Global Navigation Satellite Systems (GNSS) are widely used for positioning. An accuracy of achieved receiver coordinates depends on used equipment, external products, and processing software. Systematic errors influencing GNSS observations can be either eliminated or reduced by forming double-differences (i.e. RTK strategy), or they can be estimated or introduced by external products (i.e. PPP strategy). In the case of PPP, the initial convergence can be reduced with integer ambiguity resolution. However, initial ambiguities can be resolved as integers only if phase biases are employed from external products. Moreover, when precise ionospheric and tropospheric models are employed, the ambiguities can be resolved even faster. (read more)