This is the fifth in a series of articles looking at the impact of NextGen on GA pilots.
The term Next Generation could have been used from the very beginning as we built the National Airspace System (NAS) to the size it is today. As we have reported over the course of the last few months, the beginning started with fires then light beacons, Four Course Transmissions, NDBs, and now we will take a huge leap forward into the fifth element, the introduction of the VOR to the NAS.
This was big, I mean really big. Not only in the technology but in the bold decision by the FAA to revolutionize the NAS that inevitably exploded the victor airway system to better than 800 routes and thousands of miles in accumulated distance.
Defined as VHF Omni-directional Range Radio, VOR provided a substantial improvement for safety, accuracy, operational features, and distances. Modeled off of previous systems, this Next Generation NAS system not only provided improvements but supplied a huge set of features, including voice transmissions, Morse Code Identifiers, Distance Measurement, HIS (Horizontal Situation Indicators), and CDI (Course Deviation Indicator.)
So how does this puppy work? Initially the VOR system was somewhat crude in its technical make-up. It was assigned a frequency range of 108MHz to 117.95MHz, just above our commercial FM band, shared and supported within the first 4MHz by ILS (Instrument Landing System).
With a transmission up to 200 nm, the VOR amplitude modulates two separate identical 30Hz signals. These signals emanate out of the VOR as 360 tracks or radials, with the 360 radial assigned as leaving the station to Magnetic North. (Figure 1). Continuing with cardinal points, the 90° point is East, 180° is South, and the 270° point is West. By rotating these signals in a constant clockwise rotation, an azimuth or phase measurement can be established since the time of the rotation is constant.
Focus your attention on Figure 2 for a moment. This lighthouse is a pretty good example of how the VOR system works. Substituting the two 30Hz signals with lights will demonstrate this.
The lighthouse sends out a very sharp beam of white light. Right now the white beam is shooting towards Magnetic North, the 360° radial. As it passes by the 360 mark it trips a switch that turns on another bright red light that is omni-directional, which can be seen from any point around the entire 360° area. No matter where you are around that station you can always find your location by simply measuring the amount of time it takes for the white beam to sweep by you after the red light flashes. This time is proportional to the angular distance in degrees from Magnetic North.
Originally operated by motors, these radio beams traveled clockwise in a 360° circle at one round per minute (RPM) or 6° per second. So if 15 seconds elapse from when we first saw the red omni light flash, it would mean that we are 90° (East) from Magnetic North. (15 seconds X 6°/sec = 90°).
Figure 3 demonstrates what the phase angle looks like with the two 30Hz signals transmitted from the station. Since both frequencies are the same, the phase difference between them can always be measured in degrees. These stations were built on solid concrete pads so as to never move from their surveyed positions.
Doppler VOR stations are the latest in VOR technology providing an improved signal to noise ratio for the receiving equipment in the airplane. One 30Hz signal is Frequency Modulated (FM) and the other is Amplitude Modulated (AM).
The Doppler Effect is actually pretty simple to understand. Take a look at Figure 4. We have a person standing near a set of railroad tracks. A train is coming from the west traveling at a fixed speed. In a way the train is catching up to its own sound, compressing the waves as it moves closer to you. This compressed sound increases the audible pitch of the train’s noise. Conversely as the train leaves you, it is running away from its own sound so the wave stretches out, lowering the frequency of the train.
Doppler VOR works the same way. As the radial signal moves closer to the reference signal, it tries to catch up with its own transmission, increasing the frequency, and reduces its frequency as the signal moves away. This phenomenon acts as yet another measurement tool to calculate your phase from Magnetic North.
You can see that all the tools we have used in the past were, in some way, used with VOR. This progression of technology constantly continues through time.
So did GA pilots complain about this new generation of navigation with VORs? Sure they did. It was new, expensive, and needed to be relearned. So goes the saga of NextGen 2013.
Next stop will be GPS. Here we will use every feature and tool we have learned so far to understand GPS. NextGen takes all the work over the years and puts it together, providing the biggest advancement in aeronautical navigation.