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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 10 | Copyright
Atmos. Meas. Tech., 9, 5077-5087, 2016
https://doi.org/10.5194/amt-9-5077-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 18 Oct 2016

Research article | 18 Oct 2016

Application of the full spectrum inversion algorithm to simulated airborne GPS radio occultation signals

Loknath Adhikari1, Feiqin Xie1, and Jennifer S. Haase2 Loknath Adhikari et al.
  • 1Department of Physical and Environmental Sciences, Texas A & M University – Corpus Christi, Corpus Christi, TX, USA
  • 2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA

Abstract. With a GPS receiver on board an airplane, the airborne radio occultation (ARO) technique provides dense lower-tropospheric soundings over target regions. Large variations in water vapor in the troposphere cause strong signal multipath, which could lead to systematic errors in RO retrievals with the geometric optics (GO) method. The spaceborne GPS RO community has successfully developed the full-spectrum inversion (FSI) technique to solve the multipath problem. This paper is the first to adapt the FSI technique to retrieve atmospheric properties (bending and refractivity) from ARO signals, where it is necessary to compensate for the receiver traveling on a non-circular trajectory inside the atmosphere, and its use is demonstrated using an end-to-end simulation system.

The forward-simulated GPS L1 (1575.42MHz) signal amplitude and phase are used to test the modified FSI algorithm. The ARO FSI method is capable of reconstructing the fine vertical structure of the moist lower troposphere in the presence of severe multipath, which otherwise leads to large retrieval errors in the GO retrieval. The sensitivity of the modified FSI-retrieved bending angle and refractivity to errors in signal amplitude and errors in the measured refractivity at the receiver is presented. Accurate bending angle retrievals can be obtained from the surface up to ∼250m below the receiver at typical flight altitudes above the tropopause, above which the retrieved bending angle becomes highly sensitive to the phase measurement noise. Abrupt changes in the signal amplitude that are a challenge for receiver tracking and geometric optics bending angle retrieval techniques do not produce any systematic bias in the FSI retrievals when the SNR is high. For very low SNR, the FSI performs as expected from theoretical considerations. The 1% in situ refractivity measurement errors at the receiver height can introduce a maximum refractivity retrieval error of 0.5% (1K) near the receiver, but the error decreases gradually to ∼0.05% (0.1K) near the surface. In summary, the ARO FSI successfully retrieves the fine vertical structure of the atmosphere in the presence of multipath in the lower troposphere.

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Global Positioning System (GPS) radio occultation (RO) signals can be inverted to provide profiles of temperature. Earlier inversion method that used geometric optics were observed to have large errors in the few kilometers of the atmosphere. Different theoretical methods were developed to address the deficiencies of the GO methods. In this study, we implement the full-spectrum inversion (FSI) method for inversion of GPSRO signal received by a receiver placed on an aircraft.
Global Positioning System (GPS) radio occultation (RO) signals can be inverted to provide...
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