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.
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.
Please send your comments, questions and suggestions to:
#1. – “VHF Omni-Directional Range – Splat! Path Analysis_a”
#2. – “The Technical Wizardry of VORs – How They Work”