The principle of GPS positioning shows us that constraints exist in terms of constellation distribution.
Indeeed, a bad distribution of the satellites will generate a low precision of positioning. To qualify this geometry, we generally have indicator noted DOP (Dilution Of Precision) and whivh give, at a given moment, an assessment of the weakening of precision for:
✽ Planimetric positioning: HDOP (Horizontal DOP)
✽ Altimetric positioning: VDOP (Vertical DOP)
✽ A determination of time : TDOP (Time DOP)
✽ 3D positioning (with a minimum of three satellites) : PDOP (Position DOP)
Lastly, one often retains a last factor called GDOP (Geometric DOP) which integrates the PDOP and the TDOP. It is usually only accessible on geodetic receivers and requires the visibility of at least four satellites.
Generally, for positioning by pseudorange and if more than four satellites are visible ( which is most often the case in natural envirenments, but can sometimes be difficult in urbvan areas), the DOP parameters are used to choose the four satellites best placed, so as to affer the best results
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