Detail Surveying with GPS

Sammanfattning: Abstract In the past ten years, the Global Positioning System (GPS) has revolutionized navigation and positioning at or near the surface of the Earth. Even though it is developed and authorized by the US military forces, it has found a remarkable civilian use. In most uses of GPS, the position accuracy is in the order of metres. To achieve better accuracy (i.e. at the centimetre level), the position determination is somewhat more complicated. Firstly, centimetre accuracy can not be established with a single receiver since too many errors will affect the observations. At least two receivers have to be used, and it is the vector between the receivers which can be accurately determined. Secondly, the integer numbers of whole cycles between each satellite and receiver, called cycle ambiguities, have to be determined. This is a rather complicated procedure, which will delay the time to get a solution. Many papers have been written in the last decade about different methods for resolution of ambiguities as fast as possible and the emphasis of this thesis is put at a description of some of these theories. Ambiguities can either be determined while the receiver is at rest or while it is in motion. The latter case is called ambiguity resolution on the fly and it gives in several ways a more complicated model to solve. To aid the users, this thesis starts with a short description of the system, its history and use today followed by a list of different measuring techniques and the plans for an operational reference system in Sweden. Furthermore, observation techniques and error sources are briefly discussed together with other factors affecting the measurements such as satellite geometry and distances between receivers. In order to validate the theoretical part, some practical tests is carried out and calculated by a RlNEX-based software developed by the author for this special purpose. Parameters such as receiver compatibility, distance between receivers, number of frequencies, number of satellites and multipath effects are investigated. To enable setting out using GPS, a radio link has to send corrections to the roving receiver. This is one of the most interesting utilities for GPS in the future. To enable set outs, a real-time system has to be used. This utility is briefly explained and investigated. From the practical part, some conclusions can be drawn. Firstly, a dual-frequency receiver is superior compared to a single frequency unit when a fast solution is wanted. However, there might be some problems when initializing the second frequency due to encryption. Secondly, the time of ambiguity resolution is highly dependent of the number of satellites in view since redundant satellites give the possibility to reject false solutions at an early stage. Finally, real- time observations will significantly improve the use of GPS since otherwise, there are no possibilities to tell the user in the field when ambiguities are solved.