systems, GNSS positioning error budgets, TEC special features in volcano eruptions, and similar topics.
Currently, with the popularity of smart devices, assured Position Navigation and Time (PNT) is critical for these devices and some fundamental infrastructures, i.e., the power grid. The Global ...Navigation Satellite System (GNSS) is dominant in providing PNT information due to its coverage and high accuracy. However, its signals are weak, and it is vulnerable; multipath and None-Line-Of-Signals (NLOS) are the major errors that occur with regard to the GNSS in applications in urban areas. Advanced signal processing methods are expected to improve its resilience and assurance. In addition, the GNSS is fragile to interference and spoofing, which should be emphasized for unmanned systems and smart devices. This Special Issue aimed to provide a platform for researchers to publish innovative work on the advanced technologies for position and navigation under GNSS signal-challenging or -denied environments.
This reprint of the Special Issue “GNSS-R Earth Remote Sensing from SmallSats” includes a compilation of research articles concerning the pioneering NASA CYGNSS mission and potential future ...constellations of GNSS-R SmallSats for Earth remote sensing. This reprint is composed of different sections, covering various scientific applications such as ocean wind speed, ocean altimetry, inland water bodies, and soil moisture content. The use of GNSS-R techniques by the future constellations of SmallSats could overcome several limitations of more classical remote sensing techniques; for example, passive microwave radiometry and synthetic aperture radar. Present and upcoming GNSS-R missions include BuFeng-1, Fengyun-3 series, Spire Global and Muon Space commercial constellations, ESA’s HydroGNSS and PRETTY, TASA TRITON, etc. The outcome of this reprint aims to leverage state-of-the-art procedures and techniques by present and future mission science teams.
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