## 2 Carrier Phase Observations for Static Measurements Provided by GNSS Chipsets
The method of processing the GNSS measurements, such as the choice of correct methodology for processing GNSS baselines and managing a detailed analysis of their correct determination, also has an important influence on obtaining a reliable and accurate position. Only in the case of properly planned GNSS baselines and properly chosen reference station distance can we conduct a reliable accuracy analysis in post-processing static mode [21; 22; 23]. The baseline computation consists of a series of processing trails. Triple differencing is commonly used to determine and correct cycle slips in the datasets. A double differencing of the phase observables provides the baseline estimation. The latest post-processing algorithms involve highly advanced statistical methods for integer ambiguity determination. Combinations of the phase observations (narrow lane or wide lane) are often used as a step to confirm the final solution, according to baseline length. Permanent GNSS reference stations, widely in use for baseline determination, allow us to model the distance-dependent errors (troposphere and ionosphere) and additionally, to verify the data transmitted in real time to the user, or downloaded for post-processing purposes. In 2008, the permanent reference stations system (ASG-EUPOS) was launched in Poland offering Real-Time Kinematic (RTK) services and modules for Differential (DGNSS) surveys. This system also allows processing of the data from static measurements with a one-second measurement interval. The ASG-EUPOS reference-station system also provides virtual reference data in any location around the survey area [24]. Although, researchers found that virtual reference stations in post-processing mode do not increase the accuracy of GNSS survey at all [23]. This is related to the fact that the virtual reference stations located in reach of a few kilometers are very highly correlated to each other. When one is interested in centimeter/sub-centimeter accuracies, a rapid static survey technique will be much more reliable when conducted in relation to the local reference station [25]. Although, the virtual reference stations (VRS) in post-processing mode may still be the reference stations, in the case of rapid static measurements in which the session length is several minutes, the usefulness of a local permanent station is still recommended because the accuracy of the coordinates for the point being determined strictly depends on the accuracy of the reference station [26]. One should also mention that VRS observations rely on ionosphere and troposphere activity, which have a significant influence in rapid static surveys [27]. Therefore, for results comparison and reliability of the accuracy for the coordinates determined, the authors decided that all measurements using rapid static techniques will be much more reliable with reference to the local ASG EUPOS permanent station. In static relative positioning, both using a real reference station or a virtual one, the efficiency of determining precise baselines will be primarily affected by the accuracy of the phase measurements of the mobile receiver (in this case, smartphone) due to the quality of the phone's phase observations.