Theory of GNSS Radio Occultation - GNSS Remote Sensing: Theory, Methods and Applications - page 100

(a joint India-France mission); the SAC-D/Aquarius from Argentina; and a follow-
on MetOp-B from ESA. Moreover, a series of single-receiver RO missions have
been planned to be launched in the near future such as Chinese FY-3C equipped with
GNOS (GNNS Radio Occultation Sounder); ROHP-PAZ from Spain; KOMPSAT-5
from South Korea and EQUARS from Brazil.
In general, a single GPS RO receiver satellite can recover 500 rising and
setting RO profiles each day, distributed almost uniformly around the globe; a
large constellation would recover many thousands of profiles, which could have
a profound impact on both long term climatological studies and short term weather
predictions.
The follow-on mission of COSMIC-II are planned to have 12 satellites with
6 satellites on low-inclination (equatorial) orbits and another 6 satellites on high
inclination orbits to produce uniform global sounding coverage. The next generation
RO receiver (e.g., TriG developed by JPL) will be capable of tracking the GPS,
GLONASS and Galileo satellites at the same time and will significantly increase
the sounding densities. The likely over 10,000 daily profiles will provide extremely
valuable atmospheric observations that are essential for mesoscale weather fore-
casting, such as hurricane/typhoon, thunderstorms etc. The GNSS RO also attracts
strong interests from the private sectors. The CICERO (Community Initiative for
Cellular Earth Remote Observation) was form to seek private funds for launching
many micro-satellites in Low-Earth Orbit and providing dense RO soundings.
All these RO missions provide essential global atmospheric measurements with
high vertical resolution and significantly benefit the weather and climate research
communities. A comprehensive list of the past and current RO missions as well as
many on plan is summarized in the Table 5.2 .

5.2 Principle of GNSS Radio Occultation

As the LEO satellite equipped with a GNSS receiver orbits around the Earth, an
occultation event occurs (Fig. 5.2 ) when the received navigation signal from a
setting (rising) GNSS satellite scan through successively deeper (higher) layers of
the Earth's atmosphere until the GNSS signals descend below the Earth surface (rise
above the atmosphere). The GNSS signal is bent or delayed before arriving at the
LEO due to the Earth's atmosphere.
Strictly speaking, the propagation of the GNSS signal through the atmosphere
obeys Maxwell's equation in which the propagation medium (e.g., the Earth's
atmosphere) is characterized by a three-dimensional spatial distribution of a com-
plex and dispersive refractive index. The GNSS radio occultation technique takes
advantage of the extremely precise phase and amplitude measurement of the GNSS
navigation signals that pass through the Earth's atmosphere to provide accurate
retrieval of the vertical profiles of refractive index of the atmosphere. Consequently,
the atmospheric properties such as air density, temperature, pressure, and humidity
can be inferred (Kursinski et al. 1997 ; Rocken et al. 1997 ).