Modeling and inverting radio occultation signals in the moist troposphere

Accurate modeling of radio occultation signals is performed by solving the Helmholtz equation with the use of a multiple-phase-screen technique. Refractivity is assumed spherically symmetric, and vertical profiles are reproduced from high-resolution tropical radiosondes. As a result, the characteristics of the signals, which are important for their tracking in low Earth orbit, are evaluated: the spectral bandwidth, similar to 50 Hz, and the random phase acceleration, similar to 1000 Hz/s. The complex signals are inverted with the use of two radio holographic methods: back propagation and sliding spectral (radio optics). For the back propagation method, finding the position of the auxiliary trajectory which provides an unambiguous bending angle function of impact parameter appears to be a problem. For the sliding spectral method a simple technique, which takes into account the whole spectral content of the signal without identification and selection of local spectral maxima, is introduced and tested. The sliding spectral method allows for the stable reconstruction of bending angles and refractivity with vertical resolution of similar to0.5 km;. The small-scale laminated structure of refractivity results in propagation of radio occultation signals down to significantly lower observation altitudes than in the case of smooth refractivity. Information content of radio occultation signals at those low altitudes is important for the radio holographic inversions.