GNSS-PPP ile Elde Edilen Düşey Yerdeğiştirmenin Doğruluğu
Bu çalışmada GNSS–PPP (GNSS-Precise Point Positioning/Küresel Konumlama Uydu Sistemi-HassasKonum Belirleme) tekniği ile elde edilen düşey yerdeğiştirmenin (çökme ya da yükselmenin) doğruluğuincelenmektedir. Bu amaçla, bir test noktasında başlangıç periyoduna ve bu periyoda göre yükseklikleriçok iyi bilinen beş periyoda ilişkin GNSS (statik) çift frekans gözlemleri günlük olarak elde edilmiştir.Güvenilirliği arttırmak için 6 periyotluk deney farklı koşullarda üç kez tekrarlanmıştır. GNSS gözlemleriT=2 saatlikten T=12 saatliğe kadar değişen süreler için 4 farklı durumda (1: GPS-PPP gözlemi-sonuçyörünge; 2: GPS/GLONASS gözlemi-sonuç yörünge; 3: GPS-PPP gözlemi-hızlı yörünge, ve 4:GPS/GLONASS gözlemi-hızlı yörünge) CSRS (Canadian Spatial Reference System)-PPP internet servisiyardımıyla değerlendirilmiştir. Söz konusu 4 farklı durum ve gözlem süreleri için periyotlar arasındakigözlenen ve bilinen düşey yerdeğiştirmeler karşılaştırılmıştır. Karşılaştırmalar sonucunda belirlenenhatalar yardımıyla her bir durum ve gözlem süresi için karesel ortalama hata elde edilmiştir. Yapılananalizlere göre, GPS ve GPS/GLONASS gözlem türlerine ilişkin karesel ortalama hataların hemen hemenözdeş olduğu, gözlem süresi ve yörünge türüne bağlı olarak açıklanabileceği görülmüştür. GNSS-PPPdüşey yerdeğiştirme karesel ortalama hatası, sonuç yörünge koordinatları kullanılması durumunda3.9/(T 0.5 ), hızlı yörünge koordinatları kullanılması durumunda ise 4.2/(T 0.5 ) [cm/saat 0.5 ] şeklindemodellenmiştir. Bu sonuç, her iki yörünge koordinatları kullanılarak T=4 saatlik gözlem süresi için 6cm’lik bir düşey yerdeğiştirmenin %80 doğrulukla belirlenebileceğini göstermektedir. Bu büyüklük,gözlem süresi arttıkça küçülmektedir.
Accuracy of Vertical Displacement Monitored by Using GNSS-PPP
This study investigates the accuracy of vertical displacement (subsidence or uplift) monitored by using GNSS-PPP (GNSS-Precise Point Positioning) technique. For this purpose, GNSS (static) dual-frequency observations for an initial period and five periods in which the heights of a test point relative to the initial period were accurately known were collected. To increase the reliability, the experiment including six periods was repeated three times in different conditions at the same point. GNSS observations were processed with the use of online CSRS-PPP service for different observing-session durations from T=2 hours to T=12 hours in 4 different cases, namely, (1) GPS-PPP observation type-final orbit, (2) GPS/GLONASS observation type-final orbit, (3) GPS-PPP observation type-rapid orbit, and (4) GPS/GLONASS observation type-rapid orbit. The estimated and known vertical displacements between the periods are compared to determine the root mean square (RMS) error of PPP technique in monitoring of vertical displacements for each case and session duration. According to our analyses, the RMS errors of GPS and GPS/GLONASS are almost equivalent to each other and can be defined depending on the type of orbital coordinates and the session duration: The RMS error of GNSS-PPP is 3.9/(T 0.5 ) and 4.2/(T 0.5 ) [cm/hour 0.5 ] for final and rapid orbits, respectively. This result shows that vertical displacements of about 6 cm can be detected with the 80% power of the test for both orbit types when T=4 hours data are collected in the field. As a corollary, this detectable magnitude decreases while session duration increases.
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