# Ionospheric Remote Sensing with GNSS
###### Abstract  
**Definition:** The Global Navigation Satellite System (GNSS) plays a pivotal role in our modern positioning, navigation and timing (PNT) technologies. GNSS satellites fly at altitudes of approximately 20,000 km or higher. This altitude is above an ionized layer of the Earth's upper atmosphere, the so called \"ionosphere\". Before reaching a typical GNSS receiver on the ground, GNSS satellite signals penetrate through the Earth's ionosphere. The ionosphere is a plasma medium consisting of free charged particles that can slow down, attenuate, refract, or scatter the GNSS signals. Ionospheric density structures (also known as irregularities) can cause GNSS signal scintillations (phase and intensity fluctuations). These ionospheric impacts on GNSS signals can be utilized to observe and study physical processes in the ionosphere and is referred to ionospheric remote sensing. This entry introduces some fundamentals of ionospheric remote sensing using GNSS. GNSS; ionosphere; remote sensing +
Footnote †: _Encyclopedia_
This set of four equations, involving reception of at least four GNSS satellite signals, forms the underlying algorithm to solve a simple static positioning problem in the 3D space including the receiver clock bias. By definition, GNSS are satellite navigation systems with global signal coverage. Currently, there are four operational GNSS constellations: USA's Global Positioning System (GPS), Russia's Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), European Union's Galileo, and China's BeiDou Navigation Satellite System (BDS, formerly known as COMPASS). As of October 2021, the GPS, GLONASS, and Beidou constellations are fully operational. The Galileo constellation is expected to reach a full operational capability (FOC) stage soon. A brief status summary of four GNSS constellations is given in Table 1.  
The GPS satellites are located within six different orbital planes of medium Earth orbit (MEO) with an altitude of \(\sim\)20,200 km. Each two neighboring orbital planes are separated by 60 degrees in \(\Omega\) (longitude of the ascending node). The inclination angle of all GPS satellites is approximately 55 degrees. The orbital period of all GPS satellites is approximately 12 h. By design, a GPS receiver at any place on the Earth's open surface should be able to track at least six line-of-sight (LOS) direction satellites. The GPS constellation is designed with a total number of 32 satellites in orbit. Currently among the 31 operational GPS \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **GPS** & **GLONASS** & **Galileo** & **Beidou** \\ \hline Affiliation & U.S.A. & Russia & European Union & P.R. China \\ \hline Orbital & MEO & MEO (\(\sim\)20,200 km) & MEO (\(\sim\)19,100 km) & MEO/GEO/IGSO \\  & (\(\sim\)23,222 km) & & (\(\sim\)23,222 km) & MEO/GEO/IGSO \\ \hline Number of & & & & \\ Operational & & & & \\ Satellites & 31 (32) & 23 (29) & 22 (30) & 56 (\(\geq\) 61) \\ (Scheduled & & & & \\ total) & & & & \\ \hline Signal and & L1 (1575.42), & G1 (1602 + \(n\times 0.5625\)), & L1 (1519.795), & B1 (1561.10/1575.42), \\ Frequency & L2 (1227.6), & G2 (1246 + \(n\times 0.4375\)) & E5a (1176.45), & B2 \\ (in MHz) & L5 (1176.45) & G3 (1201 + \(n\times 0.4375\)) * & E5b (1207.14), & (1207.14/1176.45), \\  & & E6 (1278.75) & B3 (1268.52) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Current status of GNSS constellations (* \(n\) stands for GLONASS frequency channel number). Figure 1: Trilateration principle of GNSS positioning. satellites, 11 satellites broadcast the L1 (1575.42 MHz) signal only, 7 satellites broadcast the L1 and L2 (1227.6 MHz) signals, and 13 satellites broadcast the L1, L2 and L5 (1176.45 MHz) signals. The transmission of these GPS civilian radio-frequency (RF) signals is based on the Code Division Multiple Access (CDMA) spread-spectrum technology. The details of GPS signal structure can be found in the Interface Control Documents (ICD) [1]. The latest status of the GPS constellation can be found at the U.S. Coast Guard Navigation Center website [2].