Theory of GNSS Radio Occultation - GNSS Remote Sensing: Theory, Methods and Applications - page 95

Chapter 5
Theory of GNSS Radio Occultation

5.1 Introduction

Theword“ occult ” is original from Latin word “ occultus ” referring to “hide”.
The usage of “occultation” in science field starts in astronomy. An occultation
refers to an event that occurs when one celestial body is occulted (or hidden) by
another celestial body of interest that passes between it and the observer. During
an occultation event, the signal from the occulted object travel through the limb (or
edge) of the middle object could be distorted due to the existence of medium, such
as atmosphere at the limb. The change of the signal can then be used to infer the
atmospheric structure of the celestial body of interest. This is called the occultation
technique. Various signal sources at different wavelength have been used to apply
the occultation remote sensing technique. For example, by looking at the stars that is
setting or rising behind a celestial body of interest is called stellar occultation. The
way in which the star's signals bend and change relative to the observer can be used
to characterize the atmospheres and/or ring systems in the solar system or beyond.

5.1.1 Radio Occultation in Planetary Sciences

The radio occultation (RO) technique, which uses the radio signals, emerged at the
dawn of interplanetary space exploration in the 1960s. It was pioneered by two
independent research groups at Stanford University and the NASA's Jet Propulsion
Laboratory (JPL), for determining the size and atmospheric composition of distant
planets. The first planetary radio occultation profile were collected in 1965, when the
Mariner-4 spacecraft flied by Mars and sent the radio signals through its atmosphere
and ionosphere and intercepted at the Earth (Fjeldbo and Eshleman 1965 ; Kliore
et al. 1965 ). As seen in Fig. 5.1 , by precisely recording the change of the radio
signals scanning through the planetary atmosphere, the vertical structure of the
atmosphere can be precisely measured. Since then, the radio occultation technique