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ABSTRACT -
Global Positioning System (GPS) is a satellite-based radio positioning and navigation system providing accurate, common 3-D grid position, velocity
and time information to users anywhere on or near the earth. It consists of a constellation of 24 satellites with atomic clocks, along with the global satellite tacking network and the sophisticated ground processing stations,
that together provide precise navigation coordinates to whom processes a small, readily available GPS receiver. Depending on the number of GPS satellites in view and the geometry involved, the presence of nearby objects may interfere with or degrade the reception of signals
from one or more satellites, and other factors. It provides navigation services including 3-D positioning (Longitude, Latitude, Altitude), velocity and precise time under all weather and continuous real-time operation; Support to unlimited number users and areas. Including NMEA command and GIS technology, this sNdy includes the flexible GUI tinctions of LabVlEW
combining with the analysis programme of MATLAB to develop a novel real-time prototype of GPS positioning and geodetic datum transformation. Indeed, this concise module will be in its all aspect of low-cost, flexibility, and graphic-user-interface under the PC-based platform design Moreover, this system can be corporate directly with different types of GPS receiver as desired. It will functionally achieves and decodes NMEA-OI 83 sentence,
and then shows the measured [I] and calculated pseudorange positioning and .velocity in WGS84 and/or TWD97 coordinates as desired. Therefore, GPS users in Taiwan could not only simply integrate GPS applications into the geodetic/geocentric positioning and navigation, but also have a concise way to develop some versatile teaching and learning programs on GPS usage.Key Words: GPS (Global Positioning System), NMEA
(National Marine Electronics Association), GIS
(Geographic Information System), WGS (World
Geodetic System), TWD (Taiwan Datum)
1.0 INTRODUCTION
Each GPS satellite continually broadcasts a signal that says, in fact, “According to my atomic clock, the time is nowt”, and each satellite periodically broadcasts a current,precise estimate of its orbital elements. These elements are determined by ground processing, using the precise
observations of the satellite by a page link of global network of tracking station whose locations have been precisely surveyed. In addition, each satellite clock is monitor edand regularly calibrated by ground based processing
Estimates and calibrations are regularly uploaded to satellites, which in turn regularly broadcasts them to GPS Thus, in effect, user’s GPS receiver hear the message, “At that time f by its clock, its 3-dimensional position was
P.” It means a typical user is in directing line-of-sight of GPS satellites simultaneously, and consequently these positioning data, which are received from several satellites almost simultaneously. Conclusively, a straightforward computation on timeof- amval difference is sufficient to calculate receiver’s position accurately. However the measured time of signal amval was inaccurate, but this error isn’t significant because it is consistent across satellite. All that required is a good simultaneous view of at least four GPS satellites. For users with commercial GPS receivers, the main sources of error in computing 3-dimensional position are listed [4] Several of these errors can be partially compensated for by the position e stimation a lgorithm in the user’s receiver, but residual errors remain. A
significant factor in the final result is how the satellite ephemerede have been estimates and how the satellite clocks have been calibrated recently. If these uploaded estimates are not refreshed at least daily, the resulting
navigation accuracy for precision users degraded. Using a fixed GPS receiver in precisely surveyed position,Differential GPS (DGPS) can then correct the relative instantaneous bias errors in the clocks of all the GPS
satellites in view. The effect of tbis method is to compensate not only for the SA noise, hut also for the related errors that are originated from tropospheric and ionospheric transmission delays, earth tides and etc.
However, as a user moves away from the based receiver, viewing a different combination of GPS satellites than are viewed by the referred receiver, the accuracy of DGPS sharply degrades., Some users have long desired generalization of DGPS that could provide superb navigational accuracy over wide geographical areas. The reminder of this paper discusses this concept further, and describes a particular real-time prototype of such a system being implemented in LahVlEW [5,6] and MATLAB.