A Dynamic Satellite Search Scheduling for GNSS Super Constellation

Kun-Tso Chen

Abstract:	No information can be used to aid satellite searching at cold start of a satellite communication system, such as GNSS. Before the receiver position and system time are computed and the satellites almanacs are collected, the visible satellites cannot be determined. Hence, all the possible satellites must be searched. It takes a lot of time to search all the possible Doppler frequencies and PRN code phases of a satellite before determining a satellite’s existence. Therefore, the uncertain range of a satellite’s parameter is three dimensional: satellite ID, Doppler frequency and PRN code phase. The hardware speedup is usually performed to reduce the searching time of Doppler and code phase uncertainty. That is, a lot of equivalent correlators are used in the satellite acquisition process. Although many techniques are developed to reduce the searching time in acquisition of the Doppler frequency and PRN code phase, little attention has been given to process the unknown satellite ID. This is because it is difficult to reduce the number of possible of satellite IDs without any information available. When more and more GNSS systems are built up, such as GPS, Galileo, GLONASS, Compass, QZSS and so on, a generic receiver must search all the possible satellites to use the benefits brought by the super GNSS constellation. How to allocate all the satellite candidates to the search channels efficiently to reduce the Time To First Fix (TTFF) becomes a performance bottleneck even when the high speed hardware is available. We can keep using more and more correlators to search all the possible satellites but it costs a lot of ASIC area and power consumption. On the other hand, we can design a smarter scheduling algorithm to reduce the number of satellites to search. In this paper, we propose several satellite search schemes that use the search result of satellites to reduce the satellite ID uncertainty. We define conditional visibilities for each satellite. A conditional visibility is the possibility that one satellite is visible if so is the other satellite. We can use the conditional visibility and the satellite searching results to maintain searching weights for each satellite. Then, we can make sure which satellites are visible more and more confidently as more and more satellites are tried. When choosing which satellite to search next, we can always search the most possibly visible satellites first in order to reduce TTFF. Numerical examples show that the proposed scheme can attain 1/3.67 TTFF of the traditional sequential search scheme. By using this dynamic satellite search scheduling, we can use the expensive correlators more efficiently and improve TTFF performance especially when a lot of satellites are required to search.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	212 - 220
Cite this article:	Chen, Kun-Tso, "A Dynamic Satellite Search Scheduling for GNSS Super Constellation," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 212-220.
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