Aside from pilotage and dead reckoning, other forms of navigation are also available to pilots, through the use of electronic navigational aids or nav aids for short. These systems transmit signals to aircraft through radio waves, and tell pilots where they are and where to go. Before we get into what these systems are?, and how they work?, we first need to review what are radio waves and antennas?.
One of the oldest types of nav aids still in use today is called the non directional radio beacon or NDB. While NDB is are not as common in the United States as they used to be, they are still used in other countries around the world. and NDB is simply just a ground-based AM radio transmitter that transmits radio waves in all directions, in the United States these NDBs operate on the frequency range of 190 to 535 kilohertz. Because NDBs operate in this low to medium frequency band, they are not subjected to the line-of-sight limitations of space waves.
To navigate via NDBs pilots need have installed in their aircraft an automatic direction finder or ADF. The face of an ADF contains a needle, that points to the relative bearing of the NDB. The relative bearing is the number of degrees measured clockwise between the aircraft’s heading and the direction from which the bearing is taken from.
You can use this simple formula to calculate the magnetic bearing to the station. Magnetic heading + Relative bearing = Magnetic bearing. For example if your airplane is flying a heading of 030 and the ADF is indicating a relative bearing of 120 then that means that the NDB is at a relative bearing of 150 degrees. If you wanted to fly towards the NDB 150 would be the initial heading to turn to. when flying directly toward the NDB the needle will look like this pointed straight up at the NDB station, once crossing over it the needle will reverse direction, but still point at the NDB as you fly away from it.
It is possible to track both towards and away from an NDB station. This sounds really easy, right?. just keep the needle straight up and you’ll fly right towards the station. Well, in a no-win situation that would probably be just fine however, most of the time there is in fact wind if a pilot were to keep the needle straight up on a windy day as they were navigating they’d be doing a procedure called homing. Homing is not a recommended procedure to follow as you would not be flying in a straight line instead of homing, tracking should be used to fly from station to station in a straight line. Tracking involves compensating for the wind by turning slightly into the wind and thus staying on course. when you are on course and tracking to the station the airplanes wind correction angle should equal the number of degrees the ADF is deflected from straight up.
NDBs can be spotted on sectional charts with this magenta colored symbol. In the vicinity of the symbol you will find a box containing the name of the NDB, frequency, ID and associated Morse code. Before you can actually navigate via an NDB you need to tune and identify the desired station. Tuning in a station is pretty easy just look up the frequency on your chart, and enter it into your receiver, after it’s entered we need to confirm that we are receiving the correct station, and that it is operational. As part of an NDB transmission they’ll send out their ID in Morse code format, Pilots must listen to the Morse code and verify that it matches what’s printed on our chart. After we’ve identified the station we can use it for navigation however, since there is no flag on the instrument to advise us whether or not it is operating Properly, we must continue to monitor the Morse code for as long as we intend on using the station. Luckily we can turn down the volume so it’s not as obnoxious.
There are four different classes of NDBs they all operate on the same principles, but the different classes contrast and how far away their signal can be reached. The weakest of all NDBs is the compass locator, this low powered NDB uses less than 25 watts of power giving it a range of only 15 nautical mile. The other three classes are labeled as Medium-High, High and High-High, with each offering progressively larger ranges. To receive a signal from an NDB, the Aircraft’s ADF is able to determine the relative bearing from the aircraft to the NDB station. this is accomplished through the use of two antennas onboard the aircraft, one being the loop antenna the other being the sense antenna. the loop antenna is a directional antenna containing two or more stationary loops of wire, looking at just one loop if radio waves hit the loop in any direction other than directly perpendicular, a voltage will be induced over the antenna.
By using multiple loops oriented in different headings, the system can deduce down to two possible headings that the signal is coming from both 180 degrees apart. to remove this ambiguity the sense antenna is also used. this antenna which is more or less just a straight wire, looks at the electrical field of the signal receiving an identical signal from all directions, looking at the phase of the signal and not the amplitude, the ADF receiver compares the sense antenna signal with the loop antenna and is able to remove the ambiguity and deduce the relative bearing of the NDB station.
Now before you start relying on NDBs for navigation you should be aware of its limitations. The first of many errors is called the thunderstorm effect , during a thunderstorm the ADF needle will be temporarily deflected towards the lightning strikes instead of the NDB. Next is the night effect where NDB signals can be refracted by the ionosphere and returned as sky waves, this effect is largest during the dawn and dusk hours, this can cause interference with distant NDB stations. Mountains can also have an effect on the NDB signal as they can reflect the NDB signal.
Finally there’s the coastal effect, as the airplane is flying across a coastline, the ADF needle will bend slightly towards the coastline when crossing it at an angle. All of these errors result in erroneous bearing information affects the ADF needle, since the pilot has to monitor the NDB Morse code, hearing any static on that frequency along with the ADF needle acting erratically are two indicators that there may be an error and what you are receiving.