I.Global Navigation Satellite SystemThe Global Navigation Satellite System (GNSS) is asatellitesystem used to identify the geographic location of a users receiver anywhere around the globe. It has been first extensively used for military applications due the better accuracy that it produces for precise positioning and navigation as compared to the previous system that they use (e.g. TRANSIT). Over the years, GPS has also been used by civilian users, including surveyors, for the same purpose. It is efficient and can produce reliable results with relatively high accuracies.Three segments are considered in the said satellite positioning system: the space segment, the control segment, and the user segment. The space segment comprises of the network of orbiting satellites; the control segment, on the other hand, is responsible for the ground facilities that track and monitor the satellites and their performance. Lastly, the user segment consists of the receiver equipment which receives the signals from the satellites and uses the information to calculate the users three-dimensional position and time.The satellite system makes use of three different kinds of receivers. The navigation or mapping grade receivers are handheld units available from commercial retailers and are designed for recreation purposes. Additionally, the single frequency receivers are capable of receiving information from a greater number of satellites, in several frequencies, and different satellite systems. Moreover, dual frequency receivers are the survey grade antennas that that record the full-wavelength carrier phase and signal strength of the L1 and L2 frequencies and they track at least eight satellites simultaneously on parallel channels. These dual-frequency receivers limit the effects of ionospheric delay and, increase the reliability of processed results over long baselines. Satellite errors, atmospheric effects, and receiver errors are the sources of errors considered in satellite positioning systems. Satellite based error is the difference between the actual and predicted position of a satellite. Clock errors are also a contributing factor to the parameters of the satellites. For the atmospheric effects, there are the ionospheric and tropospheric delays. The ionospheric delay is the largest source of error in GPS, which can vary from 1m to 100m depending on the time of day, season, receiver location, viewing direction, solar activity and state of the Earths magnetic field. There are also phase center offset and variation for receiver errors, which is the non-coincidence between an antenna's geometric center and its phase center and the multipath error which occurs when a satellite signal arrives at a receiver via more than one path.Satellite-based augmentation systems are the regional contributions to improve the performance of GNSS systems. The performance of a satellite navigation system is assessed according to accuracy, integrity, continuity, and availability. Accuracyrefers to the difference between the measured and the real position, speed or time of the receiver. Integrityrefers to a systems capacity to provide confidence thresholds as well as alarms in the event that anomalies occur in the positioning data.Continuityrefers to a navigation systems ability to function without interruption. Availabilityrefers to the percentage of time during which the signal fulfils the accuracy, integrity and continuity criteria.II.GPS SurveyingOriginally developed for defense and military use, the GPS is now widely used and is considered part of everyday life. With its convenience and wide offer of benefits, the said positioning system was rapidly adapted for surveying, as it can give geographic locations easily. It is most commonly used in surveying coasts and waterways, making nautical charts, and building bridges and offshore oil rigs. It is also said to be efficient and can produce reliable results.

Figure 1. The GPS Network Process (Photo obtained from Trimble)A GPS Survey makes use of a GPS baseline, which uses two GPS receivers located at the end of each line to be measured. Both receivers collect data from the satellites at the same time, with the observation duration being highly dependent on the accuracy needed and the distance between the established stations. Survey data may then be post-processed and analyzed using various surveying softwares, and uncertainties are minimized using survey adjustments and calculations such as the Least Squares Adjustment Method.The main method used in processing GPS data is the least squares adjustment. It is used at two different stages in processing. First, in adjustment that yields baseline components between stations from the redundant carrier-phase observations. This procedure employs different techniques to eliminate errors present in the system and to fix the uncertainties in the cycle. In this stage, the solution includes observation equation with the difference in coordinates between stations as parameters. The reference coordinate system for this adjustment is the Xe, Ye, Ze geocentric system. Software provided by the manufacturers in the GNSS receivers prepares the observed phase changes to form the differencing observation equations. Then, Least squares adjustment is performed and will adjust the baseline vector components.

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