GPS World, March 2018

For lower quality data ambiguity fixing does not provide such quick initial solution convergence Fixing is dependent on the quality of the float solution and for PPP the latter requires time to reach acceptable levels of accuracy Therefore depending on the application PPP AR may or may not be helpful To consistently reduce the initial solution convergence period PPP processing requires additional information as is the case for network RTK in which interpolated satellite orbit ionospheric and tropospheric corrections are needed since doubledifferenced RTK baselines over 10 to 15 kilometers in length contain residual atmospheric errors too large to effectively and safely resolve phase integer ambiguities For PPP uncombining the ionospheric free code and phase measurements from the conventional model is required to directly estimate slant ionosphere propagation terms in the filter state In this form the model can allow for very quick re initialization of short data gaps by using the pre gap slant ionospheric and zenith tropospheric estimates as down weighted a priori estimates post gap making these estimates bridging parameters in the estimation filter Expanding this approach external atmospheric models can be used to aid with initial solution convergence FIGURE 6 illustrates for a large dataset that applying a spatially and temporally coarse global ionospheric map GIM to triple frequency four constellation float processing can reduce one sigma convergence time to 10 centimeters horizontal positioning error from 16 to 6 minutes If local ionospheric and tropospheric corrections are available and AR is applied PPP sometimes now referred to as PPP RTK can produce RTK like results with a few minutes of initial convergence to fewcentimeter level horizontal solutions PPP PROCESSING WITH LOW COST HARDWARE As the impetus for low cost precise positioning and navigation for autonomous and semi autonomous platforms such as land vehicles and drones continues to grow there is interest in processing such low cost data with PPP algorithms For example it has been shown that with access to single frequency code and phase measurements from a smartphone short baseline RTK positioning is possible It has also been shown that similar smartphone data can be processed with the PPP approach MARCH 2018 WWW GPSWORLD COM GPS WORLD 45