Differential GPS positioning

Miha Škerbec (2009) Differential GPS positioning. EngD thesis.

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Abstract

GPS is becoming a part of everyday life. We use it to navigate while travelling, for which low-priced GPS receivers with 10-15 meters positioning accuracy suffice. Several factors influence on positioning accuracy like ionosphere and troposphere, multipath effect, satellites geometry and rounding up at calculation. Thus other systems are developed to reduce these influences. The accuracy of GPS signal is not good enough particularly in geodesy, that is why DGPS is used to reduce influences by using a reference station placed on a location with known coordinates. Its task is to define correction, which is then used by DGPS receivers when calculating positions. With this method we can achieve an accuracy of less than one meter. Distance from the reference station is crucial. The newest RTK method is commonly used in geodesy lately. It uses a network of ground based stations that forward measurements to the central computer, which analyses and corrects information. With a NTRIP client the measurements can be acquired on the internet. Such network, named SIGNAL, is also running in Slovenia. Some newer GPS receivers have a built-in programme like this. In aviation, accurate positioning is also essential. However aircrafts are unable to receive the signal from reference stations and that is why satellite based augmentation systems were developed to enable observation corrections. Like with the RTK method, measurements are sent to the central computer, which then forwards them to geostationary satellites. From here on, corrections are transmitted to aircrafts and users on the globe as well. Other low-priced GPS solutions are being developed. Inverse DGPS is useful especially to track users as the exact location is known only to control centre. The user merely sends measurements to central computer, which applies the corrections on data from GPS receiver. As a part of my diploma assignment I tried to develop a system, which would acquire corrections with NTRIP client in real-time and use them at positioning.

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