Introduction to Radio Occultations

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Introduction to Radio Occultations
Georg Bergeton Larsen GRAS SAF Project
Manager Atmosphere Ionosphere Research Division
(AIR) Danish Meteorological Institute
(DMI) Copenhagen
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Outline of presentation

• The satellite system
• Derivation of atmosphere parameters
• Bending angle
• Refractivity
• Temperature
• Humidity
• Distribution of measurements
• Advantages and limitations
• Satellite missions
• Summary

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The satellite system
The occultation measurement
The GPS constellation
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GRAS Atmosphere profiling
Metop
t1
t2
t3
GPS
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Neutral Atmosphere Bending Angle

• The bending angle is computed from the
  atmospheric phase delay
• Ionosphere correction on bending angle

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Refractivity profile

• The refractivity is determined using the Abel
  transform
• Uncertainty 0.3 measurement range 4 -
  450 N-units

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Atmosphere Parameters

• The density of the dry atmosphere is computed by
  assuming an ideal gas
• where Rd 0.287 J/(gK) and k177.6 K/hPa, k2
  37,39?104 K2/hPa, k3 70,40 K/hPa. The
  pressure profile is now obtained by using
  hydrostatic equilibrium
• Uncertainty 0.3 measurement range
  10-1100 mb

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Temperature profile

• The temperature profile is derived by using the
  ideal gas law and the estimated profiles of
  refractivity and pressure.
• Uncertainty 1K measurement range 180K - 335
  K

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Water vapor profile

• The water vapor pressure is derived by an
  iterative process using T(z) from NWP model
  (Offline Products) and using the 1DVAR method
  (NRT Products).
• 1) Total pressure
• 2) Water vapor pressure
• 3) Total density
• Equations 1) to 3) are solved with the dry
  pressure as initial input in eq. 1) and Rw0.461
  J/(gK).
• Uncertainty lt 20 Measurement range 1 - 45
  mb
• Assumed uncertainty on T(z) less than 1K

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GRAS SAF Prototype TemperatureProfile from CHAMP
Data
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Retrieved Water Vapour Profile
GPS/MET occultation Feb 9, 1997 at UT 1615 lat
14 lon 141
Tdry
eocc
TNWP
esaturated
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Distribution of GRAS measurements
GRAS occultations during 24 hrs. Approximately
600 atmosphere profiles distributed globally
Distribution of NWP Radio sondes
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Advantages and limitations of GPS Atmosphere
Profiling

• Absolute measurement
• The basics of the observations are a measurement
  of time. Calibration of clocks can be achieved
  using assisting ground observations.
• Global coverage
• The geometry of the observation leads for one
  satellite to evenly distributed data on a 24-hour
  interval. Observations over seas and oceans
  (covering 70 of the Earth) minimize the major
  error source concerning accuracy of weather
  forecast and climate models.
• High vertical resolution
• The vertical resolution limited by the Fresnel
  zone of the observation leads to information of
  atmosphere phenomena with scale sizes less than 1
  km.
• Insensitive to clouds and precipitation
• The wavelengths applied makes the measurement
  transparent to clouds and rain hampering other
  space techniques.

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Satellite missions

• Research / demonstration
• GPS-Met (1995-97)
• Ørsted (1999-)
• CHAMP (2000-)
• SAC-C (2000-)
• GRACE
• FedSat
• Operational
• METOP/GRAS (2005-)
• NPOESS/GPSOS (2010-)
• Constellations of micro-satellites
• COSMIC (2006-)
• ACE (2008-)

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Summary

• Key parameters and method introduced
• Bending angle
• Refractivity
• Temperature
• Humidity
• Examples and distribution of RO measurements
• Advantages and limitations
• Global coverage (not synoptic)
• High vertical resolution (averaged horizontally)
• Insensitive to clouds
• Satellite missions
• GRAS on Metop - first operational RO mission