This article describes DOP - Dilution of Precision - its application to receivers and scenarios developed with PosApp software
Determination of both the three-dimensional receiver location and the receiver clock error requires pseudo-range measurements from a minimum of four satellites. Dilution Of Precision, DOP, is a measure of the “goodness” of the positions of any set of four of the currently visible GNSS satellites, relative to the receiver location. A better (lower) DOP value indicates the probability of a more accurate solution, from geometric considerations. Generally, the lower the DOP value the better the navigation solution. Values around 1 to 2.5 are good, 9 is very poor. You can calculate DOP from equations given in any good GNSS textbook.
Figure 1 presents a very simple pictorial appreciation of the effect of SV geometry on DOP. The light blue shaded areas represent measurement error from the signals of two SVs. The grey shaded areas represent regions of uncertainty. You can see how, as the angle between the two SV LOS signals changes (the SV geometry) the region of uncertainty changes, i.e. the DOP changes.
Like many GNSS receivers, PosApp (SimGEN) can be configured to make use of the DOP type as part of its satellite selection algorithm. This is particularly beneficial if there are insufficient signal generator channels to include all visible satellites. You can use DOP type, and the resulting value, to determine the most suitable set of simulated satellites for your test needs. The periodicity of the DOP calculation is equal to the satellite-sampling interval. These selections are available via the corresponding constellation signal sources file. Please consult the SimGEN software user manual for further information. The table below shows the DOP types and how they relate to a navigation solution optimised for different applications:
You can also choose which DOP type is to be displayed in the software GUI. The “best set” of four SVs and also the results from “all-in-view” SVs are displayed. Selection of the type of DOP to display is independent of the DOP type employed for the satellite selection algorithm explained above. For scenarios using multiple constellations you may also review how those different constellation geometries affect the DOP result. Please consult the SimGEN user manual for further information. Comparison of PosApp’s DOP value with the DOP value given by a receiver under test should give good correlation of the DOP numbers (but you must set SimGEN to use the same DOP type as your receiver uses). However, these may differ if the receiver applies a different algorithm to determine its “best set” satellites.