## 5 Results and Discussion
### Interpretation of GNSS Prediction Results  
The results of the viewshed analysis for a single point in time at Maynooth University are displayed in Figure 9a. This image displays the output from the proposed satellite visibility methodology after obstructions have been included. The 2D vector polygons representing the building footprints have been included as an aid to visualising these obstructions when interpreting the results. Areas in green represent areas of four satellites or more whereas areas in red are areas visible to fewer than four satellites, implying that a GNSS receiver would not be capable of calculating an accurate 3D position at that location. Upon examining the results a very definite trend towards GNSS shadows in the south and southeast of the buildings was apparent, with tall buildings exhibiting longer shadows than smaller buildings. This overall trend in shadow distribution implies that the satellite constellation was primarily in the north and northwest at the time of the survey. This assumption was verified when a plot of satellite azimuth throughout the prediction tests was created (Figure 9b), with the majority of GNSS satellite orbits visible from the quadrant of azimuth of 270 deg-350 deg. The values on the Y axis represent the total number of satellites measurements recorded at that azimuth throughout the tests--for example, 141 satellite positions were recorded throughout the tests and approximately 95 of those observations were positioned at azimuths of 270 deg-350 deg from the observer position. An initial examination of the shadows in the test area suggest that the method has successfully modelled the obstructions in the area:  
* The long slender shadow to the south of the central courtyard in the prediction results was caused by the tall spire of the cathedral on campus, which is over 80 m in height.
* Areas of the smaller courtyards were almost completely in shadow, implying that if a surveyor were operating in that area they would find it extremely difficult to acquire any GNSS signal, as would be expected in real life.  
The cause and shape of other GNSS shadows, such as the long, slender shadow in the northwest of the map were more difficult difficult to identify:  
* This shadow was potentially caused by a satellite in the southwest of the area that eliminated a shadow close to the building or a satellite high in the horizon in the northwest that was able to view part of the southern face of the building, thus eliminating the rest of the shadow in this area. Figure 9b proves that there were satellites in the southwest throughout the survey.
* Alternatively, there could be an error in the TIN in this area, as this building exhibited a triangular extension in Figure 7c. This was not apparent in the resulting raster DSM, however, so the ultimate cause is uncertain.  
Figure 10: Validation tests (**a**) validation points selected as representative of the surrounding environment (**b**) azimuth of the two predominant shadowing objects throughout the validation tests at each of the ten test locations.  
Figure 9: Results and accuracy Tests (**a**) visualising output from the proposed methodology—green areas: visible to four plus satellites, red: less than four satellites (**b**) a plot of satellite azimuths throughout the validations tests—red represents azimuth and number of satellites, grey numbers on Y axis represent total number of satellites visible from the observer location throughout the tests.