Effects Of The Troposphere Delay Mappıng Functıons And The Elevatıon Cut Off Angles On The Tıme Serıes Of Statıon And Radıo Source Coordınates As Well As Earth Orıentatıon Parameters As Estımated From The Observatıons Of The Vlbı Technıque
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Very Long Baseline Interferometry (VLBI) is a space geodetic technique that observes the radio wave signals at S and X radio frequencies using the Earth fixed antennas (nowadays broadband i.e. from 2 to 8 GHz observables are sensed using several sophisticated VLBI antenna equipments), coming from the quasars (extra galactic radio sources), along with the atomic clocks to time tag the digitized and sampled signals. VLBI technique measures the arrival time differences of the signals between the multiple VLBI antennas. A scan is an event that two or more antennas target to the same radio source for an integration time of about a few minutes to gather enough radiation for resolving delay observables. Quasars are the radio sources which are mostly 5 to 7 billions of light years far away blazars from our galaxy. Geodetic VLBI sessions are maintained by the International VLBI Service for Geodesy and Astrometry (IVS, Schuh and Behrend, 2011). IVS Intensive sessions are held with the participation of 2 to 4 VLBI stations to monitor universal time, whereas the IVS 24 hour long sessions are held with the participation of more stations for monitoring several unique geodetic and astrometric parameters. The troposphere is the most significant error source remaining in the reduced observations of the radio wave space geodetic techniques. Hence, this thesis aims to assess the effects of using various troposphere delay mapping functions, i.e. VMF1 (Vienna mapping function, Böhm, Werl and Schuh, 2006), VMF3 (Landskron and Böhm, 2018), GMF3 (Global mapping function, Böhm et al., 2006), as well as elevation cut off angles, i.e. 5, 7, 10, 15 degrees, on the estimated geodetic parameters. In the scope of this thesis, 24 hour-long (daily) VLBI sessions covering the years from 2000 to 2018 are analyzed using Vienna VLBI and Satellite Software (VieVS, Böhm et al., 2018). Troposphere zenith total delays (ZTD) at 20-minute intervals, source, and station coordinates, as well as Earth orientation parameters, are estimated daily using the mapping functions: VMF1, VMF3, and GMF3 and the elevation cut off angles as 5, 7, 10, and 15 degrees. After, the time series of the above-mentioned geodetic parameters are produced for each mapping function and elevation cut off angle, as an objective and unbiased metric, the weighted root mean square (WRMS) repeatabilities and WRMS differences were calculated for and between each set of the estimated parameters, respectively. GMF3 mapping function ZTD results exhibit high correlations with VMF1 and VMF3 mapping functions. This is expected since GMF is a global spherical harmonic seasonal approximation of VMF. ZTD agreement between VMF1-VMF3 is found to be 2 to 3 times better than GMF3-VMF1 and GMF3-VMF3 agreements in terms of the WRMS differences. Not a significant difference between mapping functions above e.g. 15 or 20 degrees elevation cut off angle were found when the ZTD and station coordinates are compared. This result indicates that relative to the other mapping functions the benefit of using VMF3 is started dominantly after reducing the elevation cut off angle to about 5 10 degrees. The mean biases (weighted mean, WM) of the stations’ daily positions are seen as not sensitive to the change of mapping functions. The station position WRMS repeatabilities between the mapping functions decrease when the elevation cut off angle increases. The repeatabilities of EOP are much more sensitive to the number and the geometry of the observations which depends directly on the selected elevation cut off angle. The total number of observations and their geometry becomes less and inhomogeneous in the southern hemisphere and the poles relative to the northern hemisphere. This worsens (increases) the WRMS repeatabilities of the source daily coordinates at the southern celestial hemisphere and the poles.
xmlui.dri2xhtml.METS-1.0.item-citationKof, B., 2020, "EFFECTS OF THE TROPOSPHERE DELAY MAPPING FUNCTIONS AND THE ELEVATION CUT OFF ANGLES ON THE TIME SERIES OF STATION AND RADIO SOURCE COORDINATES AS WELL AS EARTH ORIENTATION PARAMETERS AS ESTIMATED FROM THE OBSERVATIONS OF THE VLBI TECHNIQUE", MSc Thesis, Hacettepe University
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