This is the fourth in a series of articles looking at the impact of NextGen on GA pilots.
By understanding a little more of the technological side of the National Airspace System, GA pilots will not only improve their piloting skills, but will learn that each new advancement is a building block for more advanced systems, ultimately leading to the Next Generation Air Traffic Control System (NextGen).
This process has provided us with a state-of-art airspace system for the past 50 years. Similarly to Moore’s Law, advancements have nearly doubled about every two years.
Last month’s story on the Four Course Radio System was just a taste of the advancements set into motion providing advanced tools that gave pilots an even better ability to find their way around the massive airspace they fly in.
By incorporating these techniques, adding new ones and combining what was learned throughout the years the Postal Service had control of the airway system, a structured charting system began to evolve. Consisting of landmarks, altitudes, and symbols for depicting certain objects, these charts started showing routing lines connecting these audible navigational stations to each other.
Non-Directional Beacons (NBD) and Beat Frequency Oscillators (BFO) provided even longer range than Four Course, however these transmission systems did not provide any directional information on their own. Being transmitted with a technology that would later be known as Heterodyning, Morse Codes tones were generated by combining or “beating” two different frequencies, effectively subtracting them from each other and producing a new third frequency that was audible. Frequencies as high as 45,000Hz (45KHz) could beat with 44KHz, resulting in 1Khz difference frequency, a tone easily heard by the human ear.
These transmitters could be found with power ratings from 50 to 200 watts in output. Due to their propagation characteristics over the curvature of the earth, long distances were achieved.
An added benefit was that many commercial AM broadcast stations’ frequencies were positioned within areas of the NDB operating band, so aircraft could use these transmitters as NDB stations as well. This allowed for additional transmitters that were available to pilots for even more selectable course correction.
Although this proved to be extremely effective, they were transmitted as an AM (Amplitude Modulated) carrier, which left them very susceptible to atmospheric interference. This was received as noise, which would mask the necessary Morse Code signals that pilots listened for. A direction system more immune to the noise was needed, so next came the ADF (Automatic Direction Finder).
By incorporating the use of propagated signal transmissions and their phase relationships, a direction finder could now be developed that would offer more precise piloting and safety. This worked within the airspace system’s charts developed from the past, but also would be updated in the future, eventually becoming our Victor Airways.
A loop antenna is typically mounted at the bottom of an aircraft. Consisting of two sets of windings (Figure 1a), high gain is achieved when parallel to the transmitting station, while low gain or an offset null is produced when perpendicular to the station by way of cancellation of the two phase relationships on either side of the antenna.
In reality, your ADF systems actually look for a no signal condition to point your way home. The reason is solely due to the fact that an offset null has far better selectivity than an increase in amplitude. This type of polarization will show its head again as we move into ADS-B and NextGen.
If you ever noticed, the “clothes line” wire that is connected from the end of your aircraft to the front demonstrates just one part of this pretty abstract design for its time. It is called the sensing antenna and it allows your ADF to determine whether you are coming or going, while a loop antenna determines where the station is.
These low and medium frequency nav aid routing lines were depicted on charts with colored lines. In general, amber and blue lines represented North/South routes and green and red lines represented East/West airways and were used typically below 18,000 feet AGL.
By using these charts and knowing the position of each transmitting beacon, an aircraft can use an ADF not only to compute a fix but also to calculate distances using two or more transmitters.
Figure 2 illustrates a typical ADF receiver known as an RBI (Radio Bearing Indicator). Obviously the little airplane in the middle shows that the nose is always positioned longitudinal to the North on the circular RBI. There is also a needle that points to the transmitter’s carrier wave. By adding the Relative Bearing to the Magnetic Heading, a Magnetic Bearing can be calculated.
Next month we will dive into VORs with and without Doppler, VOR DME, and VORTAC (TACAN). The VOR technological breakthrough set the stage for today’s GPS systems. This will be followed by a look at GPS that will provide details on just how this incredible system works and why, in some cases, it doesn’t, and then along came WAAS (Wide Area Augmentation System). Now this begins to get heavy.