I understand that the DME measures the straight-line distance from the aircraft to the ground station. This is called "slant range" and is slightly more than the actual horizontal distance because of the difference in elevation between the aircraft and the station. The most extreme case of "slant range error" occurs when the aircraft passes directly over the station; instead of reading zero, the DME shows the altitude of the airplane above the station in nautical miles. In the airplane that I fly, however, the DME stops working, when in proximity (approx..+/-2 miles) of a VOR. This is inconvenient, because, if there is a change of heading at the VOR, I like to anticipate the turn at a specific distance from the VOR that depends on the magnitude of the change of heading and the airplane speed. The GPS provides guidance on this, but sometimes I keep it off for training purposes. Is perhaps my understanding of how the DME should work incorrect or something is wrong with my DME?
Thanks
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