# Ionospheric Remote Sensing with GNSS
## 5 Conclusions and Prospects  
Fundamental physics and engineering of GNSS and ionospheric remote sensing are introduced in this entry. It is important to monitor and understand the ionospheric impact on GNSS, because the ionosphere can cause delays or scintillation of GNSS signals which eventually degrade the PNT solutions from GNSS. As a reflection of ionospheric ionization level, TEC is an integration of the electron density along the LOS between two points. The larger the TEC, the larger ranging offset in the GNSS observable caused by the ionosphere. S4 and \(\sigma_{\phi}\) are the two commonly used ionospheric scintillation indexes to quantify the GNSS signal fluctuation level in the amplitude and phase domain, respectively. Ionospheric irregularities can cause scintillations of GNSS signals, which may lead to signal attenuation, carrier phase cycle slip or even loss of lock. The ubiquitous GNSS is a powerful engineering tool for ionospheric remote sensing. Ionospheric remote sensing studies using ground-based GNSS receivers have been conduced over the past several decades, while ionospheric measurement using space-based GNSS techniques is emerging rapidly and providing much higher coverage and flexibility.  
Conceptualization, Y.P. and W.A.S.; methodology, Y.P.; software, Y.P.; validation, Y.P. and W.A.S.; formal analysis, Y.P.; investigation, Y.P.; resources, Y.P. and W.A.S.; writing--original draft preparation, Y.P.; writing--review and editing, W.A.S.; visualization, Y.P.; supervision, W.A.S. All authors have read and agreed to the published version of the manuscript.  
This work is supported by the AFOSR (Grant No. 13-0658-09) and Virginia Tech.  
**Data Availability Statement:** GPS TEC data products and access through the Madrigal distributed data system are provided to the community by the Massachusetts Institute of Technology under support from US National Science Foundation grant AGS-1952737. Data for the TEC processing is provided from the following organizations: UNAVCO, Scripps Orbit and Permanent Array Center, Institut Geographique National, France, International GNSS Service, The Crustal Dynamics Data Information System (CDDIS), National Geodetic Survey, Instituto Brasileiro de Geografia e Estatistica, RAMSAC CORS of Instituto Geografico Nacional de la Republica Argentina, Arecibo Observatory, Low-Latitude Ionospheric Sensor Network (LISN), Topcon Positioning Systems, Inc., Canadian High Arctic Ionospheric Network, Institute of Geology and Geophysics, Chinese Academy of Sciences, China Meteorology Administration, Centro di Ricerche Sismologiche, Systeme d'Observation du Niveau des Eaux Littorales (SONEL), RENAG : REseau NAtional GPS permanent, GeoNet--the official source of geological hazard information for New Zealand, GNSS Reference Networks, Finnish Meteorological Institute, SWEPOS--Sweden, Hartebeesthoek Radio Astronomy Observatory, TrigNet Web Application, South Africa, Australian Space Weather Services, RETE INTEGRATA NAZIONALE GPS, Estonian Land Board, Virginia Tech Center for Space Science and Engineering Research, and Korea Astronomy and Space Science Institute.  
The authors would like to thank [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] for their comments and inputs on our work.  
The authors declare no conflict of interest.  
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