ICAO Annex 10, stipulates the required accuracy for DME and the picture to the right shows the actual performance of DME transmissions over a range of 200 NM against the ICAO requirement which is the red border. It should be noted that DME appears to perform significantly better than the standard requires.
Accuracy
Taking into account the accuracy requirement of Annex 10 and the Angle Subtended, the accuracy of a position estimation using DME/DME could be as good as 0.3 NM if the angle of cut is 90°. Immediately that angle of cut changes, either increasing or decreasing then the expected positional accuracy will drop to 0.5 NM. Below 30° and above 150° the solution is too inaccurate for use.
Procedure designers and the navigation specialists need to understand this as the navigation performance of the aircraft is affected by the DOP. There are software tools available, such as DEMETER, that allow designers and technical experts to assess the navigation infrastructure to see if it can support the planned operation.
Flight crew do not need to worry about the 30/150 Rule as the avionics will automatically switch between DME pairs provided they are available. This switching is transparent to the pilots.