Therefore, it is necessary to produce precise and reliable seabed maps, so that any hazards that may occur, particularly in shallow waterbodies, can be prevented, including the high dynamics of hydromorphological changes. Measurements are taken inter alia to detect natural obstacles or other navigational obstacles that endanger the safety of navigation, to examine the navigability conditions, anchorages, waterways and other commercial waterbodies, and to determine the parameters of the safe depth of waterbodies in the vicinity of ports, etc. Obtaining a low HDOP value, which results to a low GPS position error value, calls for providing a high mean number of satellites (12 or more) and low variability in their number.īathymetry is a subset of hydrography, aimed at measuring the depth of waterbodies and waterways. Moreover, 95% of measurements featured a geometric coefficient of 0.973 - this is why it may be assumed that in optimal conditions (without local terrain obstacles), the GPS system is capable of providing values of HDOP ≤ 1, with a probability greater than 95% (2σ). Only 2.77% of fixes indicated an HDOP value larger than 1. The experimental study and statistical analyses showed that the most common HDOP values in the GPS system are magnitudes of: 0.7 (p = 0.353) and 0.8 (p = 0.432). For HDOP values (in the range of 0.6-1.8), position errors were recorded and analysed to determine the statistical distribution of GPS position errors corresponding to various HDOP values. To this end, measurement results of a 10-day GPS measurement campaign (900’000 fixes) have been used. It is possible only statistically, based on an analysis of an exceptionally large measurement sample. The aim of this article is to determine the relation between GPS position error and HDOP value. The variability of the UERE causes the actual measurements (despite an exact theoretical mathematical correlation between HDOP value and position error) to indicate that position errors differ for the same HDOP value. However, the UERE value is a magnitude variable in time, especially due to errors in radio propagation (ionosphere and troposphere effects) and it cannot be precisely predicted. The non-dimensional HDOP coefficient, determining the influence of satellite distribution on the positioning accuracy, may be calculated exactly for a given moment in time.
2D position error in the Global Positioning System (GPS) depends on the Horizontal Dilution of Precision (HDOP) and User Equivalent Range Error (UERE).
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