ahepburn

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?

Spitoon

Can't give you a definitive answer for Prestwick but it's quite common for ILSs in the UK (don't know how common it is elsewhere) to have range info provided by DME rather than markers - this all originated before GPS, of course. For runways with an ILS at both ends it's achieved by putting the DME at the mid-point of the runway any setting it up to read zero at both ends. In the early days of FMSs this caused minor problems because aircraft equipment assumed that all DMEs said zero at their location - but I gather this issue is now covered by the navaid databases.

Don't know for sure nut I would expect the missed approach to be designed around what the DME reads - whether a new-fangled RNAV thing or a raw data display like I used to use.

IRRenewal

You'll probably find that both rwy's (31 and 13) have the same ILS frequency, and share a DME station positioned halfway between the two thresholds.

This DME station has been altered (electronically) so it reads 0 at both thresholds, as opposed to reading zero at the station location itself. This is somehow done with a delay setting within the DME station and requires no input from the crew.

Short answer, yes.

ahepburn

Thanks, guys, but there's something still missing. Looking at the IAP, the DME is quite clearly co-located with the localizer transmitter, and is down near the far end of the runway. The localizer frequency is indeed common to both runways, but the equipment is not - they just turn the one associated with the active runway on at any given time. You need to look at the EGPK IAP to see this.

I can accept that the DME can be tricked into displaying a shorter distance than the physical distance between the airborne equipment and the ground satation - though even that is quite a trick, because it will require speeding up the round trip time of the signal.

Usually on the missed approach, you just go straight ahead to some specified altitude and then make a turn, but Prestwick specifies a turn at 0.7 DME. I'd be really interested to know how a DME that is tricked into displaying distance to the approach threshold on final can be tricked into displaying distance to the departure threshold on the missed approach. If it is set to show, say, 1.5 miles less than true distance on final, then I'd expect it to show 1.5 miles less than true distance at the departure end as well. This would work if the DME ground equipment was located exactly half way down the runway, as suggested by one of the responders, but the IAP shows quite clearly that this is not the case. I guess it will take someone who is responsible for setting up the ground equipment to convince me what is going on here.

ahepburn

I'm pretty sure I have found the answer to my own question. If you look at the EGPK airport diagram, the DME associated with the ILS 13 and 31 is located very close to half way between the thresholds at either end of the runway, so given that it is tricked into showing distance from the approach threshold, it will also show distance from the departure threshold. Clever, these Brits.

Oktas8

I think it might have been IRRenewal who found the answer, not you...

If the localizer frequency is common to both runways, so will the DME and glideslope frequencies. DME & GS are frequency paired with the localiser so a particular localiser frequency will always use one particular DME frequency and one particular GS frequency. But although there will only be one DME antenna, there will be two separate GS antenna arrays, one pointing each way.

ahepburn

Yes, and the DME stays on all the time, whereas the GS and LOC are switched, depending on the runway in use. None of the ground equipment is co-located. I will now consider this one closed. Thanks, guys.

seilfly

Is the signal from the ground equipment modified to include information that the DME-equipment in the aircraft uses to correct the initial distance calculated based on time-delay?

Oktas8

Yes, seilfly.

Aircraft equipment sends a signal.
Ground transponder receives the signal a short time later.
Ground transponder deliberately delays the signal by 50 microseconds.
Ground transponder sends the signal back.
Aircraft receives the signal a short time later.
Aircraft does the time-speed-distance calculation.

To "zero-range" the signal, the 50 microsecond delay is reduced by whatever time is appropriate to adjust the zero-distance point to the end of the runway. The maximum adjustment is therefore (300 million m/s) x (50 millionths of a second) = 15km (approximately!)

Cheers,
O8