On our work FMC’s we can inhibit specific stations from being (automatically) tuned. It is pretty rare that we would actually do this, in fact I would say never for an enroute station (maybe because we probably aren’t even aware of an enroute station being on test due to it being on page 32/75 of notams). We would inhibit an on test station for terminal procedures if we anticipated that it would be be used.
Airborne Again
My guess is the FMC software will reject implausible data, with 8 stations to look at finding the one that is giving dud information should be quite easy as the system knows approximately where It is from the IRS position ( at worst ) and the GPS position ( at best ).
The DME-DME system only kicks in when the GPS fails , this I have only seen in the eastern Mediterranean.
I would think a DME station can’t do much wrong. All it does is reply to a pulse pair on one frequency by a transponder on another frequency after a specific delay. If the transponder does not transmit the pulse pair then the DME in the aircraft will not lock onto the signal. So in the aircraft, it is more like all or nothing.
NCYankee wrote:
So in the aircraft, it is more like all or nothing
That was my point in an earlier comment, but that all or nothing means an aircraft calibrating DME will not gather the require data for their calibration flights, so they will rather ask other aircrafts not tune that DME staion while they are flying radials & arcs or ILS nearby?
For a DME with a collocated VOR or LOC, the following is noted at least in the US:
f. VOR/DME, VORTAC, ILS/DME, and LOC/ DME facilities are identified by synchronized identifications which are transmitted on a time share basis. The VOR or localizer portion of the facility is identified by a coded tone modulated at 1020 Hz or a combination of code and voice. The TACAN or DME is identified by a coded tone modulated at 1350 Hz. The DME or TACAN coded identification is transmitted one time for each three or four times that the VOR or localizer coded identification is transmitted. When either the VOR or the DME is inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.
I assume that a DME station that is not collocated with a VOR/LOC would not have a means of indicating it was in a test mode other than by NOTAM.
NCYankee wrote:
I would think a DME station can’t do much wrong. All it does is reply to a pulse pair on one frequency by a transponder on another frequency after a specific delay. If the transponder does not transmit the pulse pair then the DME in the aircraft will not lock onto the signal. So in the aircraft, it is more like all or nothing.
What if the delay is wrong? Or worse, varies?
NCYankee wrote:
For a DME with a collocated VOR or LOC, the following is noted at least in the US: either the VOR or the DME is inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.
It is the same in Europe, and everywhere I would guess. But I don’t see how you can confuse the ID signals. VOR and DME have separate boxes in the aircraft with their own selections on the audio panel (at least in every aircraft I’ve ever seen). Possibly it is different with a TACAN box.
IIRC, in the US co-located DME and ILS/VOR idents alternate, with 2x VOR and 1x DME idents being broadcast at fixed intervals. So identify an ILS/DME, you can select both VOR/ILS and DME on the audio panel simultaneously. Since they have different pitch you can tell when you have identified both.
However, I may mis-remember that.
Airborne_Again wrote:
What if the delay is wrong? Or worse, varies?
Don’t you think that would be detected in the ground station monitoring system and result in a shut down? The delay is fixed at 50 microseconds.
NCYankee wrote:
Don’t you think that would be detected in the ground station monitoring system and result in a shut down? The delay is fixed at 50 microseconds.
If the station is NOTAMed as being on test, unreliable etc. then I would no be so sure that monitoring will be in place.
The delay is fixed for a particular station, but not necessarily at 50 µs – it can be shorter as the DME may be offset. E.g. a one mile offset would need a delay of 37,7 µs.
