L1 Backup Navigation for Dual Frequency GPS Receiver

Y. Yang, R.T. Sharpe, R.R. Hatch

Abstract:	GPS satellites transmit a spread spectrum signal on L1 and L2 at 1575.42 and 1227.6 MHz, respectively. These different frequencies allow the user to estimate the ionosphere delay error, which is one of the main sources of navigation error. Dual frequency GPS receivers are therefore capable of generating refraction corrected (ionosphere free) measurements which can provide more accurate positioning than single frequency receivers. However, the GPS L2 signal is more di±cult to acquire and track for two reasons: a) the broadcast signal strength is lower than on L1; and b) civilian receivers must use less efficient codeless or semicodelss techniques to recover the P2 code from the encrypted signal. This paper presents a technique for L1 backup navigation in the event of intermittent loss of the L2 signal. A three-state Kalman filter is used for each GPS satellite. When both L1 and L2 signals are available for the satellite, L1 and L2 observables are used to estimate ionospheric refraction delay, delay rate, and a combination of integer ambiguities on L1 and L2 carrier-phase measurements. When the L2 signal is lost, the L1 signal is used together with the estimated states to synthesize L2 observables during the outage. When the L2 signal is reacquired, state estimation is resumed. Special techniques are used to smoothly transition between the synthesized and real L2 observables. Experimental results are presented showing that dual frequency, refraction corrected navigation accuracy is sustained through significant periods of L2 signal loss. The technique described herein could be applied to synthesize L1, L2 or L5 measurements if any one of the three frequencies is retained. The missing measurements at one of the frequencies can be synthesized using the measurements from the retained frequency that was not lost.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1258 - 1263
Cite this article:	Yang, Y., Sharpe, R.T., Hatch, R.R., "L1 Backup Navigation for Dual Frequency GPS Receiver," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1258-1263.
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