## 2 Experimental Section
### Real-Time Approach  
A well-known and more accurate real-time positioning technique is the Real Time Kinematic (RTK) technique, where two geodetic receivers are used, and for limited inter-station distances (around ten to 15 km between master and rover), it is possible to determine the ambiguity value as an integer number in RT (Real Time) in order to obtain centimetric accuracy.  
The Ground Base Augmentation System (GBAS) positioning that uses a network of permanent stations has allowed the breakdown of this limitation and the improvement of the reliability of positioning. With the use of geodetic receivers, the differential corrections provided by these networks allow Network RTK (NRTK) positioning.  
These infrastructures are managed by control centres, which manage and broadcast differential corrections [18] to the rover: these corrections are composed of spatial correlated biases of the GNSS signal (such as ionospheric and tropospheric corrections, _etc._) that can be broadcast to the rover in various ways (Virtual Reference Station(r)--VRS, Master Auxiliary Concept--MAC, Flachen-Korrektur-Parameter--FKP, and Nearest correction-NRT) due to the Radio Technical Commission for Maritime Services (RTCM) protocol [21] that are well described in literature [22] in order to fix the ambiguity on the carrier phase measurements [23; 24].  
Permanent stations usually have inter-distances of about 60 km. These infrastructures are not designed for use with mass-market receivers; however, the VRS(r) correction may provide a useful improvement of the positioning accuracies of these receivers and with some limitations (the low accuracy and the time to fix) we can achieve the phase ambiguity fixing. Later we will demonstrate the limits and the results obtained with the two mass-market receivers.  
Also for these real-time experiments, we considered the NRTK Regione Piemonte network shown in Figure 1. The figure shows the area where the measurements were carried out. This network has a typical Italian mean inter-station distance (about 60 km) and can be used as a representative network in order to evaluate the performances of the receivers previously cited. In order to evaluate the performances of the instruments described in Table 1, we decided to compare the positioning performed with the u-blox and NVS receivers at the same time. To do this, a splitter was used: this instrument allows splitting the GNSS signal, which arrives at the antenna for more than one receiver (Figure 2).  
To perform NRTK positioning, the routine RTKLIB V. 2.4.2 was used [25]. In particular for these experiments the RTKNAVI tool was used. This tool allows input for both raw data (pseudorange and carrier-phase measurements) of the u-blox and NVS receivers (simultaneously, as in Figure 3) and stream data coming from a network with NTRIP authentication. The RTKNAVI software uses a modified LAMBDA method for fixing integer ambiguities. The LAMBDA method [26] has been widely used in GNSS positioning. For real time applications computational speed is crucial: the modified LAMBDA (MLAMBDA) method [27] reduces computational complexity of the LAMBDA method.  
Considering these experiments, we have fixed the ambiguities using a \"ratio\" equal to 3 both for the GPS and GLONASS constellations using a \"continuous\" method. This means that the software verifies whether the fix of the phase ambiguity is correct at each epoch. Moreover, we have chosen a cut-off angle of 10\({}^{\circ}\) in order to neglect the signals coming from satellites with low elevations (that are very noisy).  
Figure 3: RTNAVI interface: parallel analysis of u-blox and NVS positioning.  
Figure 2: Using the antenna splitter, the positioning results of two receivers can be properly compared.