Navigation Aids

311 - Navigational Aids

Navigational Aids

(NDB/VOR/ILS, from the concept of navigation point to point and on airways)

By Eric Stearns

There are three basic forms of navigation: pilotage, ded reckoning (yes, "ded" is spelled correctly), and electronic. Pilotage and ded reckoning haven't changed much in the 100 years of powered flight, and are covered in another lesson. Electronic navigation is always associated with IFR flying, but a VFR pilot will use it regularly too. Electronic navigation is always evolving. In the airmail days, pilots navigated by reference to light beacons (similar to the airport beacons still used today) along routes. This system had one obvious limitation: the pilot had to be able to see the beacon through his window. In the 1930s, the installation of radio ranges began. These were often called "four course" ranges because they would have four possible courses inbound and outbound which could be used for enroute navigation or instrument approaches. Around the time of World War II, the Non-Directional Beacon (NDB) was introduced. The NDB was revolutionary because it allowed an unlimited number of courses to and from the station. The Very-high frequency Omnidirectional Range (VOR) followed closely behind the NDB and began to supplant NDBs in large numbers during the 1950s and 60s. The VOR improved upon the NDB, because the VOR allowed a particular course to be selected by the pilot; an instrument in the cockpit would then display deviation from the desired course. Radio ranges disappeared decades ago, but VORs and NDBs are still with us in significant numbers. They are starting to be replaced by GPS, but will play an important role in electronic navigation for at least another decade. Knowing how to use them is important to a virtual pilot. 

NDB Navigation
Most pilots rarely navigate using an NDB, especially with GPS becoming so prevalent. However, there are still airways around the world which are defined by NDBs and there are still hundreds of NDB approaches. 

NDBs broadcast in the frequency range just below the AM radio band. In order to navigate in reference to NDB stations, the aircraft must be equipped with an Automatic Direction Finder (ADF). An ADF can also provide navigation information from an AM broadcast station. When the ADF receives an NDB station it determines the direction of the station from the airplane. It delivers a signal to a needle on the instrument panel causing the needle to point toward the station. It's a very simple display. Its simplicity makes it a more challenging form of navigation. 

When navigating using NDBs, courses are referred to by the bearing to the station. The bearing to the station is the magnetic course required to track directly toward the station.

NDB navigation information is presented in one of two ways. The first is what we'll call a "simple display", which is commonly found in light aircraft; the second uses what's known as a radio magnetic indicator (RMI), which is commonly found in larger aircraft because of the higher cost.

In order to navigate using the ADF, the pilot also needs to know the airplane's heading. Normally, in a light airplane, you'll get the heading from the directional gyro:

As was mentioned previously, when navigating using NDBs, controllers and charts will reference courses as bearings to the station. In this case, using the two images above, the pilot is on the 300° bearing to the station; to calculate bearing to the station, add the heading (135°) and relative bearing (165°  - relative bearing is the difference between the aircraft's heading and the bearing to the station...if the compass card on the ADF display is set to North then the relative bearing is read directly off the ADF indicator) to get the bearing (if the result exceeds 360°, then subtract 360 from it). An easier way to determine the bearing is to adjust the compass card to your actual heading on the ADF display; this simplifies the calculation of the bearing to the station, but increases workload by requiring the pilot to constantly update heading on the ADF indicator. Because of the complexity involved with determining the bearing to the station, it's easy to see why it's a high workload method of navigating. The workload decreases substantially with a radio magnetic indicator.

Radio Magnetic Indicator (RMI)
The RMI eliminates the mental math needed for a pilot to calculate the bearing to the NDB station. This is done by superimposing the needle over an actual compass card. The RMI ADF needle will always point at the bearing to the station because the compass card will turn as the airplane turns. This is what the RMI would look like at the same geographic position as the two images above:

There are two needles; in this case the buttons on the lower left and lower right control what is displayed by the respective needle. Notice that the button on the left says "ADF" next to it and that the button is below an arrow with a dashed line. That means the needle with the dashed line is displaying ADF information. This needle is pointed at a bearing to the station of 300°. Assuming that the compass is accurate, the needle will always point at the bearing to the station.

