Analysis of Error Sources in Phase Rate Measurements in GPS Radio Occultation

Elliot H. Barlow, Penina Axelrad, Peter Withnell, and Danielle Nuding

Abstract:	Recent years have seen a dramatic increase in the use of GPS radio occultation (RO) measurements for numerical weather prediction and ionospheric and climatological research [2]. These measurements, provided by an international series of missions, including CHAMP, METOP, and most significantly, COSMIC, have shown a demonstrable improvement in weather forecasting and space weather modeling [1]. Several new Earth-observing missions have been considered to conduct GPS RO measurements, including CICERO [9], CLARREO [15], and COSMIC-2 [4], aiming to increase the coverage, precision, and sensitivity of atmospheric soundings in the future. Driving demand for more and better atmospheric soundings to satisfy the needs of the atmospheric science community. As this need is likely to only increase in the future, it is necessary to fully understand and characterize the error sources in RO measurements and their impacts on the RO data products While past and existing GPS-RO satellites and programs have been developed primarily by governmental agencies and contractors, there exists considerable interest in deploying commercial satellites and networks to provide this kind of for government, scientific, and private industry users. The Community Initiative for Cellular Earth Remote Observation (CICERO), a planned pathfinder constellation of two microsatellites of approximately 100 kg each in a near-polar orbit [12], is an example of this type of commercial venture. The purpose of the CICERO mission is to help "bridge the gap" anticipated between the decommissioning of COSMIC and the launch and commissioning of COSMIC-2 and the higher quality data the latter will provide. This will address both the strong need for long-term continuity in climatology data, and the constant requirement for accurate and up-to-date atmospheric soundings for meteorology. The error analyses presented in this paper represents the original needs and goals of the CICERO mission, but are also generalizable, and potentially of use, to other forthcoming missions making GPS RO measurements Prior investigations into GPS-RO measurements have provided a clear connection between observables and the final science products. Work by Kursinski [11] and others [7] derived expressions for the propagation of GPS-RO phase observation errors to the science products, including bending angles, refractivity, and temperature. This paper attempts to extend that work by deriving requirements that must be imposed on a small low Earth orbit (LEO) satellite design to achieve the phase observation accuracy and precision required to support ionospheric and atmospheric climate and weather science and weather prediction. Understanding directly how the spacecraft design and performance influence the quality of the data products can help reduce the size, weight, power, and cost of future space borne GPS-RO measurement platforms.
Published in:	Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014) September 8 - 12, 2014 Tampa Convention Center Tampa, Florida
Pages:	1478 - 1491
Cite this article:	Barlow, Elliot H., Axelrad, Penina, Withnell, Peter, Nuding, Danielle, "Analysis of Error Sources in Phase Rate Measurements in GPS Radio Occultation," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 1478-1491.
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