Abstract: In　GPSINS　ti ghtl y－coupled 　navi gationlow　precision　of　GPS　pseudo－range　observation　reduces　the　navi gation 　accurac y．In　order 　to 　resolve 　the 　issuea　GPSINS　ti ghtl ycoupled 　navi gation 　algorithm　based　on　Doppler 　smoothing　pseudorange　is 　proposed．First 　the　dynamical　model　and　observation　model　of　GPSINS　ti ghtl ycoupled 　are 　introduced．Based 　on　phase　smoothing　pseudo－rangethe al gorithm　of　Doppler 　smoothing　pseudorange　and　desi gn　formulas 　of 　variancecovariance 　are 　given．At　the 　same 　time genetic　al gorithm GAis 　introduced 　to 　estimate 　the 　smooth　factor．A　vehicle 　experiment　was　made　to 　test 　the　validit y　of 　new　al gorithm．Experimental 　results 　indicate 　that 　smooth　al gorithm　is 　able 　to 　improve 　the　precision　of 　pseudorange　observationwhich　increases 　the　position　accuracy　of 　inte grated　navi gation．Compared　with　phase　smoothing　pseudo－rangeDoppler 　smoothingpseudorange　has 　litter 　better　performance 　as 　inte grated　observation．The　Doppler 　smoothing　pseudorange　eliminates 　the 　influence 　of 　cycle 　sli p　and 　increases 　computational 　efficienc y．At　the 　same 　timethe 　accurac y　of 　pseudorange　observation 　is 　improved 　to 　the 　accurac y　of 　pseudo－range　rate which　enhances 　the 　stabilit y　of 　filter 　system