NDB JAA theory question

[Dynamic Apathy]

I have a few times come across questions in the Radio Nav feedback papers questioning the various factors which will affect NDB/ADF signals. I can't remember precisely how the question was worded, but the supposed answers included snow as a factor which will reduce signal strength and range.

I am trying to find something concrete to back up this answer or deny it, but without any success. My understanding and reasoning is that the radio waves at LF/MF are far too long for rain, let alone snow to have any effect at all. For radio reflection to occur the wavelength should be no more than twice the size of the object reflecting it, as with weather radar where a wave length of 2.5 to 3 cm is required.

The only way that I can see that snow might have an effect is when there is a build up on the earial itself. Could that be why?

I would be delighted if any avionics types could shed some light, so I await with thanks.

[inbalance]

I canīt tell you why, but it is true for shure. I had to circumnavigate a snowshower today and both ADF pointing direct to the shower.

[OzExpat]

I suspect it might be because snow and rain are both wet and will both accumulate around the NDB's earth mat. Once that ground in that area becomes soggy, the strength of the NDB's signal is reduced.

[OneMileHigh]

Inbalance:

I suspect that what you saw was as the result of static charges built up within the cloud, much like the effect of flying past a thunderstorm and was therefore not due directly by the snow itself.

Weather radar will pick up very low or no returns from snowflakes because they are dry and crystalline, as opposed to large raindrops or wet hail.

Ozexpat seems to have a pretty good idea as to why snow may have an effect and it is interesting to note again that this is not due directly to falling snow but to the fallen snow lying on the ground.

I would hope that any reference to the effect of snow on NDB/ADF in JAA questions would be a little more specific, and make the distinction between falling snow and fallen snow.

[Don D Cake]

I'm not an avionics engineer but I've been an RF electronics engineer for a few years now. You are correct in saying that precipitation is too small to cause reflections (or scattering) of low frequency RF. Generally, attenuation of RF by precipitation is proportional to frequency. However, water is a poor dielectric compared to air and attenuation will occur due to absorbtion - the water absorbs the power and dissipates it as heat. Rain being denser will aborb more energy than snow.

[Dynamic Apathy]

Thanks Don,

This subject has a habit of becoming more and more confusing. If we relate your statement specifically to NDB/ADF will falling snow have any effect at all on the frequency used.

Any further comments on Ozexpat's idea about the fallen snow affecting the transmission signals.

Has the JAA question bank got this wrong. Does it or doesn't it affect the NDB/ADF signals.

[aztruck]

Without consulting text books my brain cell seems to remember NDB having a Ground Wave. If you had a very long range NDB and you were attempting to use it for navigation over the horizon(ie the curve of the earth)then the surface over which it travelled would be quite important, and some surfaces would absorb its energy more than others. A short range NDB like the ones we are used to at Airports might not suffer quite so much because we using mainly the Space wave bit of it, although having said that, it can still be bent by coastal refraction and other local terrain effects. Southends' NDB is bent if you ever try an approach...well that was my excuse anyway.

[ATPMBA]

NDB/ADF signals are affected by static interference. This interference can be caused by precipitation, including falling snow. Range and bearing accuracy can be reduced.

Atmospheric attenuation in which particles, droplets, and ice crystals will reflect and scatter radio waves but I believe this happens at higher radio frequencies and does not affect NDB signals.

[Don D Cake]

I would have to say that falling snow would attenuate an HF signal but to what extent I couldn't say. A heavy fall of large flakes of wet snow would be a lot worse than that fine dusty stuff.

How much a ground wave is attenuated depends on the conductivity of the ground it's travelling over. The more conductive the ground, the less the attenuation. Saline water is quite conductive, granite is almost an insulator, pure water is an insulator. You couldn't possibly say what effect snow on the ground had, there's too many variables.

The conspiracy theorist in me suspects that JAA have planted these type of questions because they don't like people scoring 100% in their tests.

In reality you either have to second guess what the examiner wants or better still find the correct answer and learn it by rote. Not ideal but it is one of the failing of multiple choice exams.

In my PPL Nav there is a question that asks "how is VDF information displayed to ATC?". Apparently "radial lines on a CRT" is correct, "a digital display" is wrong and "not at all because the VDF equipment is broken and there's no money in the budget to fix it" is not listed at all....

[OzExpat]

"not at all because the VDF equipment is broken and there's no money in the budget to fix it"

I wonder where I've heard THAT before...

[Tinstaafl]

re. the VDF display: As a pilot, who the F. cares how it's displayed? We don't look at it.

It could be analog, digital or a chimp pointing out of the window. Who cares as long as it points in the correct direction.

And some people wonder why I find the UK aviation system such a joke at times...

[steamchicken]

a chimp pointing out of the window

I suppose an extension of that principle would create a "primate radar display"

[Tinstaafl]

Add another chimp to hold up a number for each different a/c & that would be Second Radar?

[Sheep Guts]

Now now Tinstaffl, some ATC Employers could be reading this, may give Serco some ideas on revised Work place agrrements.


Regards
Sheep

P.S. Speaking on behave of all Chimps in the field of Aviation and Space Technology.

[cactus37]

Essentially, signal attenuation due to precip (snow and rain in particular) is nill below about 4 GHz. However, there can be significant static build up on or around the A/C which effectively reduces the signal to noise ratio at the A/C receiver. There are additional effects of precip due to the change of impedance of the
NDB ground station antenna.

Regards

[john_tullamarine]

Cactus37,

You might like to expand a little on your post for the benefit of those for whom electrons are a black art topic ...

[Dynamic Apathy]

Thanks Cactus, what you have said squares with what I've read, but it still leaves a small dilemma when related back to the JAA question which mentions just the snow.

Now is the static created by the cloud development (Cb, Ns), or by the snow itself.

If the latter then, the answer to the JAA question must be yes, but if the static is a product of the static charges within the cloud itself then the answer is no.

Of these two options which would you pick.

I have a feeling this question will soon be laid to rest. Thanks everyone for your input so far,

DA

[ATPMBA]

From an ATPL theory course book:

Precipitation static is generated when rain or snow impinges on an airframe. It can degrade both the range and accuracy of ADF systems.