SpaceBorne GPS Radio Occultation Data Assimilation

1
Space-Borne GPS Radio Occultation Data
Assimilation
Shao H. and X. Zou
4. What Quantities Are Measured?
For RO observations, a GPS receiver placed in a
low earth orbit (LEO) satellite (hundreds of km
height) keeps track of the signals transmitted
from the existing constellations of higher
orbiting GPS satellites ( 22,000 km height).
When the GPS satellite is occulted by the Earth,
due to the atmospheric refraction, the signal has
a propagation delay (Doppler shift) that can be
related to the bending angle of the ray path
(Fig. 2). Each bending angle reflects the
integrated effect of refraction of the ray along
its path. The relative motion of the satellites
enable the variations of the measured bending
angle with the tangent height of the path. Since
the atmospheric refractivity is related to the
pressure, temperature, and moisture as well as
the ionospheric electron density along the ray
path, the bending angle measurements can be
inverted using an Abel inversion to yield
vertical profiles of refractivity.
Fig. 2 The geometry of the GPS RO measurement
(from GFZ-Potsdam, Germany) .
2
5. What Data Do We Assimilate?
A series of quantities can be derived from the
raw GPS RO measurements phase measurements ?
bending angle ? refractivity ? pressure,
temperature and moisture content. The advantages
and disadvantages of assimilating these
quantities into the model analysis are discussed
in the following table. It is preferable to
assimilate the GPS RO bending angle or
refractivity.

• References
• About GPS measurements
• Fjeldbo, G. and V. R. Eshelman, Planet. Space
  Sci., 16, 1035-1059, 1968.
• GFZ-Potsdam CHAMP Website http//op.gfz-potsdam.
  de/champ
• NASA Jet Propulsion Laboratory (JPL) website
  http//genesis.jpl.nasa.gov/html/missions/missions
  _index.html
• Kursinski, E. R., et al., Science, 271,
  1107-1110, 1996.
• Ware, R., et. al., Bull. Am. Meteorological
  Soc., 77, 19-40, 1996.
• About our GPS data assimilation work
• Zou, X., F. Vandenberghe, B. Wang, M. E.
  Gorbunov, Y.-H. Kuo, S. Sokolovskiy, J. C. Chang,
  J. G. Sela, and R. Anthes, J. Geoph. Res., 104,
  22,301-22,318, 1999.
• Zou, X., B. Wang, H. Liu, R. A. Anthes, T.
  Matsumura, and Y.-J. Zhu, Quart. J. Roy. Meteor.
  Soc., 126, 3013-3040, 2000.
• Liu, H., X. Zou, R. A. Anthes, J. C. Chang,
  J.-H. Tseng, and B. Wang, J. Geoph. Res., 106,
  31771-31786, 2001.
• Zou, X., H. Liu, and R. A. Anthes, JTECH., 19,
  51-64, 2002.
• Shao H. and X. Zou, J. Geophy. Res., 107 (D23),
  4717, 2002.
• Liu, H., and X. Zou, J. Geoph. Res., 108(D17),
  4548, 2003.
• Zou, X., H. Liu, R. A. Anthes, H. Shao, J. C.
  Chang, and Y.-J. Zhu, Journal of the
  Meteorological Society of Japan, (accepted),
  2003.