There is international precedent to consider that you don’t bust. I distinctly remember reading that commercial flights do that flying “precisely at the limit” between the Jeddah FIR (EOJD) and one or several of the Cairo (HECC)/Khartoum (HSSS)/Asmara (HHAA) FIRs, at one level northbound and another southbound, broadcasting information messages, but being controlled/cleared by neither side. I can’t find back my source now…
I kid you not. I also wouldn’t advise that this precedent would hold up in court in Europe.
By the way: its even WORSE if radar is not precise enough. Because in this scenario your are inside one of the airspaces for sure if radar doesn’t show a precise position.
That ICAO rule that is referenced here applies only to airspaces adjoining vertically, at least in Belgium it does.
Where ATS airspaces with a different class of airspace adjoin vertically, flights at a common level shall comply with the requirements of, and will receive the services applicable to, the less restrictive class of airspace (class B is considered less restrictive than class A; class C is considered less restrictive than class B; etc.)
Now let’s look at it differently. Say P2 was not there, so you just flew past the corner of P1, just touching it. Would that be enforced? I don’t think so.
Almost certainly not. Then you could rightly argue that it’s class G under ICAO, but also you’re having to argue about the accuracy of the radar reading. Even Dave’s 300mtrs is enough of an error margin to put you into class G :)
It appears that nobody has yet come up with any examples of enforcements of such “point” crossings?
Now let’s look at it differently. Say P2 was not there, so you just flew past the corner of P1, just touching it. Would that be enforced? I don’t think so.
I thought the DME L-band frequency was chosen to minimise that, or at least the weather-related stuff.
Well, people seem to be using GPS signal bending to measure atmospheric water content.
acp_5_1665_2005_pdf
I’ve personally witnessed ducting on 1.2GHz (i.e. close to the DME band at ~1GHz) which went on for hours. I made more than 100 contacts to the south of england from eastern switzerland from an altitude of about 8200ft, where you would expect a maximum range of around 120NM, yet the signals traveled more than 400NM. So they must have been heavily bent.
tomjnx, the atmosphere must have some effect, but I thought the DME L-band frequency was chosen to minimise that, or at least the weather-related stuff.
I found this Link this morning, which seems pretty well written (so far as I can tell :-), and describes the effects you mention.
Warning, some physics required!
ICAO Doc 8071, Vol1 Ch3 is the source document. Clearly the USA (or any other state) may differ from these SARPs but one would then expect to see such a difference filed in the appropriate AIP.
Presumably range accuracy depends on accurately measuring when the echo returns. Surely this should be very accurate until the range-weakened return signal starts to get drowned in noise, which must tend to happen near a maximum range.
This is true for free space propagation in vacuum. We seldom operate DME’s in the vacuum, however. Even though there’s little water (vapour) in the atmosphere, it can still have an effect due to the high dielectric constant of water (otherwise, we wouldn’t see rain drops and weather radar would not work). It causes radio waves to bend, which means the distance travelled by the wave will be greater than the straight line distance. Also, the presence of inversion layers may cause ducting. Rain clouds may scatter radio waves, as well as local scatterers, which cause the pulses to widen. I haven’t found a reference with good quantitative data, but I’d still be cautious about stating that there’s no distance effect on DME error. Also, the FAA AIM even gives 3% of the distance as error.
Yes. Come to East Anglia in a couple of weeks when we’re doing just that. We will fly various radials/heights recording our position utilising DGPS. Our equipment accuracy (i.e. knowing where we are) is in the order of millimetres although the pilot is only required to achieve about +/-25ft or so in all directions as a measurement of uncertainty factor can be applied The data collected is then compared against that recorded at site.
The same system and principles can apply an extreme level of accuracy on equipment such as ILS. I’m relatively new to the job but am stunned by the granularity we can achieve.
