Department of Physics

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Browsing Department of Physics by Author "Bab, Irekere Hellen"

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    Characterization of the multipath environment of the global navigation satellite system receivers over Uganda.
    (Busitema University, 2024) Bab, Irekere Hellen
    Errors in the data tracked by the Global Navigation Satellite System (GNSS) receiver consist of errors from the satellites, errors from the propagation medium such as the ionosphere and multiple signal paths surrounding it, known as the multipath errors. Some efforts to curb multipath were made such as setting a fixed elevation cutoff for the receivers and placement of receivers in an open environment. However, this does not cater for the different heights of the obstacles in the receiver environment implying that a fixed cutoff elevation cannot be set for such environment. In addition, placement of receivers in an open environment may not be possible most especially in cities where there are many buildings and yet data from such places is also needed for ionospheric studies. Therefore, in this study the possibility of a variable elevation cutoff for each receiver environment in Uganda was investigated. Potential sources of multipath were first identified for the different receivers in Uganda. The results show that there exists a stationary object at an azimuth of 00-300, 700-1100, 2400-3000 and 3300-3500 from Mbarara-GPS receiver. This object can block satellite signals as far as 2000 elevation. Similarly for the case of the receiver at Entebbe, stationary objects exist at an azimuth of 00-200 and 2200 3300. The stationary structures can obstruct satellite signals at an elevation of 00-200. Basing on the variation of elevation against azimuth, obstacles at Mbarara-SCINDA exist at azimuth angles of 00-200 and 3400-3600. For this receiver, the satellite paths were not visible at elevation angles of 100 -200. The receiver at Makerere has stationary obstacles at azimuth angles of 00-200 and 3400-3600, as observed from the variation of elevation against azimuth for this receiver. The satellite signals were obstructed at elevation angles of 100-200. Likewise, the receiver at Mt. Baker has obstacles at 00-1400 azimuth. At elevation angles of 00-200, the satellite signals were obstructed. Amplitude scintillation together with the S4proxy were used to identify multipath events in space and time. Before using the S4proxy, it was validated against amplitude scintillation from collocated receivers. The results show that majority of the scintillation events occurred 0 between 18:00 Universal Time (UT) and 22:00 UT during the selected days of the year. These scintillations occurred during post sunset because they were initiated by the Rayleigh Taylor Instability (RTI) mechanism. This mechanism occurs when light plasma supports dense plasma. The mechanism occurs in the F layer of the ionosphere. It is during post sunset that the bottom side of the F layer contains light plasma while the topside of the F layer contains denser plasma. This is because during that time, the ionosphere is high enough to overcome recombination of ions and electrons, thus making the topside F layer denser than the bottom side. When light plasma supports dense plasma, it becomes unstable, which makes the dense plasma to sink into it, while the light plasma to drift upwards. This creates a fluctuation in the electron density in the ionosphere, which causes ionospheric scintillations. The days considered in this study were selected on the basis that they had high scintillation occurrence for both amplitude scintillation and the S4proxy. The seasonal pattern reveals higher likelihood of occurrence of both amplitude scintillation and the proxy in the equinox months. The S4proxy highly correlate with the amplitude scintillation, with a correlation coefficient ranging between 0.8323 and 0.9312. The S4proxy was then used to characterize the environment of the receivers where scintillation monitors were not available. Multipath events were identified on the different satellite links. These events varied from one receiver to another, suggesting differences in the characteristics of the environment of the various receivers. The scintillation events around MBAR receiver were observed by satellite number 22 and took place between 5:00 UT and 6:00 UT on the DOY; 002, 003, 004, 005 and 006. Satellite number 22 was selected because it was in view for all the five consecutive days that were selected for the above receiver. The PRN 18 observed scintillation events near MAK occurring between 2:00 UT and 3:00 UT, spanning from Day of Year 041 to 045. Between Days of Year 003 and 007, PRN 25 detected scintillation events near EBBE from 2:00 UT to 4:00 UT. Similarly, PRN 18 observed scintillation events near BAKC on days 017 through 021 of the year. Satellite number 31 detected scintillation events near the MBA receiver from DOY 289 to 293 between 5:00 and 10:00 UT. These scintillations were observed by only one satellite at each station simply because that particular satellite was in view for all the five consecutive days that were selected for the scintillation occurrence. Amplitude scintillation was then mapped on an azimuth- elevation space to identify elevation threshold for each azimuth. The azimuth-elevation threshold values were then modeled using splines. The model was then used to determine the varying elevation threshold for each receiver. Comparing the varying elevation threshold with the fixed elevation threshold, more data was obtained using the varying elevation threshold and the quantity of that data varied from one receiver to another.