Open Loop Tracking of Radio Occultation Signals from an Airborne Platform

T.D. Lulich, J.L. Garrison, J.S. Haase, Y-M. Yang, J. Voo, F. Xie, P. Muradyan

Abstract:	Radio occultation (RO) is a remote sensing technique that uses signals from the Global Positioning System (GPS) to determine electron density in the ionosphere, and temperature and water vapor content in the neutral atmosphere. In the lowest part of the atmosphere where there are strong refractivity gradients due to boundary layer structure, the signal may experience rapid phase accelerations, extending beyond the operational range of traditional receiver tracking loops. Open loop (OL) tracking is a method for tracking occulted GPS signals by replacing a traditional feedback loop with an algorithm for tracking the signal using a model-based estimate of Doppler frequency and a record of the GPS data bits. The OL tracking method described in this work operates using data recorded by an airborne receiver. Occultation data were recorded by the GISMOS system aboard the NSF research aircraft HIAPER during flights over the Gulf of Mexico and Southeastern United States in February 2008. Rising and setting GPS occultations were recorded during five flight missions. The OL tracking method was demonstrated on GPS signals recorded from setting occultations during this campaign and was shown to successfully track signals to lower elevations than a highquality commercial receiver using a traditional phase-lock loop method. OL tracking was able to extract continuous phase information down to approximately -3.9, -4.2, and - 4.5 degrees elevation relative to the local horizon, on three setting occultations observed on 22 February 2008. Finally, the accuracy of OL measurements, averaged over a Fresnel zone, is shown to be on the order of 3 mm/s, which is well within the limit for extracting useful scientific measurements.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	1049 - 1060
Cite this article:	Lulich, T.D., Garrison, J.L., Haase, J.S., Yang, Y-M., Voo, J., Xie, F., Muradyan, P., "Open Loop Tracking of Radio Occultation Signals from an Airborne Platform," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 1049-1060.
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