Why Doppler vor uses in place of conventional vor?

Answer 1

Conventional VOR

A conventional VOR (CVOR) has three Amplitude Modulated (AM) signals encoded on a VHF carrier:

1) a 30 Hz variable (VAR), which is modulated by the antenna, not the transmitter;

2) a 9960 Hz subcarrier, which is in turn frequency modulated (FM) with a 30 Hz reference (REF) signal;

3) and a voice / identifier channel, which includes 1020 Hz "Morse Code" identifiers and aural voice signals.

The CVOR antenna is a slightly directional antenna, which means it works best in one direction and worst in the opposite direction. This antenna is physically rotated clockwise at 1800 rpm (30 Hz).

Imagine one observer (receiver) on a line that is magnetically North of the VOR and another observer on the line that is magnetically East of the VOR. Suppose the VOR station transmits a constant amplitude carrier (in reality, the VOR carrier amplitude isn't exactly constant). The VOR carrier is fed to the spinning antenna. The observers see the VOR carrier increase in amplitude when to antenna is pointed toward the observer (peak) and decrease when the antenna points in the opposite direction (valley). Since the antenna rotates 30 revolutions per second, the observer sees 30 peaks and valleys in the carrier amplitude, the carrier is amplitude modulated with a 30 Hz signal. The phase of the 30 Hz modulating signals perceived by the two observers in our example differ by 90 degrees (North observer sees peak 90 degrees before East observer sees peak). Since this signal's phase varies with position relative to the VOR, the signal is called the variable channel (VAR).

In order for the VAR channel to be useful, we need a reference 30 Hz signal (REF). This signal must be perceived by all observers as the same phase, regardless of position relative to the VOR. Here is the problem: the VAR 30 Hz signal is already modulated on the carrier. If the REF 30 Hz signal is modulated onto the carrier without processing, a receiver would find two 30 Hz signals (just one signal if REF and VAR signals are in phase). How would the receiver know which signal is the REF and which is the VAR?

To get around this problem, the VOR takes a 9960 Hz carrier and frequency modulates this carrier with the REF 30 Hz signal. The modulation index is 15, meaning the 9960 carrier has a deviation of 450 Hz (30 Hz times 15). In other words, the subcarrier varies between 9510 Hz to 10410 Hz (9960 +&- 450 Hz). This frequency excursion occurs 30 times per second (30 Hz). The subcarrier signal spectrum does not overlap with the spectra of the VAR or aural signal; therefor it can be amplitude modulated on the RF carrier.

The reason for frequency modulation of the REF signal on the 9960 carrier, as opposed to amplitude modulating the REF signal, is that the AM detector in a VOR receiver would still output two 30 Hz ambiguous signals and a 9960 signal, all summed together.

The VOR receiver has an AM detector which recovers the VAR, 9960 Hz subcarrier, and aural information (called the VOR composite video signal (COMP)) from the RF carrier. The VOR instrumentation processor takes the detected VOR signal , and processes the signal as follows:

1) COMP is processed through a low pass filter that preserves 30 Hz to get the VAR signal;

2) COMP is processed through a high pass filter to reject the VAR and aural signals, then an amplitude limiter, and then though an FM detector to get the REF signal. The FM detector could be a discriminator (used in the bad old days), or a phase lock loop (used in modern equipment);

3) COMP is not processed by VOR instrumentation; however it may be filtered to please the listener, i.e. range filter (1020 Hz bandpass), voice filter (200 to 3000 Hz bandpass).

VOR bearing (magnetic direction away from the VOR) is simply the phase angle of the VAR signal minus the phase angle of the REF signal.

Doppler VOR

The difference between Doppler VOR (DVOR) and CVOR is in the method of encoding the VAR signal on the VOR carrier. The REF and aural channels are the same for both VOR types.

To understand DVOR, one must understand the Doppler effect. The classic example is of a stationary observer standing near (not on) a railroad track. The train's horn (source) is moving at a positive velocity toward the observer. The observer hears the horn at a higher pitch than some one on the train hears. As the train passes, the observer on the ground hears the horn at a lower frequency than the person on the train because the velocity of the horn is negative (moving away from the ground observer). This is an example of Doppler effect for pressure (sound) waves.

Doppler also applies to radio waves (and light for that matter). To understand how DVOR works, here is a ridiculous illustration: suppose a complete CVOR station, except with a non-spinning omnidirectional antenna (antenna works the same in all directions) is placed on a rail car. The rail car is on a circular track with a diameter of approximately 13.4 meters. The rail car runs really fast: 30 laps per second! (I told you the example is ridiculous.)

An observer some distance away from the moving VOR will observe the VOR carrier frequency increase as the rail car comes toward the observer and a decrease as the rail car move away. Since the VOR comes and goes 30 times per second, the carrier frequency is frequency modulated by a 30 Hz carrier. Moving a VOR around a track at a tangential velocity of 1260 meters per second isn't practical. The way a DVOR "moves" the VOR is to have an array of evenly spaced omnidirectional antennas mounted on the 13.4 meter diameter circle. The number of antennas can be as many as 48. Except for "make-before-break" overlaps, only one antenna is connected to the transmitter at any given instant. Each antenna in the array is activated one at a time, in sequence (next antenna on the circle). If the number of antennas in the array is 48, each antenna will be on for 7.5 degrees of 30 Hz (694.4 microseconds). Less expensive systems would use fewer antennas, and each antenna would be on for a longer period of time. The 48 antenna array would require a 48 throw rotatry switch, that can be switched electronically or by a synchronous motor. Each antenna would have to be fed by a transmission line that is the same length as the other antennas. It is important to understand that the VAR signal is encoded by the time-domain spacial velocity of the signal caused by switching individual antennas. It would be a mistake to believe the other antennas are used as a phase array to make a rotatable directional antenna. If the antenna "rotates", the VAR signal is amplitude modulated; therefore a CVOR. If the antenna "moves" spatially in time, the VAR signal is frequency modulated; therefor a DVOR.

Does it take a different VOR receiver to process DVOR? No, a VOR receiver does not "care" if it receives DVOR or CVOR. The spectrum of the CVOR REF signal is a narrow signal at the RF carrier frequency (fc) (between 108 to 117.95 MHz), and two side bands, one at fc + 30 Hz and the other at fc - 30 Hz. The DVOR REF signal has the same spectrum components as the CVOR REF signal, with the addition of sidebands at (plus and minus) 60 Hz, 90 Hz, 120 Hz ... and on (with diminishing in amplitudes). The VOR receiver does not react to the sidebands at 60 Hz and above because the VAR signal is separated by a low pass filter. After this low pass filtering, the spectra of the CVOR and DVOR VAR signals are the same.

Answer 2

The Doppler VOR is the second generation VOR, providing improved signal quality and accuracy. The REF signal of the DVOR is amplitude modulated, while the VAR signal is frequency modulated. This means that the modulations are opposite as compared to the conventional VORs. The frequency modulated signal is less subject to interference than the amplitude modulated signal and therefore the received signals provide a more accurate bearing determination.