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Abstract:
Precise point positioning (PPP) is a convenient, accurate and reliable means for GNSS positioning and has been used in a variety of scientific and engineering applications such as crustal deformation determination, hydrographic surveying, and ionosphere monitoring. This study introduces Taiwan’s first online PPP service -- Taiwan Online Precise Point Positioning Service (TOPS). TOPS is a self-developed, Matlab-based software package capable of performing cm-level GNSS positioning. Because Taiwan has its own coordinate system called Taiwan Geodetic Datum 1997 (TWD97), the positioning results obtained from TOPS, which are intrinsically defined in the International Terrestrial Reference Frame (ITRF), must be converted to TWD97 via a suitable coordinate transformation procedure. To accomplish the coordinate transformation, TOPS utilizes a transformation model that incorporates IERS-published global transformation parameters, local inter-seismic velocity models, and local co-seismic displacement models associated with earthquakes greater than magnitude 6.0. Experiments using data collected at global CORSs show that cm-level positioning results compatible with other online PPP service such as CSRS-PPP, GAPS and magicGNSS-PPP can be achieved with TOPS. Experiments using data collected at local control points show that after converting positioning results to TWD97[2010] (TWD97 at epoch 2010.0) and comparing them with pre-determined fiducial coordinates, a horizontal agreement level of ~2 cm and a vertical agreement level of ~9 cm have been achieved. The horizontal component is acceptable, considering that the accuracy of the fiducial coordinates is also cm-level; however, the vertical component suggests that the transformation model has not yet characterized the local vertical motion to a satisfactory accuracy level.
