Analysis of Timing Errors under Limited Sky View Conditions

S.N. Shankara-Murthy, G. Vyasaraj, B.S. Nandini, B. Srinivasa

Abstract:	Accurate timing source is very important in those applications that need a precise synchronization over a wide geographical area. The 1 Pulse Per Second (PPS) output of GPS receiver provides timing and frequency synchronization for applications in the field of navigation, mobile communications, remote sensing and intelligence. Advancements to the timing solution in providing accurate and continuous time dissemination and synchronization would provide better overall time coordination between distributed elements in the system. Timing GPS receivers earlier enjoyed the advantage of having their antennae located with a clear view of the sky and of being stationary. With the adoption of such receivers in a myriad of timing applications, the demands on such receivers have increased manifold. Some applications such as Femto cells require accurate timing in indoor conditions. Even those applications that once had open sky views are now facing obscuration from growing urban development. It is in such circumstances that there is an increasing demand being placed on Timing GPS receivers to deliver improved accuracies on a continuous basis. This paper describes the restricting factors in getting a reliable timing output under indoor and limited sky view conditions and provides an overview of the probable efforts to reduce these errors. A Study of GPS measurements in indoors is conducted and their effect on timing accuracy is discussed. The design of tracking loops has a large influence on the Pseudo-range measurement accuracy. They must be designed to mitigate the impact of various GPS error sources on measurements. In spite of having sophisticated tracking loops, it would not be possible to ensure error free GPS measurements under urban canyons / indoor conditions. Hence, it is necessary to incorporate error reduction/elimination mechanisms in the navigation solution. Towards this an assessment of the signal characteristics in terms on measurements errors and Signal to Noise Ratio (SNR) variations is conducted under indoor or limited sky view and open sky conditions. Tests were repeated to arrive at a statistical estimate of the performance. A comparison of these estimates is presented and the impact of signal parameters on the timing output is discussed. The study explores the feasibility of using the timing receiver in limited sky view conditions and provides a stimulus for developing newer algorithms to improve the timing performance.
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:	2126 - 2134
Cite this article:	Shankara-Murthy, S.N., Vyasaraj, G., Nandini, B.S., Srinivasa, B., "Analysis of Timing Errors under Limited Sky View Conditions," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2126-2134.
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