Ionospheric Sounding Using GNSS-RO - GNSS Remote Sensing: Theory, Methods and Applications - page 160

Chapter 7
Ionospheric Sounding Using GNSS-RO

7.1 Introduction

The GNSS signal will be bent from GNSS transmitters and LEO satellites when
the signal goes through Earth's atmosphere and ionosphere. As the movements
of GNSS and LEO satellites, the signal paths finish scanning the atmosphere and
ionosphere, and one occultation event is generated. If the scanning is up to down,
it is called as the descending occultation, and in the contrary it is called as the
ascending occultation. According to the occultation altitude from occultation point
(i.e., tangent point) (Fig. 7.1 ), occultation events can be divided into two categories:
atmospheric occultation events (lower than 90 km) and ionospheric occultation
events (from 90 km to LEO altitude). Atmospheric occultation events' time span
is 1 - 3 min, and ionosphere occultation events' time span is 10 - 20 min. With the
improvement of GNSS-RO technique and more GNSS-RO missions, a huge number
of occultation observations are available. Table 7.1 just lists the number of total
atmospheric occultations and ionospheric occultations till January 15 23:25:02 in
2013. It is a useful database not only for military and civil users but also for Earth's
space environment researchers.

7.2 Ionospheric Inversion

Figure 7.2 shows the geometric sketch of GNSS Radio Occultation. The GNSS
satellites, LEO satellites and the ray path locate in the same plane, which is called
occultation plane. The O, L, G, P are the refraction center, the LEO satellite's
location, the GNSS satellite's location and the tangent point on the ray path,
respectively. O is consistent with Earth's center with assuming the Earth as a sphere.
The other parameters are showing below, respectively: