Earth Science covers sciences related to understanding the planet Earth, being the use of GNSS signals most relevant on the aspects of Earth physics (as opposed to chemistry or biology). The main fields of this science that are studied using GNSS as tools are:
- Geodesy and Geodynamics;
- Remote Sensing Using GNSS-R;
- Physics of the Ionosphere and Troposphere.
On Geodesy and Geodynamics, GNSS is an essential tool for Geodesy because provides to the scientists a stable reference frame based on the accurate knowledge of the orbits of the GNSS satellites and the precise position of the receivers. The International GNSS Service (IGS) provide the highest quality data and products as the standard for Global Navigation Satellite Systems (GNSS) and it extensively used for Geodetic applications. IGS is one of the services of the Global Geodesy Observing System (GGOS) of the Internal Assotiation of Geodesy (IAG). The usage of the GNSS data in Geodesy enables research in three fundamental areas of geodesy:
- The geometric shape of the Earth (land, ice and ocean surface) as well as its variation in time;
- The orientation of the Earth in inertial space as a function of time;
- The Earth’s gravity field and its temporal variations.
On Remote Sensing, the GNSS signals are well specified and stable signals that are obiquos on the Earth. Those make them perfect signals of opportunity for bi-static radar applications where the transmitter is not part of the remote sensing instrument. The concept of GNSS-R (GNSS-Reflectometry) or PARIS (Passive Reflectometry and Interferometry System) is that of a bi-static radar, where the transmitter is a GNSS satellite and where the receiver can receive both the signal coming directly from the source and the signal reflected from the Earth’s surface. In spite of the fact that the properties of GNSS signals have been optimized for navigation applications, the reflected signal contains information about the state of the reflecting surface that can be applied in areas like:
- Ocean surface altimetry
- Monitoring sea state
- Soil moisture
- Sea ice and dry snow
GNSS is also a powerful tool for the analysis of the ionosphere and troposphere. The carriers and signals recorded by receivers on/ or near the Earth’s surface have crossed the Earth’s atmosphere causing signal delay. These delays, namely Ionospheric Delay and Tropospheric Delay, are compensated through measurements or by models in order to achieve the performance expected from the system. As a consequence the derived models constitute a valuable input in the study of Earth’s atmosphere behavior as well its effects on radio signals. On the other hand the ionosphere’s effects are frequency dependent and since GNSS satellites transmit signals in more than one frequency this effect can be quite minimized also improving ionospheric analysis. GNSS signals are a well-established source of data for the study and modelling of Ionosphere and Troposphere.