There are some excellent articles by the Swedish meteorologist Anders Persson on the effect. See here for a short list.

Thanks Paco, sounds good!

Can anyone tell me how many total marks there are available for the UK ATPL Mass and Balance exam? I'm aware there are 25 questions but I assume there will be some worth more marks, just can't seem to find the totals listed anywhere.

Q. Why does doppler VOR have less site error compared to CVOR?

Because the mountains (or similar fixed hard objects) don't affect the propagation. Instead of going round in a circle, the signal moves back and forth. The receiver can't tell the difference, though.

Phil

thanks for the input. But can you please elaborate what you just said?

OK:

Using Doppler allows the frequency of a signal to decrease when the distance between the beacon and aircraft increases, and vice versa. The Doppler shift makes the transmitter look as if it is advancing and retreating 30 times a second. That is, the aircraft sees a varying frequency rather than varying power.

As mountains don't move, they don't affect the reception of the signals and the VOR doesn't suffer from propagation errors. Do you need an explanation of Doppler?

phil

are you implying that just because its vari phase signal is FM and that of CVOR is AM, the site error is less. And if yes is that the only reason?

No. As mentioned above the Dopper principle removes the site errors because anything that does not move has no effect on the signal propagation. From a "normal" VOR the aircraft sees a change of power even though FM is involved. With a doppler VOR it sees a change in frequency instead.

phil

I read somewhere that site error in doppler VOR is less because of its bigger aperture compared to CVOR. By aperture they meant the diameter of the circle in which the antennas of the doppler are arranged. Does that make any sense to you? I dont really know what tht means.

You mean wide-aperture? This averages out the local distortions which would normally be much more noticeable with the more narrowly focussed CVOR antenna (at about half a wavelength as opposed to about 5), so a theoretical tenfold reduction in site errors is possible (something to do with space diversity).

The Doppler shift comes from the relative motion of the antenna and the receiver. It is used because the wide aperture system needs Doppler to work properly, in that it creates the direction-dependent FM signal. The frequency change observed as the antenna rotates towards or way from an observer on the same circle is in proportion to the azimuth.

Phil

all you could possibly need on youtube

thanks for the input alex, so thats how bigger aperture leads to less site error. Nicely explained. But I believe there's also another phenomenon here acting to reduce the site error called "Capture effect". This refers to the capability of aircraft receiver to demodulate only the strongest of the variable phase VHF signals from doppler VOR, ignoring the reflected but weaker variable phase signals. As opposed to the CVOR wherein the average of the direct and the reflected weaker signal is taken into account to derive a wrongful "average" bearing. Would like your comments on this.

The capture effect is an FM phenomenon and partly the reason for the system being designed that way. AM allows multiple signals to be broadcast on the same channel.

Phil

so If am not mistaken, there's two effects now "capture effect" and "Bigger aperture" that is leading to lesser site error with DVOR compared to CVOR. Is that correct?

Sort of - as I see it, the site errors are minimised with the wide aperture, the Doppler effect makes that work and the capture effect just makes for a stronger signal. Simples!

Phil

The aircraft maintains the magnetic heading of 060 degrees. The declination is 8°W, the drift angle is +4°. What is the true track heading of the aircraft?

a. 056 degrees
b. 048 degrees
c. 064 degrees
d. 072 degrees

It's not ATPL level, but I am not sure 072 degrees is actually the correct answer... Shouldn't it be 048 degrees?

Declination? Do they mean variation? And why is drift shown as +4, not 4 left or right? How confusing can you get?

If variation is 8W and drift is 4 port, yes, true heading is 052, true track is 048. Who wrote this question?

Polish CAA for PPL... It's a mess. Luckily ATPL questions are from a centralized QB.

Hi Alex, both in BGS (i've been told) and atplonline(confirmed) they have this question wrong:

Which of these statements about a reaction turbine are correct or incorrect?
It's considered that it's the same question as this one:
Which of these statements about an impulse turbine are correct or incorrect?

But they are different things. Can you confirm me that it's like this in BGS too? In class we have been taught that are different things and pressure in stator and rotor difers depending if its reaction turbine or impulse turbine.

Cheers.