说明：  Precise Point Positioning (PPP) is currently based on the processing of only GPS observations. Its positioning accuracy, availability and reliability are very dependent on the number of visible satellites, which is often insufficient in the environments such as urban canyons, mountain and open-pit mines areas. Even in the open area where sufficient GPS satellites are available,the accuracy and reliability could still be affected by poor satellite geometry. One possible way to increase the satellite signal availability and positioning reliability is to integrate GPS and GLONASS observations.