AniaraUnderstanding the Amplitude and Phase Signal in Radio Occultation Data
Department of Space Science
Luleå University of Technology
The talk summarizes work that was carried out during a two month stay at EUMETSAT as a visiting scientist. The goal of the stay was a better understanding of the data from the GRAS instrument on Metop. GRAS is a GPS occultation instrument, with some special features compared to other such instruments. In particular, GRAS provides a good measurement of the signal amplitude, not just its phase. Furthermore, in raw sampling mode, GRAS provides high resolution spectral information, which can be used to characterize multi-path effects, or to some extent even correct for them. These advanced GRAS features are not yet fully exploited.
In the first part of the stay, the main factors influencing the signal amplitude were studied. These are defocusing and atmospheric extinction. Defocusing was modelled based on the bending angle gradient, following a standard relation from basic theory. Extinction was modelled with a state-of-the-art radiative transfer model (ARTS).
In the second and larger part of the stay, it was investigated whether the Empirical Mode Decomposition (EMD) method can be used to spectrally analyze GRAS data. EMD is a new method for non-stationary and nonlinear time series analysis, with some theoretical advantages over established methods such as FFT spectrograms or Wigner-Ville distributions.
The talk presents work in progress, not a finished result. It will start with a brief general introduction on the GRAS instrument, its operational products, and the main features of the GRAS raw sampling (open loop) tracking. The main part will focus on the results obtained during this visiting scientist stay.
Created 2011-03-09 10:41:11 by Uwe RaffalskiLast changed 2011-03-17 17:21:30 by Mats Holmström