Measurement System and Experimental Results of Airborne-based Downward-looking GPS Occultation

Takayuki Yoshihara, Naoki Fujii, Kazuaki Hoshinoo, Keisuke Matsunaga, Shinji Saitoh, Takeyasu Sakai, Toshitaka Tsuda, Yuichi Aoyama and Satoshi Danno

Abstract:	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/INS hybrid positioning systems. The sensitivity of the developed GPS receiver system with two RF front ends was improved as –142 dBm to track GPS L1 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.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	290 - 297
Cite this article:	Yoshihara, Takayuki, Fujii, Naoki, Hoshinoo, Kazuaki, Matsunaga, Keisuke, Saitoh, Shinji, Sakai, Takeyasu, Tsuda, Toshitaka, Aoyama, Yuichi, Danno, Satoshi, "Measurement System and Experimental Results of Airborne-based Downward-looking GPS Occultation," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 290-297.
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