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 (Ref.1). It is gradually being replaced by newer GNSS systems. Nevertheless, I decided that it would be an interesting experiment to receive VOR signals and attempt to decode them. I chanced upon a tremendous YouTube video that went into a complete technical description of their operation (Ref.2). VORs are based on the Alford loop which I had investigated for use in directional finding systems used by early PanAm clipper aircraft (Ref.3). I researched the Toronto area and found a VOR listed on the Our Airports website (Ref.4). Figure 1 shows a Google Earth picture of the site. Now the question remained would I be able to receive the signal at my home QTH.
Free Space Loss
In order to determine whether I could receive the YYZ VOR signal at 112.15MHz, I first worked out the free space loss just to give me an idea of signal levels. I laid out the path on Google Earth using the NavAids & my QTH coordinates as shown in Figure 2. Next I used a Scicos program Figure 3 to determine the receive signal level in free space at a distance of 19.4Km from the transmitter with a power of 10W. Finally I used a calibrated Signal Hound spectrum analyzer to measure the carrier and ambient noise levels at that frequency as show in Figure 4. The receive carrier was measured at -106dBm, with the subcarriers perhaps about 15-20dB below. This signal was far weaker than the free space calculation. To understand why, I had to do a Splat! path analysis and follow the path using Google Earth.
Tx Power = 10W = +40dBm
Lfs = 99.2dB
Rx Power = -59.2dBm
Rx Power Measured = -106dBm
Attenuation Difference = 46.8dB
Splat! Path Analysis
Splat! (Ref.5) is a tremendous program for analyzing propagation, and I have used it in many previous posts. I created two QTH files, one for the Toronto VOR and one for my QTH using the Lat/Long coordinates. The path profile Figure 5 shows that the line of sight path is blocked and also that there is no first Fresnel Zone clearance or even 60% clearance. So this means that there will be considerable loss on top of the free space loss. The free space loss of 99.6dB Figure 7 agrees with my Scicos calculation. The Longley Rice model adds an additional 22.8dB of terrain shielding giving a receive level of -76.3dBm. However, this is still much higher than the actual receive level of -106dBm.
Google Path Analysis
|Obstruction||Distance from QTH Km|
|2 x 20 story apt bldg||12.9|
|1 x 10 story apt bldg||10|
|4 x 30 story apt bldg||9.7|
|1 x 10 story apt bldg||3.3|
|2 x 10 story apt bldg||2.2|
The Splat! profile is based on DEM elevation data. It does not take into account man made structures that may be present along the path. The Google path, however, allows us to follow the azimuth and see what buildings/structures are found. Figure 8 shows 4 large apartment buildings right in the middle of the path at 9.7Km. Table 1 lists the obstructions. There are 5 sets of apartment buildings. Taking a highly simplistic view that each building is exactly on the line of sight path and causes a 6db knife edge diffraction, then we have an additional = 5 x 6db = 30dB of attenuation. This knocks our signal down to -106dBm which is what was measured.
VOR Signal Capture
Clearly the VOR transmit signal is designed for flying aircraft having direct line of sight to the transmitter. Signal strength is basically limited by earth bulge at a given flying altitude, not free space loss. By driving close to the VOR site and passing all the obstacles listed in Table 1, I received a strong signal shown in Figure 9. I used the RTL-SDR as a receiver (Ref.6)
Please send your comments, questions and suggestions to:
#1. – “VHF Omni-Directional Range”
#2. – “The Technical Wizardry of VORs – How They Work”
#3. – “Celestial Navigation Basics – Aim Off”
#4. – “YYZ @ Our Airports”
#5. – “Splat! RF Signal Propagation Software, Loss and Terrain Analysis Tool”
#6. – “RTL-SDR for VHF Air and Marine Bands”