## 4 Research Test  
To analyze the application of the presented research method in aviation, GPS/GLONASS satellite data were used from a flight test performed with a Seneca Piper PA34-200T aircraft at the civil airport EPML (European Poland Mielec) in Mielec, Poland. The test flight took place in the morning and lasted from 12:47:50 to 14:02:24, according to GPST time. The horizontal trajectory of the test flight is shown in Figure 1.  
A Topcon HiperPro dual-frequency dual-system geodetic receiver was deployed on-board the Seneca Piper PA34-200T aircraft to record GPS/GLONASS code-phase observations on the L1/L2 frequency. The Topcon HiperPro receiver was located in the cockpit. The Topcon HiperPro receiver recorded GPS/GLONASS code-phase observations with an interval of 1 s. The flight test was performed in real time. The receiver was collecting real-time GPS/GLONASS data for the numerical analysis.  
The stage of the first scientific research for this paper was carried out in geodetic software CSRS-PPP v.3 [42]. CSRS-PPP software used the PPP absolute positioning method to determine the coordinates of the aircraft. The position of the Seneca Piper PA34-200T aircraft was determined from the GPS and GLONASS solutions and expressed in geocentric coordinates, according to Equations (1)-(3). The IGS products, such as precision ephemeris, precision satellite clocks and satellite and receiver antenna phase center characteristics, were used in the calculations. In addition, the P1/P2 codes and L1/L2 phase observations in GPS and GLONASS from the Topcon HiperPro on-board receiver were used in the calculations. The calculations assumed a pseudo-range measurement error of 0.3 m in GPS and 0.6 m in GLONASS and a phase measurement error of 0.003 m in GPS and 0.006 m in GLONASS. The elevation mask was 10 degrees for the GPS and GLONASS observation cut-off. For the stochastic modeling of the determined parameters, the following settings were assumed [38; 39]:  
* the coordinates of the aircraft are modeled from white noise,
* the receiver clock offset is modeled from white noise,
* the phase ambiguity for each satellite is modeled as a constant value;
* the ZWD is modeled as a random walk parameter.  
Other model parameters, such as the multipath effect, tidal and geodynamic corrections and phase slip, were used in the calculations.  
The second part of the research was carried out in Scilab v.6.0.0 [43] for Equations (4)-(9). In the Scilab program, the Seneca Piper PA34-200T aircraft's position coordinates were imported from the GPS and GLONASS solutions, and then, the alignment model was set up according to Equation (4). The final calculations in Scilab were performed for mathematical Equations (4)-(9). Calculations in Scilab were performed for all readings of the aircraft position from the GPS and GLONASS solutions. The reference position of the aircraft was calculated using the RTK-OTF solution in Trimble Total Control v.2.700 software.  
Figure 1: The aircraft trajectory in-flight experiment in Mielec airport [own material].  
Numerical calculations in the CSRS-PPP software, Scilab software and Trimble Total Control program were performed post-factum after a flight test of a Seneca Piper PA34-200T aircraft.  
For the purposes of scientific research in aviation, the state of the ionosphere was determined by the VTEC (Vertical TEC) parameter. The VTEC value was determined based on the GIM (Global Ionosphere Maps) ionosphere model from the CODE (Center for Orbit Determination in Europe) Analysis Center in Switzerland [44]. The VTEC parameter fluctuated during the flight tests from 16.8 TECU to 17.3 TECU.