Ground GNSS Ionosphere Sounding - GNSS Remote Sensing: Theory, Methods and Applications - page 63

Chapter 4
Ground GNSS Ionosphere Sounding

4.1 History

The ionosphere is about 60-1,000 km above the earth's surface, which is actually
plasma of ionized gas of the upper atmosphere by solar radiation and high energy
particles from the Sun. The ionized electrons concentrations vary with the height
above earth's surface, location, time of the day, season, and amount of solar activity.
In the past decades, different observing instruments have been developed and used to
gather information on the ionosphere and plasmasphere, such as ionosonde, scatter
radars (Tsunoda 1988 ), topside sounders onboard satellites (Reinishch et al. 2001 ),
in situ rocket and satellite observations (Klobuchar 1991 ). But most instruments are
restricted to either the bottomside ionosphere (ionosondes) or the lower part of the
topside ionosphere (usually lower than 800 km), such as ground based radar mea-
surements. The Global Positioning System (GPS) has provided an unprecedented
high accuracy, flexibility and tremendous contribution to navigation, positioning,
timing and scientific questions related to precise positioning on Earth's surface,
since it became fully operational in 1994. The Global Positioning System (GPS) is a
satellite microwave technique whose signals are transmitted on microwave (L-band)
carriers through the Earth's atmosphere, and it inevitably suffers the ionospheric
effect. When the GPS signal propagates through the Earth's ionosphere, it is delayed
by the ionospheric refractivity, which results in lengthening of the geometric path
of the ray, i.e., ionospheric delay. Ionospheric delay will bring errors for GPS
navigation and positioning as the electromagnetic wave signal goes through the
earth's ionosphere from satellites to receivers. The amount of ionospheric delay
of GPS varies from a few meters to decades of meters, but it could reach more
than 100 m during severe ionosphere storms. In contrast, the GPS ionospheric delay
may provide some useful information on the ionosphere. In 1993 , Bassiri and Hajj
introduced the model for GPS ionospheric delay based on realistic modeling of the
electron in the ionosphere using a dipole moment approximation for the Earth's