Zero Ranged DME
I am a "foreign" ferry pilot, just returned to Canada after ferrying a Cirrus to Switzerland.
I noticed a couple of unusual features on the UK AIP version of the ILS/DME 31 approach at Prestwick, which I'd appreciate if someone would explain.
First, there's a note that the DME I-KK is "zero-ranged" to the threshold of runway 31. Presumably, this means the signal has been tinkered with so that it shows diatance from the runway threshold, rather than from the DME antenna, which appears to be near the west end of runway 31. Looking at other ILS approaches in the UK, this appears to be common practice, and certainly makes sense, although it appears unusual to a "foreigner".
Second, an this seems to be more unusual, the first waypoint in the missed approach sequence (designated MM13 in the Garmin, and I-KK D0.7 on the IAP) does not appear to be referenced to the threshold of runway 31. In fact, it looks like it is referenced to the threshold of runway 13. If I'm flying the waypoint sequence in the Garmin, everything is fine, but if I'm looking at the old-fashioned DME, what happens? Is the ground equipment somehow smart enough to show the distance from either threshold, depending on whether you're east of west of the transmitter, or what?