Patent claim: An improved vehicle navigation system comprising a GPS receiver carried aboard a vehicle and subject to interruptions of signal transmissions from orbiting navigation satellites a DRcomputer coupled to the GPS receiver and providing for range and heading information computed by dead reckoning during periods of said signal interruptions and such that vehicle navigation solutions and assistance are continually available through a screen display to a user wheeltick sensors coupled to the DRcomputer and providing for rotational data related to the turning of individual wheels that carry said vehicle a road map database having road segment information that can be matched to said vehicle navigation solutions the improvement characterized by a network server for providing road segment information from the road map database for a limited local area on request by a motor vehicle to a look ahead database carried by the vehicle an estimate of the true radii of said individual wheels wherein estimates of the true radii of said individual wheels are combined with said rotational data from the wheeltick sensors to generate change in range deltarange and change in heading deltaheading information related to movements of said vehicle wherein said deltarange and deltaheading information are propagated by the DRcomputer from absolute position information periodically obtained from the GPS receiver with thedead reckoning results alone sometimes being output as vehicle navigation solutions wherein road segment information stored in said look ahead database is matched to said vehicle navigation solutions and is used to correct drifts in dead reckoning propagation during periods of GPS outages wherein a combined GPS and dead reckoning position is corrected for drift each time a snapping error exceeds a predetermined value wherein said vehicle navigation solutions and assistance are continually made available through a screen display to a user and that are derived solely from the GPS receiver solely by the DRcomputer or from a mix provided by the GPS receiver and the DRcomputer that have been periodically corrected for drift away from a road segment by feedback correction and wherein a map matching function is based on server provided map line segments.

Related passage: In the present example typically since the GPS sensor 10 and the DR sensor 20 include an error in the measured value the current location detector 40 performs mapmatching to correct the error.For instance the GPS sensor 10 may have errors such as ionization layer delay error satellite clock error multipath and the DR sensor 20 may have an initial alignment error and a conversion factor error.Particularly when the car passes high buildings trees tunnels etc.the car may not sufficiently receive the GPS signal and thus the error becomes larger.Also when the error is accumulated it is difficult for the current location detector 40 to accurately determine the location of the car.When location information of the car measured using sensors which have errors as described above is indicated on a map the location information does not agree with the actual location of the car.Accordingly in order to correct errors a typical current location detector 40 performs mapmatching using a digital map.