| Some problems are solved or thorough explored in sensing Earth's atmosphere via GPS radio occultation methodology. Firstly, an analytic expression for the angle of refraction in GPS radio occultation in the general case is presented. The angle of refraction without assuming a circular orbit is calculated iteratively starting with the value from the circular assumption. The atmospheric parameters are then calculated by inversion with and without the circular assumption. It is shown that the bias caused by the circular assumption is up to about 1 mbar in the pressure and up to about one degree Celsius in the temperature. On one hand, this result supports the practice of using the circular assumption in qualitative error analyses; on the other hand,it shows the necessity of foregoing the circular assumption when the highest accuracy is required of the atmospheric parameters. Secondly, the way of remote sensing of terrestrial atmospheric parameters using spaceborne GPS radio occultation technique is discussed. Based on this method, data analysis software has been coded and is used to reduce real radio occultation data. It is confirmed that exponential-function extrapolation contributes much to the improvement of inverted temperature; however it does much less for pressure inversion. Compared with relevant published results from abroad, our results are, at large, agreeable with foreign investigators' relevant works. Thirdly, MSISE90 atmospheric density model is briefly introduced and the amendments in this model by Hedin are pointed out. Based on MSISE90, a priori temperature series are produced for input parameter in GPS radio occultation data reduction. Fourthly, orbit information of the Low Earth Orbiter carrying a GPS receiver is needed in the conception of atmosphere monitor by GPS occultation. It is unavoidable to analyze the effects of orbit uncertainty of the low Earth Orbiter on the observable in GPS occultation for atmosphere monitor. Based on the existent Earth atmosphere model, three dimensional ray-tracing techniques are employed to simulate five representative complete GPS occultation events. The effect of the orbit error is quantified on the key observable in occultation. Fifthly, based on Haselgrove's and Budden's Equations, the propagation process of GPS signal in the ionosphere is simulated. Five groups of orbit data for GPS and LEO satellites are employed to simulate and generate the ionospheric phase delay respectively in two cases, one is LEO orbit without forced perturbationand the other is LEO orbit with forced perturbation. The response of ionospheric phase delay to the LEO satellite orbit biases is assessed and analyzed. The preliminary results have shown that the response of ionospheric phase delay to the LEO orbit biases is much greater in the radial direction than those in the transverse or the normal directions. Sixthly, the propagation of radio signal in the ionosphere is simulated. Based on five data sets of real satellite state vectors from radio occultation measurements, the phase delays are respectively calculated with four different FLUX, i.e., FLUX=0, 70, 160 and 240. It corresponds to four representative conditions, that is to say, no solar activity, low solar activity, average solar activity and peak solar activity. The simulation has shown such features as follows: ionospheric phase delay is obviously correlated with solar radio flux, and the greater solar radio flux, the greater ionospheric phase delay; as for a rising occultation event, at the beginning, the phase delay increases, then it reaches a peak, finally it decreases till the end of occultation event. The phase delay differences among three different solar radio flux conditions tend to enlarge in the first most part of occultation event, however, they tend to narrow in the rest of the occultation event. Finally, some force models are adapted for LEO orbital design dedicated to the regional atmospheric monitor by radio occultation. The orbit design are renewed.
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