1 / 16

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

GPS / RO for atmospheric studies Panagiotis Vergados Dept. of Physics and Astronomy. Outline. Objectives Introduction Description of the techniques Fresnel diffraction theory Radio-holography Back-propagation theory

laurenc
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


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

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

  3. 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

  4. 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 Earth’s atmosphere • The era for GPS RO observations of the Earth’s atmosphere began with the GPS Meteorology (GPS/MET) experiment on April 3rd 1995 [Ware et al., 1996; Kursinski et al., 1996, 1997]

  5. Introduction (2) The RO technique • Bending angle, α • Impact parameter, a • Spacecraft distance, D Radio occultation (RO) experiment geometry

  6. 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.

  7. 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

  8. 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

  9. Fresnel Diffraction (cont’d) • Error estimates: • ± 2oC (between 5 and 25 km) • > 2oC (below 5 km) Vertical resolution: • Few hundreds of m to 1 km 20 15 10 5 a b Vertical temperature difference profiles: a) f=52o N b) f=70o N (Mortensen et al., 1998)

  10. Radio-holography (1) • Approximations: • 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 equations:ak = am + Dak(the bending angle) pk = pm + Dpk(the impact parameter)

  11. Radio-holography (cont’d) • 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)

  12. 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

  13. Back-propagation (cont’d) • 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)

  14. Atmospheric 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

  15. 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 (< 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

  16. 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)

More Related