Ionospheric Propagation Effects on GNSS Signals and New Correction Approaches - Global Navigation Satellite Systems: Signal,Theory and Applications - page 381

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Ionospheric Propagation Effects on GNSS
Signals and New Correction Approaches
M. Mainul Hoque and Norbert Jakowski
German Aerospace Center (DLR), Institute of Communications and Navigation
Neustrelitz,
Germany
1. Introduction
The ionosphere is the ionized part of the earth's atmosphere lying between about 50 km and
several earth radii (Davies, 1990) whereas the upper part above about 1000 km height up to
the plasmapause is usually called the plasmasphere. Solar extreme ultraviolet (EUV)
radiation at wave lengths < 130 nm significantly ionizes the earth's neutral gas. In addition
to photoionisation by electromagnetic radiation also energetic particles from the solar wind
and cosmic rays contribute to the ionization. The ionized plasma can affect radio wave
propagation in various ways modifying characteristic wave parameters such as amplitude,
phase or polarization (Budden, 1985; Davies, 1990). The interaction of the radio wave with
the ionospheric plasma is one of the main reasons for the limited accuracy and vulnerability
in satellite based positioning or time estimation.
A trans-ionospheric radio wave propagating through the plasma experiences a propagation
delay / phase advance of the signal causing a travel distance or time larger / smaller than
the real one. The reason of the propagation delay can be realized considering the nature of
the refractive index which depends on the density of the ionospheric plasma. The refractive
index (n ≠ 1) of the ionosphere is not equal to that of free space (n = 1). This causes the
propagation speed of radio signals to differ from that in free space. Additionally, spatial
gradients in the refractive index cause a curvature of the propagation path. Both effects lead
in sum to a delay / phase advance of satellite navigation signals in comparison to a free
space propagation.
The variability of the ionospheric impact is much larger compared to that of the
troposphere. The ionospheric range error varies from a few meters to many tens of meters at
the zenith, whereas the tropospheric range error varies between two to three meters at the
zenith (Klobuchar, 1996). The daily variation of the ionospheric range error can be up to one
order of magnitude (Klobuchar, 1996).
After removal of the Selective Availability (SA, i.e., dithering of the satellite clock to deny
full system accuracy) in 2000, ionosphere becomes the single largest error source for Global
Navigation Satellite Systems (GNSS) users, especially for high-accuracy (centimeter -
millimeter) applications like the Precise Point Positioning (PPP) and Real Time Kinematic
(RTK) positioning. Fortunately, the ionosphere is a dispersive medium with respect to the