Airborne GPS Down-Looking Occultation Experiments

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

Abstract:	GPS down-looking occultation method, a kind of GPS occultation observation, is expected to be able to estimate tropospheric water vapor profile below a receiver location with aids of temperature profile obtained from local-scale atmospheric numerical model. It is required to continuously observe Doppler shift of the radio signal transmitted from the occultation GPS satellite at the location as high altitude as possible, because the altitude limits the upper bound of resulting profile. With a GPS receiver installed on the top of mountain, “mountain-based GPS down-looking occultation” campaigns have been already performed on the top of Mt. Fuji and a technique to measure water vapor profile from observed Doppler shift have been developed. We aim to apply this novel technique to “airborne GPS down-looking occultation”, which can (i) expand resulting profile range up to a flight level; and (ii) be performed almost everywhere unlike the mountain-based method. The observational principle of airborne GPS down-looking occultation is fundamentally identical to mountain-based method except the movement of receiving location. Occultation GPS satellite should be tracked to the elevation angle as low as possible even below the horizon, so that the lower bound of resulting water vapor profile will be expanded almost to the ground. A special receiver called “down-looking receiver” or “DL receiver” was designed for continuous tracking of occultation GPS satellite for this purpose. The receiver system has two antennas, one is at the top of aircraft (for timing and positioning) and another is installed side of aircraft (for tracking occultation satellite). The receiver also has special receiving functions, for example, it can performs the signal reception of the occultation GPS satellite using the side antenna with synchronization to the signal from the top antenna. In order to accomplish airborne GPS down-looking occultation experiment, we have to estimate accurate velocity of the side antenna motion with an accuracy of several mm/s even in post-processing. Therefore, GPS/INS hybrid positioning system was employed and installed on the experimental aircraft to estimate aircraft velocity and attitude. We have conducted six flight experiments since October 2003 and obtained more than 30 datasets of occultation events using the equipment consisting of the previous two elements. We investigated data qualities using the first experiment data. Our receiver successfully tracked the signal from occultation GPS satellite continuously down to the elevation angle of –3.5 degrees at a flight level of about 6 km. The sea surface reflection significantly affected the received occultation GPS satellite signal, so we have investigated some correction method to remove this effect using a special function of the DL receiver. We have to extract atmospheric contribution from Doppler shift measurement with the side antenna using aircraft velocity and attitude from GPS/INS measurements. So far, we have investigated the accuracy of aircraft attitude derived from real time solution of IMU, comparing with side-top single difference in phase measurements of occultation GPS satellite.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1713 - 1723
Cite this article:	Yoshihara, Takayuki, Fujii, Naoki, Hoshinoo, Kazuaki, Matsunaga, Keisuke, Saitoh, Shinji, Sakai, Takeyasu, Tsuda, Toshitaka, Aoyama, Yuichi, Danno, Satoshi, "Airborne GPS Down-Looking Occultation Experiments," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1713-1723.
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