An integration of GPS and GLONASS can effectively increase the number of visible satellites and thus improve reliability,availability and accuracy of GPS-based PPP solutions.In the combined GPS/GLONASS PPP (GGPPP),the estimated unknowns include three coordinate components,one receiver clock offset,one system time difference between GPS and GLONASS (STDGG),one zenith tropospheric delay (ZTD) and ambiguities.Theunknown parameters to be estimated are obviously too many and thus demand adequate number of visible satellites to obtain a precise position solution.However,the number of visible satellites is often insufficient in some open-pit mines.Therefore,we propose a modified algorithm for the GG-PPP to reduce the required amount of visible satellites by removing partial unknowns.The STDGG and ZTD values are firstly estimated under an open sky condition using the traditional GG-PPP algorithm.They are then used as priori known values to correct observations in the modified algorithm instead of estimating them as unknowns.The proposed algorithm is tested using observations collected at BJFS station in a simulated open-pit mine environment.The results show that the modified algorithm can effectivelyimprove the PPP performance in the open-pit mines with limited satellite visibility.