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

GPS RO for atmospheric studiesGPS RO for atmospheric studies

Panagiotis Vergados

Dept of Physics and Astronomy Dept of Physics and Astronomy

OutlineOutline

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

ObjectivesObjectives

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

Introduction (1)Introduction (1)

There is an increased interest in high vertical and horizontal resolutionhigh vertical and horizontal resolution observations and global ndash scale coverageglobal ndash scale coverage of temperature and water vapour

Yunck et al (1988) suggested that the Global Positioning SystemGlobal Positioning System (GPS)(GPS) be used to make Radio Occultation (RO) observations of the Earthrsquos atmosphere

The era for GPS RO observations of the Earthrsquos atmosphere began with the GPS Meteorology (GPSMET) experiment on April 3April 3rdrd 1995 1995 [Ware et al 1996 Kursinski et al 1996 1997]

Introduction (2)Introduction (2)

Radio occultation (RO) experiment geometry

The RO techniqueRO technique

Bending angle α

Impact parameter a

Spacecraft distance D

Introduction (3)Introduction (3)

Standard method to calculate refractivity profiles Able Inversion TransformAble Inversion Transform of bending angle profilesof bending angle profiles

HOW do you calculate bending angle profilesHOW do you calculate bending angle profiles

Through measurements of the Doppler-shifted phase of the received electric field and observation geometry of the experiment

ProblemsProblems Diffraction and Multi-path effect

FACT 1 strong gradients of water vapourstrong gradients of water vapour in the lower troposphere cause diffraction and multi-pathdiffraction 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

Description of the techniques (1)Description of the techniques (1)

ApproximationsApproximations

Thin screen Thin screen [[Melbourne et al 1994 Mortensen and Hoeg 1998] and] and

Spherical symmetrySpherical symmetry

Fresnel Diffraction (1)Fresnel Diffraction (1)

AdvantagesAdvantages

Introduction of a weighting functionIntroduction of a weighting function

Vertical resolution is not diffraction limited

Multi-path effects can be reduced

Fresnel Diffraction (contrsquod)Fresnel Diffraction (contrsquod)

Vertical temperature difference profiles

a) =52o N

b) 70o N (Mortensen et al 1998)

Error estimates

plusmn 2oC (between 5 and 25 km)

gt 2oC (below 5 km)

Vertical resolution

Few hundreds of m to 1 km5

10

15

20

a b

Radio-holography (1)Radio-holography (1)

ApproximationsApproximations

Account for a reference electric field Em(t) = exp(iφ(t))

Construct a radio-hologram ΔE(t) = E(t) Em (t)

Assume the radio-hologram is consisted of complex sine-waves

Governing equationsGoverning equations m (the bending angle)

ppm + p(the impact parameter)

Radio-holography (contrsquod)Radio-holography (contrsquod)

Vertical temperature difference profiles a) 28o b) 36o and c) 48oN (Hocke et al 1999)

Error Estimates

plusmn 17 ndash 33 oK (between 5 and 25 km)

plusmn 5 oK (below 5 km)

Back propagation (1)Back propagation (1)

ApproximationsApproximations

Multiple Phase Screen (MPS) [Karayel et al 1997]

Spherically symmetric atmosphere

AdvantagesAdvantages

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

Back-propagation (contrsquod)Back-propagation (contrsquod)

Vertical temperature profile of a terrestrial atmosphere(Karayel et al 1997)

Error estimates

range 02 oK to 2 oK

Vertical resolution

Around 250 m (terrestrial atmosphere)

Around 40 m (Martian atmosphere)

Atmospheric parametersAtmospheric parameters

After the refractive index profile has been constructed atmospheric parameters can be calculated through

N = a1∙P T + a2∙Pw 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

RemarksRemarks

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 02 and 2 K and of the refractive index profile between 410-6 and 1410-5

Work in progress and future workWork 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 andor 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 (eg 1D-VAR method)