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

[D driver]

no one liners please. If you can answer this kindly elaborate thanks :)

[vapilot2004]

DVOR is an FM and Phase Modulated system, while CVOR is primarily AM. FM (and phase modulation) is less affected by physical installation variables of terrain, nearby structures, and aircraft movement as well as atmospheric factors. By design, AM varies according to all of the above (and more), while frequency modulation radio transmissions over the same locations may vary in signal strength, but the modulated carrier remains unchanged.

[vapilot2004]

Expansion on above:

The point of radio, including radio navigation is to transmit information. That information is typically transmitted over a carrier wave via modulation. AM radio modulates the amplitude (or strength/EMF potential) of the carrier while FM (or phase modulated) systems vary the frequency (or phase) of a sub-carrier or carrier.

While both radio systems are technically prone to interference, Phase-modulated Doppler VORs are universally more stable in all but a relatively small number of problematic installations, where terrain, man-made structures, or even bodies of water or atmospheric phenomena create inaccuracies that go well beyond the set standards, rendering the system unreliable for navigation in select areas of coverage.

[D driver]

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.

[m39462]

FM receivers exhibit a behaviour known as "capture", in that when they receive interfering signals they tend to pay attention to the strongest and ignore the weaker ones. This is often observed with commercial broadcasts where you may hear two competing AM stations at the same time but rarely do you hear two FM stations together. Since DVOR uses an FM signal it benefits from this capture phenomenon and rejects the weaker reflected signals, where CVOR using AM would detect a combined signal with a combined phase that is decoded into an incorrect bearing.

[aterpster]

Believe this is why they stayed with AM for the VHF comm band. It is apparent when someone steps on someone else. No so, were it FM.

[D driver]

so there's two effects now "capture effect" and "Bigger aperture" ? anybody comments?

[vapilot2004]

Capture is what the FM system does in locking on to "one" carrier wave - other waves can also arrive at the antenna through reflection. The phenomenon is known as multipath distortion. Wide aperture antenna systems increase accuracy and in a number of ways including the reduction of multipath interference.

[m39462]

D driver, it is my understanding that both effects contribute. Increased aperture even helps with CVOR site errors, there were papers written on this in the '40s, but as antenna spacings approach 0.5 wavelength bearing ambiguities appear and the result is not usable. So, DVOR has an inherent ability to discriminate against weaker reflected signals, plus it removes a constraint that lets you really go to town on the array size.
I was hoping to find some quantitative comparison of these effects but have not had any luck. This research report "Effects of scattering by obstacles in the field of VOR/DVOR" (http://www.tc.faa.gov/its/worldpac/techrpt/rd76-21.pdf) looked promising, but despite its title it doesn't actually contain any DVOR results.

[D driver]

thanks for the inputs everbody. really helful!!

[Alex Whittingham]

For those not following your parallel thread on Wannabees there is a useful video on YouTube at a technical depth that would suit an instructor. https://www.youtube.com/watch?v=tZ2gG1v9Xg8