## 1. Introduction

[2] The interpretation of GPS meteorology (GPS/MET) radio occultation data in areas of multipath propagation was recently addressed in a series of papers [*Gorbunov et al.*, 1996; *Karayel and Hinson*, 1997; *Gorbunov and Gurvich*, 1998a, *1998b*; *Pavel'ev*, 1998; *Hinson et al.*, 1998; *Hocke et al.*, 1999; *Gorbunov et al.*, 2000; *Gorbunov*, 2001]. Multipath propagation arises in the presence of complicated structures of the atmospheric refractivity, which are typical for the ionosphere and the lower troposphere. All the data processing methods, discussed in the above papers, were designed in order to reconstruct profiles of geometric optical refraction angle from measurements of wave field. These methods use complete records of complex wave fields, or radio holograms. Because of that, these methods can be termed radio-holographic. Some of the basic ideas of radio-holographic data processing were introduced in earlier papers [*Marouf et al.*, 1986; *Lindal et al.*, 1987] discussing the radio occultation sounding of planetary atmospheres.

[3] The radio-holographic methods are based on the synthesis of the wave optics and geometrical optics. This approach is very effective because of the following reasons: (1) the wavelength of GPS/MET observation is short enough for the short-wave asymptotic solutions of the wave problem [*Mishchenko et al.*, 1990] to be applicable and (2) the inverse problem is formulated much more easily in the geometric optical approximation than in the wave optics.

[4] In this paper we analyze the GPS/MET data using the radio-optics method and the canonical transform method. The radio-optics, or sliding spectral, method was already used for processing of radio soundings of planetary atmospheres [*Lindal et al.*, 1987]. It uses the spectral analysis of the wave field in small sliding apertures. This method can be very easily numerically implemented, but it is not convenient for the accurate computation of refraction angle profiles. However, the local spatial spectra of wave field can be plotted in the ray coordinates (refraction angle and impact parameter), and the spectral maxima must then trace the refraction angle profile. This is a very simple means of data visualization, which allows for very fast processing of a big number of occultations.

[5] The canonical transform (CT) method was recently developed as a significant generalization and improvement of the back-propagation technique [*Gorbunov*, 2001]. It utilizes the theory of Fourier integral operators. In this method the wave field is transformed from the representation of the spatial coordinates to the representation of ray coordinates. The transformed wave function depends on the impact parameter. The derivative of its phase is then the ray direction angle. The amplitude of the transformed wave function is then nearly constant outside the geometric optical shadow zone, and it drops very abruptly at the shadow-light border.

[6] The application of these two methods for the analysis of the GPS/MET radio occultations allows also for data quality control. Many of the occultations contain corrupted data in their lowest parts. This corruption must be due to the phase lock loop failures [*Sokolovskiy*, 2001], and it manifests itself in the negative bias of reconstructed refractivities [*Rocken et al.*, 1997]. Corrupted fragments of radio occultations are seen as very chaotic patterns in the visualized local spatial spectra. The application of the canonical transform method to such data also results in strong scintillations of the amplitude and phase.

[7] We processed the complete Prime Time 4 of the Microlab-1 data. We found a series of occultations, where the spatial spectra in multipath areas behave in a regular way and trace nonmonotonous refraction angle profiles, which are characteristic for multipath propagation. These results are consistent with the results of the canonical transform method. This allows for classifying such data as suitable for further utilization. In many occultations the spectra reveal the ray reflected from the Earth's surface [*Beyerle and Hocke*, 2001]. We processed such occultations reaching the Earth's surface, and the canonical transform method allowed for the correction of effects of multipath propagation due to reflected rays.