VORs allow for more accurate navigation and provide an easier display to interpret. VORs are assigned the frequency range of 108.0-117.95 MHz (however, some of these frequencies are reserved for localizers). The signals transmitted by a VOR allow the receiver to determine its bearing from the station. Note that when navigating by reference to NDBs, the pilot references bearings to the station; VORs use bearings from the station. Bearings from a VOR station are referred to as "radials". Deviation from a VOR radial is displayed on a course deviation indicator (CDI). More advanced aircraft use a horizontal situation indicator (HSI).

Course Deviation Indicator (CDI)
Most aircraft equipped for IFR flight have two VOR receivers and two CDIs, which look like this:

The needle oriented vertically on the indicators displays deviation from the course selected. Full scale deflection in either direction is reached when the aircraft is offset by 10 degrees from the selected course. The VOR receivers in the example above are tuned to two different VOR stations. Notice that the upper one has a course of 308° selected; because the needle is centered that means you're on a course of 308° to or from the station. To determine if it's to or from, you need to look at the ambiguity indicator which is marked with the "A" on both CDIs. The upper CDI's ambiguity indicator is pointing up (which indicates a course toward the station); that means a 308° course will take you directly to the station (in this case, the needle would also center with a course of 128° selected; with 128° selected the ambiguity indicator would reverse because this would be the course directly from the station). The lower CDI is set to a course of 033°, and its ambiguity indicator is pointing down (or from the station). That means that a course of 033° will take you directly away from the station (in this case, the needle would also center with a course of 213° selected, with 213° selected the ambiguity indicator would reverse because that would be the course directly to the station). Like we discussed above, VOR courses are referred to as "radials". The radial is always a course from the station; the upper CDI is displaying a course to the station, so the radial in that case is the value shown at the bottom of the instrument (128°); the lower CDI is displaying a course from the station, so the radial in that case is the value shown at the top of the instrument (033°). If the pilot were to fly a course of 308°, he would be tracking the 128° radial inbound to the station displayed on the upper CDI. If the pilot were to turn to a course of 033°, he would be tracking the 033° radial away from the station displayed on the lower CDI.

Horizontal Situation Indicator (HSI)
Similar to the RMI, the HSI provides a combined display of heading and course guidance. The CDI needle within the HSI rotates with the compass card and the relationship of the aircraft's heading and the desired course becomes more obvious.

The Navy determined that the VOR wasn't suitable for its operations and developed a seperate system called TACAN (TACtical Air Navigation). All of the standard MSFS aircraft are not equipped with a TACAN, but you may occasionally encounter the term. From a pilot's point-of-view, the operation of a TACAN is the same as a VOR. Most Naval installations do not have a VOR on the field, but instead have a TACAN. The FAA often combines VOR installations with TACAN and terms these VORTACs.

ILS / localizer
The main purpose of an instrument landing system (ILS) is to provide a very accurate instrument approach to a runway. Instrument approaches will be covered in several other lessons, but localizers are occasionally used for enroute navigation. Mainly they are used to identify intersections along airways. Localizers use frequencies between 108.1 and 111.95 MHz. Note that this range is shared with VORs. A VOR receiver has to know whether it is looking for a VOR or localizer on a particular frequency. This is achieved by assigning localizers the frequencies in the range which use an odd tenths digit (i.e. 108.10, 108.35, 108.55, 108.70, 111.95, etc.). The display of localizer information is similar to the display of VOR information. The same CDI is used; but there is only one course provided by the localizer...the course selector on the CDI will not affect the display needle; the needle centers when the airplane is centered on the localizer regardless of selected course.

Marker Beacon
Marker beacons were first used to define positions along a radio range airway. The white marker beacon light was used for this purpose; some marker beacon installations still use an "A" on the white marker beacon light for "airway". Airway marker beacons are quite rare these days, but there are probably still a few left in the world. The main purpose of marker beacons is for use with ILS approaches and they are covered further in that lesson. A typical marker beacon installation looks like this (a typical DME installation is shown to the right of the marker beacons):

Distance Measuring Equipment (DME)
DME came along in the early 1960s. Using a UHF radio frequency, the airplane's DME transceiver sends a signal to the DME station and the DME responds. The time it takes for the airplane's DME to receive the signal back is used to calculate the distance from the station. DME stations are always tuned using a VHF frequency which would be associated with a VOR or localizer. The DME system converts that VHF frequency to the proper UHF frequency. Most VORs have DME associated with them. It's also common for localizers to have an associated DME station. A few NDBs have colocated DME transmitters as well. A typical DME installation is shown in the image above. 

