Airborne-based downward-looking GPS occultation is expected to estimate atmospheric refractivity profile below a flight level and to provide water vapor information in the lower troposphere. This observational method requires continuously and accurately observing Doppler shift of an occulting GPS radio weak signal due to the atmospheric propagation delay. The observed Doppler shift consists of the atmospheric excess Doppler shift and relative motion between an occulting GPS satellite and a receiving GPS antenna on aircraft. Therefore, it is also required to measure accurate velocity of a receiving GPS antenna motion on aircraft. To realize this novel technique, we developed a high sensitivity GPS receiver and a GPS/FNS hybrid positioning systems. The sensitivity of the developed GPS receiver system with two RF frontends was improved as -142 dBm to track GPS LI radio signal and demonstrated to receive the weak signal of the occulting GPS satellite down to an elevation angle of-4.5 degrees on aircraft at an altitude of 5,800 m. The GPS/INS hybrid system including post-processing software achieved within several mm/s accuracy to exclude the influence of the aircraft motion from observed occulting GPS signal. We have set up them on the experimental aircraft (Beechcraft B99 Airliner) of Electronic Navigation Research Institute (ENRI) and conducted nine flight experimental campaigns from October 2003 to February 2005. Consequently, we obtained more than 47 datasets including more than 75 occulting events. This paper presents the observational systems and the experimental results of the received GPS signals and the determined aircraft velocity. We also show the preliminary results of the atmospheric refractivity profile retrieved from the observed GPS occultation data at the aircraft.