الفهرس 
Disadvantages of classical GNSS relative positioning versus CORS networks
GNSS CampaignsOnly 14 pages are availabe for public view
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Abstract
GPS was becoming an indispensable geodetic tool during the 1980s and early 1990s. Governmental agencies looked for ways to replace traditional geodetic control networks initially with ground marks surveyed using GPS technology. Based on this concept, the Egyptian Survey Authority (ESA) established in 1995 the High Accuracy Reference Network (HARN), which is zero order. In 1997, ESA established the National Agricultural Cadastral Network (NACN), which has been tied to the HARN. from this date till now, ESA has depended on the NACN for GPS surveying, mapping, and positioning activities… etc. On the other hand, and due to the increasing demand for high accuracy relative positioning using GNSS, ESA established the first Egyptian Continuously Operating Reference Stations (CORS) network in January, 2012. The established network was processed and adjusted relative to the International Terrestrial Reference Frame (ITRF) 2008 at epoch October 23, 2011.
3D positional discrepancies are found between both NACN and CORS networks. Such discrepancies are referenced to the difference in fixation of each network. These discrepancies can lead to serious problems in surveying activities in Egypt unless it is considered and accounted for. In this research, the relation between NACN and CORS networks was studied as well as the possibility of direct transformation between such two networks using Molodensky transformation model.
Hence, to achieve this goal, two GPS campaigns were established, located in Cairo and Nile Delta, to study the discrepancies in the obtained 3D coordinates in different four cases of study for each campaign related to NACN and CORS networks. Then, the 3D transformation parameters between the two networks were calculated in two different cases (considering shift only in the first case and seven parameters transformation in the second).
The obtained results indicate that, there are no significant differences in the processed 3D coordinates relative to the NACN network when using one or two NACN ground control points. On the other hand, it was found that the processed coordinates for any unknown point relative to the CORS network, when tied using one or two CORS stations are nearly the same with small values of coordinate’s discrepancies.
There are significant differences in coordinate systems between both CORS and NACN networks in Egypt. The RMS of coordinate discrepancies obtained from the stand alone solution relative to each network (CORS and NACN) are 0.346m in X direction, 0.169m in Y direction, 0.232m in Z direction and 0.450m in position. The three transformation parameters solution decreases the discrepancies between the two networks as the RMS of discrepancies are 0.036m in X direction, 0.018m in Y direction, 0.022m in Z direction and 0.046m in position. On the other hand, the seven transformation parameters solution also decreases the discrepancies between the two networks as the RMS of discrepancies are 0.015m in X direction, 0.010m in Y direction, 0.009m in Z direction and 0.020m in position.
The second solution (seven transformation parameters), which is more accurate than the first solution (three transformation parameters) can be recommended for transforming the obtained coordinates from the CORS network to the NACN network. Finally, for any GNSS derived coordinates processed with respect to CORS stations, it should be transformed correctly before using it in any further surveying works related to ESA maps.