# Integrity Analysis for GPS-Based Navigation of UAVs in Urban Environment
## 3 Simulation  
### Simulation Setup  
A large amount of data is required to understand the impact of environmental parameters on navigation performance. These data can be simulated, collected by a real receiver, or found from publicly available databases. It should be noted though that isolation of the error sources in the datasets from real scenarios is difficult due to uncertainties of the transmission environment. Moreover, it is challenging to set a local urban environment in a way that addresses our test requirements entirely. Even if it is possible, another challenge occurs at the space segment and transmission environment arrangements since we have no control over them. Thus, a combination of software and hardware simulation devices were selected in this study in order to simulate all segments of the GPS navigation environment instead of experiments in a physical environment, where a real flight test in this sort of GNSS challenging environments under high cut-off elevations will cause high safety risks. The simulation environment employed in this study for integrity analysis consists of Spirent GNSS Simulator GSS7000 controlled by the SimGen software as outlined in Figure 6. SimGen [41] provides a variety of options to define scenarios and parameters related to all GNSS segments. Based on the definitions set in SimGen, GSS7000 generates not only navigation messages but also the navigation environment with all details, such as the position of satellites and receiver, potential faults, tropospheric or ionospheric effects and obstructions. With scenario definitions in the GSS7000 simulator, effects of different error sources in the navigation environment can be separated and studied independently. At the second step of the simulation environment, a MATLAB-based receiver model was used to obtain position information based on the environment simulated by GSS7000 for the following analysis. When using real receivers for this type of experiment, results may change dramatically from case to case depending on the quality of the receiver and specifics of the implemented algorithms. Therefore, the software receiver has been selected to conduct experiments in order to minimize effects of unknown receiver-based errors, ensure reproducibility of results and allow for future algorithms improvement studies. In this work, an open source receiver model, goGPS [41], was used for fundamental receiver calculations. In our tests, inputs and outputs of the model were modified to integrate it with GSS7000 and observe desired navigational parameters. The input data for the receiver model are presented in Radio Technical Commission for Maritime Services (RTCM) format; outputs are DOP coefficients, with receiver position expressed in Latitude-Longitude-Altitude (LLA) and in East-North-Up (ENU)  
Figure 6: Simulation environment. coordinates. For simplicity, the receiver's initial position was selected as (0,0,0) based on LLA coordinates, and the urban environment was produced in the environment editor of the simulator. Simulations in this study were conducted based on a single frequency (L1) GPS constellation scenario for a generic urban environment defined through the corresponding elevation map in the simulator. Assuming that the receiver is at the ground level, the cut-off elevation angles in urban areas vary between 20 and 60 degrees. A value of 30\({}^{\circ}\) was taken as the maximum value for cut-off elevation during the simulations; beyond this value, the GPS-based navigation is not usable according to preliminary results. The other assumptions made were as follows: UAV is flying a maximum of 10 m below the roof level and in the middle of 30 m wide street with a 30\({}^{\circ}\) maximum cut-off elevation. The flowchart of the proposed approach for analyzing integrity is shown in Figure 7.  
At the first simulation, the reference model of DOP impact on the accuracy was established. Positioning results were acquired and analyzed statistically under 5\({}^{\circ}\) cut-off elevation, which represents the open sky condition.