# VOR VHF Omni-Directional Range – Scicos Simulation

## Introduction

VOR VHF Omni-Directional Range is an aircraft navigational system developed just before WWII and implemented after the war. It operates in the VHF band from 108.0 – 118MHz. In the previous post (Ref.1) I described my attempts at receiving a local VOR signal. The near field VOR receive spectrum is shown in Figure 1. I find VOR to be a fascinating technology, so I decided to get an understanding of its operation by simulating it in Scicos and later in real time on GNU radio. Ref.2 is a tremendous overview of the system.

## DVOR Principle of Operation

DVOR or Doppler VOR operates on the Doppler Frequency shift principle. 48 Alford loop antennas are mounted in a circle above a counterpoise as shown in Figure 2. A centre located omnidirectional Alford loop transmits a 30Hz reference signal. This 30Hz reference is AM modulated on the carrier Fc and represents the magnetic north azimuth. Diagonally opposite antennas are fed with sub-carriers at Fc+9960Hz & Fc-9960Hz. The antennas are selected such that there is a counter clockwise 30Hz rotation.

Figure 3 illustrates the system operation. Imagine you are a plane exactly on a magnetic east azimuth. You monitor the frequency of the 9960Hz subcarrier. Initially the Fc+Fs/Fc-Fs antennas are perpendicular to the azimuth, so there is no Doppler shift and the subcarrier is 9960Hz. But as the selected antennas shift, they approach (Fc+Fs) & move away from (Fc-Fs) the plane and raise/lower the subcarrier frequency due to the Doppler effect. This is maximum when the top/bottom antennas are selected. The circle diameter of 13.5m and rotational frequency give rise to a subcarrier deviation of Fs+/-480Hz. The FM demodulated 30Hz wave differs in phase from the centre reference signal 30Hz, the phase angle giving the azimuth.

## Scicos DVOR Baseband Simulation

Figure 4 shows the baseband schematic of a DVOR transmitter. Three signals are combined to amplitude modulate the DVOR carrier Fc. The first signal is the north magnetic reference 30Hz tone. The second signal is a voice identifier 250-2500Hz which may or may not be present (YYZ no voice). The third signal is a Morse identifier that modulates a 1020Hz tone. Finally for completeness, a 9960Hz sub-carrier tone is added. This does not amplitude modulate the carrier but is added later on. Figure 5 shows the Morse modulated 1020Hz tone. Figure 6 shows the output spectrum, showing the reference 30Hz tone, the 1020Hz tone and the sub-carrier 9960Hz tone.

## Scicos DVOR RF Simulation

Figure 7 shows the full transmitter schematic. The three tones are shown that AM modulate the carrier at Fc (108-118MHz) which for simulation purposes is shown as 50KHz. Additionally two sub-carriers are added in phase sync with Fc at (Fc+Fs) & (Fc-Fs). Note that these sub-carriers are not FM modulated, as the FM modulation occurs in the aircraft receiver as the Alford Loop antennas virtually rotate around the counterpoise causing Doppler shift.

## Conclusion

So far so good. In the next post we will use hardware – RTL-SDR with GNU Radio to build a DVOR receiver and calculate the azimuth.