We specifically have problems in some Middle East regions. Ordinarily, there are emissions which may be associated with military surveillance activity. DME often takes a bit of a bashing as well. Indeed, I was at an international airport somewhere the other week where both DME and GPS were unusable (dropouts/nonsense signals etc) within 10nm of the airport.
How much of an accuracy loss have you seen, David, without the GPS or DME flagging something?
DME packets have a CRC in them so a long term duff reading should not be possible.
Oh you invariably get an indication of error, eventually.
The DME error I saw last week manifested itself by initially giving an accurate range then counting-up/down in the wrong direction (i.e. number increasing as you were getting closer, vice-versa). This error would persist for 0.5-1.0 nm before the DME would drop-out, resuming some seconds later with the correct range before plodding-off in the wrong direction again. This is not a unique fault, normally being brought about by either RF interference or reflection. In this particular case we discounted RF interference and identified a set of reflections being caused by building works being undertaken some 800m away from the DME antenna.
GPS errors tend to be all-or-nothing. Position errors are normally a matter of a few tens of meters (vertical going awry before lateral) before the receiver recognises it is giving you junk and presenting a ‘flag’, this normally being an LOI message on a G1000. Interestingly, the loss of accuracy doesn’t always tie-in with loss of satellites although this is generally a very good indicator. The said, I have observed a real error of almost 8km being generated during a specific GPS signal management trial despite the presence of 9+ satellites.
I also observed an ILS GP signal error a few days back. The ILS GP course signals (90Hz and 150Hz) are normally spaced +/-4kHz around the carrier frequency, which is somewhere in the 300MHz range. If the transmitter freq shifts, you may end-up with something like +8/-0kHz. In itself, this would not necessarily be a problem as your aircraft receiver isn’t tuned finely enough to discount the shifted signal. However, the tolerance levels of the systems alarm modes would no longer be met and you could have a scenario whereby either the upper or lower lobe failed to meet tolerance limits (signal strength) and the system alarm would not trip, resulting in inaccurate fly up/down indications without a flag.
All off the above were observed within a week or so at major international airports sporting the latest equipment and extensive safety management systems. I suppose my overwhelming point is that all systems can and do present errors, some of which are subtle and not immediately recognisable. Personally, I never put all my eggs in one basket, be that ILS, GPS, DME or VOR.
Peter wrote:
I wonder what the examples might be?
Dave_Phillips wrote:
We specifically have problems in some Middle East regions. Ordinarily, there are emissions which may be associated with military surveillance activity. DME often takes a bit of a bashing as well. Indeed, I was at an international airport somewhere the other week where both DME and GPS were unusable (dropouts/nonsense signals etc) within 10nm of the airport.
Within a large area around the city center of Cairo, GPS is basically not working at all (altitude ca 10,000). This means directly overhead the main airport and the main VOR that >80% of all traffic passes by. I have observed this consistently on about 10 occasions. It does not feel comfortable, especially in IMC.
Peter wrote:
DME packets have a CRC in them so a long term duff reading should not be possible.
What “DME packets”? Do you mean on a bus between the aircraft DME unit and a display?
The DME System works via challenge-and-response between the airborne interrogator unit and the ground station.
Each airborne unit “asking” for the distance to the ground station recieves an individual response packet.
AFAIK there is a limit to the number of interrogations a single DME ground station can handle at one time, so in theory it could be saturated. I don’t know what the display on board the aircraft would show in that case. This shouldnt’t happen too often anyway, the limit is somewhere in the hundreds, if I recall correctly.
Anyway, the aforementionned packets have a built-in check (the cyclic redundancy check) to protect the reading from static noise.
However, it does NOT help against deliberate interference. Military exercises and jamming come to my mind.
I know the Egyptians have a reputation for this…
When a DME gets saturated, you get no indication at all. It prioritises the strongest signals it gets, which means that (typically) closer aircraft get priority over ones far away. Hence you cannot tell if you are “out of range”, or approaching a saturated DME. With lots of high-flying airliners constantly interrogating 3 or more DME in parallel as part of their area navigation systems, I suspect saturation is more common than you think.
CharlieRomeo wrote:
The DME System works via challenge-and-response between the airborne interrogator unit and the ground station.
Each airborne unit “asking” for the distance to the ground station recieves an individual response packet.
AFAIK there is a limit to the number of interrogations a single DME ground station can handle at one time, so in theory it could be saturated. I don’t know what the display on board the aircraft would show in that case. This shouldnt’t happen too often anyway, the limit is somewhere in the hundreds, if I recall correctly.Anyway, the aforementionned packets have a built-in check (the "cyclic redundancy check :https://en.wikipedia.org/wiki/Cyclic_redundancy_check) to protect the reading from static noise.
That is not at all how a DME works! There are no “packets” and there is no CRC.
The DME interrogator (airborne unit) sends pulse pairs on the interrogation frequency associated with the DME channel. The DME transponder (ground unit) responds with a pulse pair on the response frequency.
There is absolutely nothing that distinguishes pulse pairs from different aircraft and no distance information is transmitted. The DME interrogator determines the distance by measuring the time it takes to get a response after it sends an interrogation.
So how does an interrogator know what pulses are replies to its own interrogations?
When you first tune a DME channel, the interrogator is in “search mode” where it will flood the transponder with pulses. Thus a large amount of the responses it gets will be to its own interrogations. It uses this fact to determine its distance to the ground station. Once it knows the distance it will switch to “track mode” with many fewer interrogations. In track mode it will only consider replies with a round-trip time that nearly correspond to its previously known distance. In that way replies from other interrogators will be filtered out.
This is a simplified explanation, but it has the essence of how a DME works.
