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GPS Precise Point Positioning Technique: A Case Study in Iranian Permanent GPS Network for Geodynamics Troposphere errors are largely removed by either applying a model which attempts to mathematically simulate the signal delay as in most commercial software or by estimating the signal troposphere delay along with the receiver coordinates(as in most research software). Ionosphere errors are removed by observing both GPS frequencies(L1 and L2) and combining the two observations to derive an ionosphere-free observation. Errors in satellite positions can be reduced by using precise satellite orbits available from the IGS and any remaining error(except multipath) largely cancels over short distances. That leaves satellite and receiver clock errors as the dominant errors to be death with and this is where relative positioning comes to the fore. Precise Point Positioning The vast majority of commercially available software utilizes the principles of relative positioning. However ,in the late 1990s, the Jet Propulsion Laboratory (NASA) pioneered a new technique that did not require differencing to obtain precise position. The labeled it Precise Point Positioning (PPP) and implemented it in their, GIPSY/OASIS II GPS processing software .(Zumberg, webb etal. 1997) The largest difference between relative processing and PPP is the way that the satellite and receiver clock errors are handled. Instead of between-receiver differencing to remove the satellite clock errors, PPP uses highly precise satellite clock estimates. These satellite clock estimates are derived from a solution using data from a globally distributed network of GPS receivers. Instead of between –satellite differencing to remove receiver clock error, PPP estiamates theses as part of the least squares solution for the coordinates. Consequently, precise absolute coordinates for a single receiver at an unknown location may be obtained without the need of a second receiver at a known location may by obtained without the need of a second receiver at a known location. A note of caution at this point is necessary. It may be possible to get PPP confused with another from a point positioning that many GPS users will be familiar with i.e, Single Point Positioning(SPP). SPP is different to PPP in two ways. Firstly SPP does not use precise satellite clock values and secondly., only the pseudo rage observations are used. PPP uses both the pseudo range and more precise carrier phase observations. The difference between these methods in terms of coordinate accuracy is larger; SPP producers coordinates accurate at the 1-3 m level while PPP can produce coordinates accurate at the 0.01 m level with 24hours observations. Consequently, PPP allows coordinate determination with a precision that is comparable to relative processing. Since no base station is required in PPP, a further question is:" what datum are the coordinates in?" For PPP , the datum is hidden in the satellite coordinates-the satellite reference frame(datum) will be the unknown ground site reference frame. This means that to obtain coordinates in a different reference frame the user needs to perform a usually straight forward coordinate transformation. GPS Observable and PPP The basic GPS observables used for estimating position, velocity and time are: Pseudo range; and Carrier phase or difference of carrier phase. Precise point positioning(PPP), together with relative position, provides a significant contribution to geodynamic applications. One of the advantages of PPP is that positions are independently derived, whereas in relative positioning, an error in the base station coordinates would translate into the other stations. In PPP, dual-frequency data is essential. Therefore, there are two observation equations , for both pseudo range and carrier phase. Considering a stationA and satellite J, the linearized observations can be written as(Monico and perez, 2001: Monical 2000a):   approximated parameters: This procedure involve four observables for each of the visible satellites in each epoch. The two pseudo range and carrier phase observables can be linearly combined, thus reducing the effects of the ionosphere refraction. The use of a troposphere model, together with parameterization techniques, can reduce the troposphere refraction effects. The IGS ephemerids supply satellite coordinates and clock errors, with accuracy in the order of 5cm and 0.3 ns, respectively, and are essential in PPP. However, variations due to geophysical phenomena should be removed using appropriate models. These corrections include (McCarthy,1996). Polar motion; Atmospheric load: Earth body tides and ocean tide loading. According to Zumberge et al. (1997), with PPP it is possible to obtain precision of a few millimeters and a few centimeters in the horizontal and vertical components, respectively. Such levels of accuracy can be obtained for static point position, using a period of 24 hors of data (Monico and Perez, 2001; Monico 2000a). Once the coordinates for all stations are daily estimated using PPP, a solution for a specific epoch/can be obtained , As there is no correlation between the coordinates of different stations, such a solution may be obtained indepenendetly for each station. Bernese The Bernese GPS Software was developed at the Astronomical Institute University of Berne (AIUB),Switzerland, in the late 1980s and is widely used around the world. The Center for Orbit Determination(CODE) analysis centre at the AIUB uses the Bernese GPS software for the analysis of the global IGS network. The software is particularly well suited for the rapid processing of small-size single and dual frequency surveys, permanent network processing, ambiguity resolution on long baselines, ionosphere and troposphere modeling, clock estimation and time transfer, combination of different receiver types, simulation studies, orbit determination and estimation of Earth rotation parameters and the generation of so –called free network solutions. In may 2004, the latest version of Bernese GPS software, version5.0, was released. The Bernese GPS software Versions 4.2(BSW4.2) and 5(BSW5.0) are primarily focused on processing double-differenced GPS observations, however, since Version 4.2, it is also possible to carry out undifference processing, which allows precise point positioning(PPP) to be carried out. Bernese GPS software Version 5.0 in PPP mode The combined IGS final orbits are obtained from the individual IGS analysis center minimum constraint solutions. As a consequence of this the implied IGS orbit origin may be offset from the real geo center by up to 10mm. Therefore, when using IGS final products for PPP, the obtained daily coordinate solutions need to be corrected for these apparent geocentric variations. The final analysis approach for PPP is as follows:
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