GPS / RO for atmospheric studies Panagiotis Vergados Dept. of Physics and Astronomy

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GPS / RO for atmospheric studiesPanagiotis
VergadosDept. of Physics and Astronomy
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Outline

• Objectives
• Introduction
• Description of the techniques
• Fresnel diffraction theory
• Radio-holography
• Back-propagation theory
• Atmospheric parameters retrieval
• Remarks
• Work in progress future work

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Objectives

• Develop knowledge and expertise in GPS / RO
  studies
• Review and understand currently used methods and
  models
• Choose and improve the method which gives the
  best vertical resolution of refractive index
  profiles
• retrieve atmospheric parameters (such as
  temperature and water vapour) from refractive
  index profiles

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Introduction (1)

• There is an increased interest in high
  vertical and horizontal resolution
• observations and global scale coverage
  of temperature and water
• vapour
• Yunck et al. (1988) suggested that the
  Global Positioning System
• (GPS) be used to make Radio Occultation
  (RO) observations of the
• Earths atmosphere
• The era for GPS RO observations of the
  Earths atmosphere began
• with the GPS Meteorology (GPS/MET)
  experiment on April 3rd 1995
• Ware et al., 1996 Kursinski et al.,
  1996, 1997

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Introduction (2)

• The RO technique
• Bending angle, a
• Impact parameter, a
• Spacecraft distance, D

Radio occultation (RO) experiment geometry
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Introduction (3)
Standard method to calculate refractivity
profiles Able Inversion Transform

of bending angle profiles
HOW do you calculate bending angle profiles?
Through measurements of the Doppler-shifted phase
of the received electric field and observation
geometry of the experiment
Problems Diffraction and Multi-path effect.
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Description of the techniques (1)

• FACT 1 strong gradients of water vapour in the
  lower troposphere cause diffraction and
  multi-path, which limit the vertical resolution
  of the measurements
• FACT 2 First-order ionospheric correction not
  sufficient
• (L1 and L2 follow two different paths)
• Various methods have been introduced in order to
  overcome these
• limitations
• Fresnel diffraction theory
• Radio-holography
• Back-propagation theory

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Fresnel Diffraction (1)

• Approximations
• Thin screen Melbourne et al., 1994 Mortensen
  and Hoeg, 1998 and
• Spherical symmetry

• Advantages
• Introduction of a weighting function
• Vertical resolution is not diffraction
  limited
• Multi-path effects can be reduced

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Fresnel Diffraction (contd)

• Error estimates
• 2oC (between 5 and 25 km)
• gt 2oC (below 5 km)
• Vertical resolution
• Few hundreds of m to 1 km

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15
10
5
a
b
Vertical temperature difference profiles a)
f52o N b) f70o N
(Mortensen et al., 1998)
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Radio-holography (1)

• Approximations
• Account for a reference electric field, Em(t)
  exp(if(t))
• Construct a radio-hologram, ?E(t) E(t) / Em
  (t)
• Assume the radio-hologram is consisted of
  complex sine-waves

Governing equations ak am Dak
(the bending angle)
pk pm Dpk (the impact
parameter)
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Radio-holography (contd)

• Error Estimates
• 1.7 3.3 oK
• (between 5 and 25 km)
• 5 oK (below 5 km)

Vertical temperature difference profiles a) 28o,
b) 36o and c) 48oN (Hocke et al., 1999)
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Back propagation (1)

• Approximations
• Multiple Phase Screen (MPS) Karayel et al.,
  1997
• Spherically symmetric atmosphere

• Advantages
• Diffraction and multi-path effects are
  mostly removed
• Much better vertical resolution, below the
  sub-Fresnel scale
• Back-propagation of the electric field rays
  to an auxiliary plane

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Back-propagation (contd)

• Error estimates
• range 0.2 oK to 2 oK
• Vertical resolution
• Around 250 m (terrestrial
• atmosphere)
• Around 40 m (Martian
• atmosphere)

Vertical temperature profile of a terrestrial
atmosphere (Karayel et al., 1997)
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Atmospheric parameters
After the refractive index profile has been
constructed, atmospheric parameters can be
calculated through
N a1P / T a2Pw / T2

where P and Pw are the atmospheric and water
vapour pressure, T is the temperature at the
respective pressure level and a1 and a2 are
constants
Known Refractive index profile and either P or T
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Remarks

• Fresnel Diffraction Theory,
  Radio-holography and Back-propagation
• remove mostly the diffraction and
  multi-path effects
• The vertical resolution achieved from all
  three methods ranges
• approximately from a few hundred meters
  to 1 km
• The back-propagation method is capable of
  achieving vertical resolution
• at sub-Fresnel scales (lt 250 m)
• The error estimates of the retrieved
  temperature profiles with the
• back-propagation method range between
  0.2 and 2 K, and of the
• refractive index profile between 4?10-6
  and 1.4?10-5

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Work in progress and future work

• Second and third order ionospheric
  correction in the calculation of
• bending angle profiles
• Abel inversion investigation and possible
  improvement
• Modification and/or development of
  software for
• ionospheric correction and Abel inversion
  transform
• Investigation of the non-spherical symmetry
  and how it affects the
• refractive index profile
• Investigation of other possible methods and
  development of an improved
• model for the retrieval of atmospheric
  parameters from
• refractive index profiles (e.g. 1D-VAR
  method)