The ionosphere, radio navigation, and global navigation satellite systems
This article is a review of Global Navigation Satellite Systems (GNSS) for space scientists who are interested in how GNSS signals and observables can be used to understand ionospheric dynamics and, conversely, how ionospheric dynamics affect the operational capabilities of GNSS receivers. The most common form of GNSS is the Global Positioning System (GPS); we will first review its operating principles and then present a discussion of errors, of which ionospheric propagation is the most significant. Methods and systems for mitigating errors will be introduced, along with a discussion of modernization plans for GPS and for entirely new systems such as Galileo. In the second half of this article the effects of the ionosphere on GPS signals will be examined in more detail, particularly ionospheric propagation, leading to a discussion of the relation of TEC to ranging errors. Next, the subject of scintillations will also be introduced and connected to the presence and scale sizes of irregularities. Scintillations will be examined as spatial and temporal structures. The method of measuring scintillation pattern drift and ionospheric velocity will be discussed. We conclude by examining ionospheric effects on GPS at midlatitudes.