A Broadcast Ephemeris Extension Method for Standalone Mobile Apparatus

C-T. Weng, Y-C. Chien, C-L. Fu, W-G. Yau, Y.J. Tsai

Abstract:	GNSS users usually expect to obtain their locations immediately once they initially started up their navigation functionality of their mobile devices; however, in poor signal conditions, there can be a significant delay for a receiver to provide the first fix. Typically, it takes 18 to dozens of seconds to receive and decode the broadcasted messages from visible satellites. Currently, with network connections available, an AGPS server can help to diminish the aforementioned delay. The remote GNSS receivers must connect to a source of assistance information to download and determinate a fix. A major portion of the assistance information is a satellite navigation message, such as ephemeris or satellite trajectory prediction data. However, broadcast ephemeris is typically valid for a few hours, as such, the GNSS receiver must periodically connect to the AGPS server. Recently, there is a trend in the art to provide the satellite orbit and clock data which are valid for an extended period. In this way, users can access the connection in a longer period. However, users need an option of falling back to standalone devices when the network is not available or especially, when using low-end devices without network connectivity. Furthermore, without relying on network assistance, the benefit is the zero cost of data exchange and network maintenance. The orbit determination technology and satellite trajectory prediction can be implemented by using a complicate yet very precise orbit model, which is equipped with a plenty of observations that require strong calculation capability to execute the processing. However, the computational burden may not be acceptable for onboard orbit determination. In this paper, a method of Broadcast Ephemeris Extension (BEE) technology for a standalone mobile apparatus is proposed and developed by MediaTek. MediaTek has developed its own force models and so far the algorithm can be achieved and demonstrated on highend portable devices with plenty of MIPs and memory such as PNDs, and the same with low-end (16 MHz) devices by simply reducing the process of forces models. BEE has been able to maximize the extendable ephemeris for as long as 7 days and the results demonstrate a 68th percentile of five-second TTFF (Time to First Fix) and a horizontal 30-meter accuracy (at the 90% confidence level), which have been achieved within 5 days. In addition, sub-ten seconds of TTFF with a horizontal accuracy of 50 meters can be achieved in one single day.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	2108 - 2114
Cite this article:	Weng, C-T., Chien, Y-C., Fu, C-L., Yau, W-G., Tsai, Y.J., "A Broadcast Ephemeris Extension Method for Standalone Mobile Apparatus," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 2108-2114.
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