Geoscience Reference
In-Depth Information
the path. Using the RO measurements onboard a LEO satellite the vertical refrac-
tivity profile from the LEO satellite orbit height down to the Earth's surface can
be computed. Since the index of refractivity depends mainly on the number of free
electrons within the ionosphere, the refractivity profile can be inverted to obtain the
vertical Electron Density Profile (EDP) (Jakowski et al.
2002b
).
Here we will not go into details about the RO technique and the inversion pro-
cedure. For more details about the RO technique refer to e.g. Ware et al. (
1996
),
Rocken et al. (
1997
), and Jakowski et al. (
2004
). Details about the inversion proce-
dure could be found in e.g. Schreiner et al. (
1999
), Hernández-Pajares et al. (
2000
),
and Garcia-Fernandez et al. (
2005
). In the following some of the LEO missions
capable of ionosphere monitoring are briefly described:
The German CHAllenging Mini-Satellite Payload (CHAMP) was mainly used
for geophysical research and application. The satellite was successfully launched
by a Russian COSMOS rocket in July 2000. Although the mission was scheduled
for five years, providing a sufficient observation time to resolve long-term temporal
variations in the magnetic field, the gravity field and within the atmosphere, the mis-
sion lasted more than ten years and the satellite re-entered the Earth's atmosphere
on September 2010. The advanced “Black Jack” GPS receiver developed by the JPL
could measure GPS carrier phases in the limb sounding mode, starting at CHAMP
orbit tangential heights down to the Earth's surface (Jakowski et al.
2002a
). The RO
measurements performed on board CHAMP were used to retrieve vertical tempera-
ture profiles of the global troposphere/stratosphere system (Wickert et al.
2001
). The
first ionospheric radio occultation (IRO) measurements were carried out in April
2001 yielding reasonable electron density profiles (Jakowski et al.
2002b
).
The Gravity Recovery and Climate Experiment (GRACE) is a NASA and
German Aerospace Center (DLR) science mission satellite system, established to
measure primarily variations in the Earth's gravity field. The system consists of two
satellites in a near-polar orbit at about 500km altitude in the same orbital plane
220km apart. The twin satellites were launched in March 2002 with an expected
life of five years; however the satellites are still operating by the end of 2012. The
dual-frequency Blackjack GPS receivers were used for precise orbit determination
and atmospheric occultation on each of the satellites, providing capability of global
monitoring of the vertical electron density distribution (Wickert et al.
2005
).
The Formosat-3/COSMIC-Formosa Satellite Mission-Constellation Observing
System for Meteorology, Ionosphere and Climate (F3C) is a joint project between
Taiwan and the U.S.A. for weather, climate, space weather, and geodetic research.
The F3C mission was successfully launched in April 2006. The mission consists of
six micro satellites, each carrying an advanced GPS RO receiver, a Tiny Ionospheric
Photometer (TIP) and a Tri Band Beacon (TBB) (Rocken et al.
2000
). The satellites
were gradually raised from their launched orbit to reach their final orbit altitude of
800km. F3Cmission is currently providing between 1000 and 2500 daily RO profiles
in the neutral atmosphere, 1000 and 2500 daily electron density profiles and total
electron content arcs, and TIP radiance products (COSMIC
2011
).
