Fundamentals of Satellite Navigation
The rapid development in the aerospace field resulted in first attempts to set up Global Navigation Satellite Systems (GNSS) in the late 1970s and mid-1980s. At the beginning, the need for such global navigation systems was exclusively of a military nature. However, over the years, these systems have gained increasing importance in the civil arena, too, and now we cannot imagine doing without them in everyday life.
The functional principle of satellite navigation consists essentially in measuring signal propagation delays. For a high-precision position determination in three-dimensional space, only three fixed reference points are required. In satellite navigation, the different satellites serve as reference points. Additionally, the satellite orbits as well as a common reference period are needed. To determine the latter, a fourth satellite is used. The exact position of the receiver at the time of measurement can be deduced from the current satellite position and the signal propagation delays (slant ranges) (see figure below).
General Functional Principle of Satellite Navigation
However, position accuracy depends directly upon the respective satellite geometry. This means that, if satellites are situated too close to each other, angular resolution deteriorates which, in turn, causes a greater positional error. For this reason, the positioning space of satellites should be very large.
To be able to make a statement about the current quality of a satellite geometry, the Dilution of Precision (DOP) is determined. These are parameters which, multiplied by the measuring accuracy, yield the current positional accuracy at the time of measurement. A DOP of one thus corresponds to optimal accuracy.
Apart from the respective satellite geometry, there are other factors affecting the measuring accuracy; these are briefly outlined below.
• Disturbance of the navigation signal by the ionosphere – This atmospheric influence causes an extension of the signal’s runtime. Since it is frequency dependant, it can be compensated in the receiver by using two frequencies. This disturbance is dominant.
• Disturbance of the navigation signal by the troposphere – This atmospheric influence causes an extension of the signal runtime and is especially dependent on temperature and humidity. Models for compensation in the receiver are available.
• Disturbance of signal reception due to multipath – This disturbance is caused by reflections of the navigation signal in the environment of the receiver. This generates ambiguities which can, in extreme cases, cause position divergences up to 100 m (data valid for GPS). Modern receivers are able to identify such multipaths to a certain extent. However, strategies to compensate such influences are still part of scientific studies, like for instance at the German Aerospace Center (DLR).
• Other disturbances like for instance receiver-sided noise and phase shift should be mentioned at this point.
Source: Skript Navigation I (Beyer/Wigger) TU Darmstadt