THE GPS LIMB SOUNDING TECHNIQUE FOR REMOTE SENSING OF THE ATMOSPHERIC REFRACTIVITY AND SOME INSIGHT ABOUT THE NEW ITALIAN SPACE AGENCY GPS RECEIVER FOR RADIO OCCULTATION

One of the most recent and promising atmospheric remote sensing technique is the GPS radio occultation. The inversion of phase measurements collected by a GPS receiver placed on a Low Earth Orbit when it is rising or setting with respect to the transmitter, allows the retrieval of accurate and high vertical resolution atmospheric profiles. One of the main problems affecting the tracking of occulted GPS signals is the premature phase lock loss, due in particular to anomalous propagation phenomena (i.e., multipath, defocusing, tropospheric scintillations, etc.) occurring through the lower troposphere. Such effects may cause the refractivity close to Earth’s surface to be unretrieved or negatively biased (when compared with the meteorological analysis). One of the possible solutions to mitigate the signal fading through the lower atmosphere is the adoption of open-loop tracking channels in the hardware design of the receivers. This is the strategy implemented in the new GPS radio occultation receiver (ROSA) developed by the Italian Space Agency. Some insight about the prediction model for the incoming frequency needed for open-loop purposes and its validation with real occultation data are given. Moreover, a measurement campaign with the ROSA bread-board is planned for the next 2006 year on the top of a mountain, in order to test and to validate the receiver. The measurement scheme is discussed.