SiRF InstantFix II Technology

Wentao Zhang, Vijay Venkatasubramanian, Hongxin Liu, Makarand Phatak, Shaowei Han

Abstract:	SiRF InstantFix II is an autonomous (no need of any network connectivity) version of SiRF’s original InstantFix technology (that needs network connectivity for assistance or updates). It is available for both SiRFstarIII GPS receivers and SiRFAtlas and SiRFPrima multifunction SoC navigation processors. The technology allows navigation warm starts in under-five seconds. Further it allows use of weak signals down to 15 dB-Hz to obtain pseudo-range measurements under certain conditions. SiRF InstantFix II continuously refines GPS orbit and clock calculations based on the latest broadcast ephemeris data it receives from the satellites being tracked every time the GPS receiver is used. This is done by starting with one broadcast ephemeris and numerically integrating satellite dynamical equations by using efficient integration methods and simplified force models. If new broadcast ephemeris data becomes available, typically over next one or two days when the same GPS satellite again becomes available, the old and the new broadcast ephemeris information is used together to estimate some of the force model parameters and to achieve even better accuracy of orbit prediction. Observed 68 percentile satellite errors along line of sight (LOS) are 22 m (with one broadcast ephemeris for 3 days prediction), 13 m (with broadcast ephemeris over past two days for 4 days prediction) and 9 m (with broadcast ephemeris over past three days for 5 days prediction) for satellite position along line of sight and 11 m for clock for 5 days of prediction (mostly independent of number of broadcast ephemeris). SiRF InstantFix II also allows flexibility to make use of network connectivity if one is available by automatically and optimally mixing orbit and clock information from SiRF Instant Fix and SiRF InstantFix II. SiRF InstantFix has better position accuracy and currently supports up to 7 days of orbit and clock predictions whereas SiRF InstantFix II has less position accuracy and currently supports up to 5 days of predictions. If both predictions are present then selection is done by using age of each of the predictions to decide which position and which clock prediction will be more accurate and then blending the minimum error position and clock predictions to arrive at the optimal prediction. The so obtained GPS position and clock predictions are used in place of broadcast ephemeris to perform navigation when four or more satellites are available. The predictions are also used to achieve signal acquisition of weak signals down to 15 dB-Hz. In recent work the predictions are further used to perform decoding of broadcast ephemeris with weak signals down to 21 dBHz.
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:	1840 - 1847
Cite this article:	Zhang, Wentao, Venkatasubramanian, Vijay, Liu, Hongxin, Phatak, Makarand, Han, Shaowei, "SiRF InstantFix II Technology," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 1840-1847.
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