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- MakaleYalnız künye2018Multipath interference cancelation in GPS time series under changing physical conditions by means of adaptive filtering
Üstün, Aydın; Yalvaç, Sefa
Multipath, a highly autocorrelated signal is observable phenomena during time periods longer than the sidereal period of Global Positioning System (GPS) satellites in their constellations. Multipath pattern and sidereal repeat time interaction are examined in terms of time series correlations of topocentric coordinates using GPS code and phase observations collected at high and low frequencies. The horizontal and vertical components at 5 and 30 s sampling rates are analyzed to detect and remove multipath classified by their MP1 levels and baseline lengths. For the selected 11 stations that have 24-hour data from CORS stations in the USA and Turkey, the repeat times of the GPS constellation were identified as a function of maximum cross-correlation of adjacent time series. The multipath interference signal from two-day time series data is removed by an adaptive filter to improve time series of coordinate estimates. After the filtering process, an effective epoch based position tracking has been accomplished, especially in the height component. The position errors disappeared to maximum possible extent in the pseudo-range measurements. Up to 70% improvement for the phase derived coordinates in reflective environment has been achieved over the baselines ranging from 1 km to 175 km. For the phase observables, the filter success directly related to the multipath level which is specific to the station.
- BildiriYalnız künye2017Detecting and removing repetitive errors from PPP time series by means of adaptive filter
Yalvaci, Sefa; Ustun, Aydin; Berber, Mike Mustafa
PPP (Precise Point Positioning) is a relatively new GNSS (Global Navigation Satellite System) method that enables position determinations using a single receiver. Being able to survey with a single receiver makes this approach more attractive and affordable than other GNSS positioning techniques. Since a single receiver is used, a drawback of this technique is propagation of errors such as orbit and clock uncertainties, atmospheric disturbances etc. Due to these errors, precision of point positioning may reach up to decimeter levels. Fortunately, some of these error sources such as orbital errors, multipath etc. are repetitive errors and therefore can be detected by using digital filters. Adaptive noise canceling filters are used to detect the correlated part of the time series. These filters optimize themselves using the Least Mean Square (LMS) algorithm which is a powerful tool for Geosciences and are also used for many engineering applications. In this study, it is aimed to detect repetitive errors on coordinate time series by using adaptive noise canceling filter. For the application, four stations have been selected from NGS CORS (National Geodetic Survey Continuously Operating Reference Station) network. Two of these stations are highly affected by multipath error and the other two are not. The coordinate time series of the GNSS sites have been obtained using PPP technique on kinematic mode (epoch by epoch) for several consecutive days. The cross-correlation analysis has been performed and up to 92% correlation has been detected between the daily time series. The correlated part of the data set has been captured by means of adaptive noise canceling filter and removed from the data set. After the filtration process, up to 50% precision improvement has been achieved on coordinate time series, especially for the stations affected by multipath.