الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 223 |  |  |
| | | from | 223 | | |
Abstract
Nowadays, precise application of geodetic control works is presenting one of the most important demands. To provide precise positioning with a few centimeters accuracy for baselines of length over several hundred kilometers, the phase ambiguity should be resolved. The atmospheric and orbital error delays are considered the most critical factors that hinder the ambiguity resolution over long baseline. The effect of the atmosphere has been identified as the major problem of long baseline carrier phase positioning. With regard to the control network and other engineering networks which aims to achieve centimeter-level precision, data processing is usually performed either with commerce GPS receiver processing software, like LEICA Geo-Office ”LGO” and TRIMBLE Geomatic Office ”TGO” or scientific software such as Bernese. The question now is whether the commercial software is able enough to give the required accuracy? Or it is better to pay over twenty thousands of dollars to have scientific software to use it in processing the control network.
To evaluate the GPS processing performance by the aforementioned software over long baseline, a chosen group of baselines were selected from International GPS Services ”IGS” stations with lengths ranged from 300 km to 1200 km approximately. The evaluation strategy was based on two factors. The first factor is physically characterized by the positioning error which is obtained by computing the 3-D vector of the differences between the software solution and the IGS predefined solution that is assumed here to be a true solution value. The second factor is characterized by statistical definition of the baseline solution accuracy. The regression analysis for the positioning errors obtained from the processing of 10 baselines ranged from 300 km to 1200km approximately shows 10.55 mm+ 0.127 PPM for LGO , while TGO gives 10.12 mm+0.059 PPM, so we can say that both software can work well for the control works that need such or less than these size of errors. In spite of the regression line fitting TGO is more accurate of its equivalent of LGO, both software results give a good view about the feasibility of using them in processing the control networks. On the other hand, the positioning errors obtained from the processing of 10 baselines ranged from 300 km to 1200 km approximately by Bernese shows 5.424 mm + 0.050 PPM. Hence, scientific software (Bernese) yields the best computed positioning results and less relative baseline errors compared with the other two Commercial software (LGO & TGO).
In 1992, an ESA steering committee developed a plan for observing approximately 30 stations at approximately 200 km interval, covering all of Egypt, creating a High Accuracy Reference Network (HARN). They also planned to establish the Notational Agricultural Cadastral Network (NACN) relative to these 30 stations, covering the green area of Egypt (Nile Valley and Delta) at 30-40 km intervals. To study the situation of the HARN & NACN Network after passing more than 10 years, an observation campaign for the existing stations of both HARN and NACN networks in great Cairo extended to about 100 km over the surrounding highways were done by NRIAG staff managed by Dr. Mostafa Rabah at September 2003. Five stations were observed for a continuous three days.
To see the quality of the HARN and NACN networks, the selected GPS stations were estimated in its International Terrestrial Reference Frame (ITRF) at the day of the observing campaign and site velocities given by the International Earth Rotation Service (IERS) and then transformed to the original processed ITRF datum, namely ITRF1994, epoch 1996. To perform the required transformation processing, two different GPS processing techniques were utilized in the transformation process as well as a three parameters kinematic rigid plate model. The first technique is based on using Precise Point Positioning (PPP) as a GPS processing engine and the second is based on the classical GPS network processing. To evaluate the PPP approach, several (PPP) tests on several African IGS station were performed to transfer them to ITRF2005 epoch 2000 using the simplified three parameter kinematic rigid plate model and comprising the results of the IGS stations published IERS values in the same epoch. The differences were just a few centimeters. The results confirm the usability of PPP with the kinematic rigid plate model in updating the frame.
The aforementioned two approaches were used to evaluate the ESA control networks; The two approaches that are used in transferring the frame to the local HARN sites are: The first approach is called ”the Direct Approach” which is based on using the resulted values of PPP updating frame, we use the resulted PPP coordinate values of OZ97 as a reference station in transferring the specified part of HARN to the ITRF1994 epoch 1996. On the other hand, the second approach, is called ”the Non-Direct Approach”, which is based upon using the resulted values of IGS stations, as a reference
in transferring the specified part of HARN to the ITRF1994 epoch 1996,. The results show that the critical factor in transferring and updating the frame is dependent on how close the reference stations to the site.
By Transferring the values of HARN & NACN networks that were defined in ITRF2000 epoch 2000 to the original ITRF frame of HARN, namely ITRF1994 epoch 1996 and compare the resulted values with the original coordinates values given by Scott (1997), ,find that, the differences in X-component ranged from 12 to 18 cm, and Y-component ranged from 1.8 cm to 6.8 cm and Z-component, the differences were ranged between 3 cm and 7.5 cm. approaching the derived values resulted from the defects of ITRF1994.
Finally, in most developing countries there is no a regular maintenance of its networks which represent datum. Many countries such as Egypt Region have yet to modernize their geodetic datum to a geocentric realization of ITRF. Maintenance is divided into two parts, the first is the physical part which is responsible for the maintenance of the network in terms of the restoration, protection. The second part (mathematic) describes the process of updating the network according to the latest frame. As it was shown before, HARN and NACN networks whose last updated remains at 1994 in the epoch 1996. But, all modern GNSS use geodetic reference systems closely aligned with ITRF. The latest realization of ITRF (ITRF2008) has a precision of a few millimeters and forms a robust basis for any regional or national geodetic datum. As the ITRF continues to be stabilized, it is anticipated that differences between future realizations of ITRF will differ from one another by less than a few millimeters at a common epoch. Transformations from instantaneous ITRF to a fixed reference epoch of ITRF are straightforward using a measured ITRF site velocity for each station defining the geodetic network, by using a deformation model, or by using a model of rigid plate motion to compute a site velocity. Also, Precise Point Positioning (PPP) of Bernese SW and GNSS post-processing services are now used extensively to provide realizations of ITRF and WGS84 globally with a precision of a few centimeters. In the current research, PPP of Bernese SW and model of rigid plate are used to update the part of High Accuracy Reference Network (HARN) and also by GNSS points.