Embedded GNSS Receiver Re-Acquisition Failure Due to Thermal Variation

Aiden Morrison and Gerard Lachapelle

Abstract:	The objectives of the study discussed in this paper were to determine the failure mode of an embedded GNSS based animal tracking system that was observed to fail unpredictably, with excessive power consumption by the GPS sub-system, despite the fact that all system and ex- ternal environmental conditions were within those specified by the receiver module manufacturer. The results of the subsequent investigation showed that when operating in ’low-power’ energy conserving modes, some models of modern GNSS receiver will fail to re-acquire visible satellites in a reasonable period of time when the receiver has experienced a moderate thermal variation during the inactive period of its power saving measurement cycle. The consequence of the extended duration search is a greatly increased power consumption up to 20 times higher than the figure advertised by the manufacturer, resulting in drastically altered system operating lifetime. The study also shows that while different models of high sensitivity GPS chipsets are affected by these variations in differing degrees, most popular receivers are affected adversely by this combination of conditions. The conclusions of the study are that the reacquisition algorithms of some popular receiver chipsets are too aggressive in their assumptions of stable frequency frequency search space during their sleep intervals, and in reducing their search space too tightly around the known correct’ Doppler frequencies. Due to these overconstrained search spaces they are unable to compensate for the relatively high thermal sensitivity of low cost quartz oscillators driving the receiver, and will not reacquire in an acceptable period. Testing confirms that the sensitivity of a receiver to the effects discussed is correlated with the temperature/frequency curve of the individual quartz crystal used within the receivers under test. The consequences of the study are the indication that the designers of receivers which are to be operated in embedded environments at low power levels should qualify their re-acquisition processes that take into account the variation in local frequency that can occur due to expected temperature shifts in the environment. This testing should lead to a performance metric specifying the maximum allowed drift of the reference oscillator during power-saving operation, otherwise some thermally unstable yet highly common applications such as E911 enabled cellular handsets will encounter unacceptable power usage and potentially dangerous lack of position information, due to the thermal variations they experience during normal use.
Published in:	Proceedings of the 2008 National Technical Meeting of The Institute of Navigation January 28 - 30, 2008 The Catamaran Resort Hotel San Diego, CA
Pages:	240 - 247
Cite this article:	Morrison, Aiden, Lachapelle, Gerard, "Embedded GNSS Receiver Re-Acquisition Failure Due to Thermal Variation," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 240-247.
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