Acquisition and tracking of weak GPS signals as received by cellular telephones
Grant, Howard Alexander
This thesis investigates the suitability of global navigation satellite system (GNSS) signals for cellular phone location. The requirement is to determine and report the location of a phone during an emergency call. The thesis analyzes acquisition and tracking techniques suitable for very weak signals as received by a cellular phone indoors. The L1 and L5 signals from GPS satellites and the L1 signal from Galileo satellites are considered. It is shown that long integration times and coherent integration are required for the weakest expected signals. Long coherent integration times require a precise knowledge of the Doppler shift due to the range rate of the satellite. The tolerance to Doppler shift can be increased by using FFTs in the analysis of the data. Non-coherent averaging techniques improve the Doppler tolerance but compared to coherent averaging, the loss of signal to noise ratio is too large for the weakest signals. Coherent averaging of the GPS L1 signal requires data removal that can be accomplished with assistance from the cellular network. The GPS L5 and Galileo L1 signals include a data-less or pilot channel. The GPS L5 pilot channel includes a 20 bit Neuman Hoffman code with a bit period of 1 ms. This code has to be acquired or removed before coherent averaging. Similarly the Galileo pilot channel includes a 25 bit short code. Once code acquisition has been accomplished, it is necessary to track the signals from at least four satellites for long enough to compute a position estimate. A discussion of tracking techniques is included to show the signal to noise ratio limitations for adequate tracking accuracy. The results show that GNSS signals are suitable for cellular phone location in a large number of situations. Increased receiver sensitivity would permit location in additional situations. In rural situations GNSS may be the only available option.
DegreeMaster of Science (M.Sc.)
Copyright DateDecember 2010
Acquisition and Tracking
Cellular Phone location