論文

2006年

Performance requirement of velocity estimation for airborne-based downward-looking GPS occultation

Proceedings of the Institute of Navigation - 19th International Technical Meeting of the Satellite Division, ION GNSS 2006
  • Takayuki Yoshihara
  • ,
  • Naoki Fujii
  • ,
  • Shinji Saitoh
  • ,
  • Takeyasu Sakai
  • ,
  • Kazuaki Hoshinoo
  • ,
  • Keisuke Matsunaga
  • ,
  • Toshitaka Tsuda
  • ,
  • Yuichi Aoyama

2
開始ページ
653
終了ページ
661
記述言語
掲載種別
研究論文(国際会議プロシーディングス)

Airborne-based downward-looking (DL) GPS occultation measurement provides atmospheric refractive index profile in the lower troposphere. This observational method is based on accurately and continuously observing carrier Doppler shift of an occulting GPS satellite due to the atmospheric propagation delay. Observed GPS carrier Doppler shift consists of components due to atmospheric excess phase delay and relative motion between the occulting GPS satellite and a receiving GPS antenna on aircraft. Therefore, it is necessary to estimate both of the occulting GPS satellite's and the receiving antenna's velocities to extract the atmospheric component from the observed data. Then, it is required to estimate atmospheric Doppler shift with an accuracy of 5mm/s, which is corresponding to an accuracy of 1K temperature in retrieved refractive index profile. Although velocity of the occulting GPS satellite can be derived from IGS (International GNSS Service) precise orbital information, it is necessary to measure velocity of the receiving antenna on aircraft with complex motion during a flight. In this paper, we investigated and discussed performance requirement for velocity estimation of the receiving antenna in airborne-based DL GPS occultation measurement. There is a tradeoff between a simplification of velocity sensor and qualities of retrieved atmospheric refractive index profile, which are accuracy, time and height resolution. A theoretical time resolution of 5-10 seconds is derived from Fresnel zone analysis for the occulting GPS signals. It is corresponding to height resolution of 50-100 meters. GPS/INS (Inertial Navigation System) hybrid system is enough to determine the receiving antenna's velocity with both the comparable time resolution and the requirement accuracy. However, it is complex and expensive system. In contrast, stand-alone GPS velocity has random noise and errors, however, it has a possibility that these random noise and errors will be reduced by smoothing with a suitable time window although time resolution is decreased. It has also advantage to simplify the observational equipment. From comparison of stand-alone GPS velocity using an ordinary fine-satellite-view GPS antenna at aircraft top (Top) with GPS/INS hybrid results, 30-seconds-averaged result of stand-alone GPS analysis could completely provide reliable velocity enough to use for retrieval of atmospheric refractive index profile with height resolution of about 300 meters. However, velocity derived form the same processing using a side GPS antenna (Side), which was pointed to the horizontal direction to observe weak signal of the occulted GPS satellite, was not so good because of a few number of satellite to estimate velocity. It was also examined to estimate an error due to aircraft attitude variations for extraction of atmospheric Doppler shift of the occulting GPS satellite in a case of compensating Side motion with 30-second-averaged result of Top. As a result, the maximum error of 40 mm/s was found during the flight with a five-meter distance between Top and Side. From these results, we also discussed to construct simple equipment for airborne-based DL GPS occultation measurement.

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