Although a GNSS receiver requires only four satellite signals to provide a complete positional fix in three dimensions, the accuracy of this fix depends to some degree on the exact positions of the four satellites relative to the receiver. If the four signals acquired come from satellites spread throughout the sky relative to the receiver, then the fix should be highly accurate. But if all four are observed in close proximity to each other within a single quadrant, then the fix will be less accurate. This phenomenon is known as dilution of precision (or DOP).
Indeed, if two or more of the satellites are aligned so as to appear to occupy the same space, then it may be impossible to obtain any fix. Alternatively, the fix obtained may be out to the tune of 150 or even 200 metres.
There are five different types of dilution of precision: GDOP, PDOP, HDOP, VDOP and TDOP refer to geometric, positional, horizontal, vertical and time dilution of precision, respectively. And the different classes of DOP have differing effects on the accuracy of a receiver. For example, a poor VDOP rating implies that the visible satellites are low in the sky. Hence urban canyon performance will be poor.
Spirent's SimGEN™ software allows designers to test and optimise the DOP performance of their receivers by allowing the user to excluding certain satellites from the simulated visible constellation.