Identifying navigation aids
Each navigation aid discussed above (except marker beacons) will transmit a morse code signal to identify itself and indicate that it is serviceable. Finding what the morse code signal should be is discussed briefly in "charting" below.

Service Volume
Each navaid will have what's known as a "service volume". This is the maximum distance from which reception of the navigation aid is assured. This distance can vary considerably depending on the type of navaid, the power with which it transmits, and obstructions surrounding it. MSFS tends to more strictly apply the service volumes which isn't totally realistic. Although they may not be real useful for the virtual pilot, details on service volumes are published by each country in publications like the United States' Airman's Information Manual.

Charting varies considerably around the world. The example below is an IFR low altitude chart produced by the U.S. Department of Defense for the area northeast of Nassau, Bahamas:

Three navigation aids (commonly abbreviated "navaids") are circled in green (this has been added for clarity...they don't appear on the real chart). The NDB circled at the top is called "Marsh Harbor". It will transmit a morse code identifier of "ZMH" (many charts will include the "dots" and "dashes" used for each letter in morse code) and broadcasts on 361 kHz. There is a second NDB circled at the bottom right; its name is "Governor's Harbor", it transmits a morse code identifier of ZGB and uses a frequency of 224 kHz. The third circled navaid is a VOR named "Nassau". It transmits on a frequency of 112.7 MHz and uses a morse code identifier of ZQA. 

Several airways are charted above as well. Note that the airways defined by NDBs use courses to the station; for example BR10L, which connects Marsh Harbor and Governor's Harbor, is defined by a course of 336° to Marsh Harbor and a course of 156° to Governor's Harbor. VOR airways use courses from the station (called radials); BR58V which heads north from Nassau is defined by the 003° radial from Nassau. There can also be mixed airways defined by both VORs and NDBs. An example is BR53V which connects Nassau and Governor's Harbor. Departing Nassau, the pilot would track the 083° radial from Nassau and then at GUAVA intersection switch to Governor's Harbor and track inbound on the 083° bearing to Governor's Harbor.

Practical application
Let's use the image below to apply what's been discussed so far. The "X" marks the airplane's location. Parts of the chart not needed for our discussion are intentionally obscured to make reading the discussed portions of the chart a little easier. Red and green arrows are used on the instrument panel below the chart to show which radio on the left side is connected to which indicator on the right side.

The top course deviation indicator is tuned to the Treasure Cay VOR near the top of the chart. The CDI shows a course of about 308° to the VOR (remember the ambiguity indicator shows whether the course at the top of the CDI is to or from the VOR). A course of 308° to the station represents a radial of 128° (remember radials are always from the station). The lower CDI is tuned to the Nassau VOR (lower left of the chart); it shows a course of about 210° to the station (again remember radials are from the station, so you're on the Nassau 030° radial). The third navaid selected is the Marsh Harbor NDB. The ADF indicator is pointing 15° to the left of the tail (left if you were looking rearward); using the heading indicator, we can see that a 300° heading is 15° left of the tail. This indicates that the airplane is on the 300° bearing to the Marsh Harbor NDB. Each of the radials/bearings discussed is marked on the chart above along with an "X" marking the aircraft's location. If the pilot desired to proceed direct to the Treasure Cay VOR, he/she should fly a course of 308°; the course direct to Nassau would be 210°; the course direct to Marsh Harbor would be 300°.

The following questions refer to the aircraft panel below (answers come at the end of the questions):

1.Where is the aircraft located on the following chart? The four possible answers (numbered 1-4) are green numbers enclosed in boxes. The upper VOR is tuned to the Nassau VOR (lower right of chart); the lower VOR is tuned to the Bimini VOR (upper left of chart), and the ADF is tuned to the Chub Cay NDB (circled in red since it doesn't stand out very well).

2.Using the aircraft panel above, which Bimini radial is the aircraft on?

3.Which Nassau radial is the aircraft on?

4.What is the bearing to the Chub Cay NDB?


1.Based on the navaids tuned, the aircraft's approximate location is #3.
2.The aircraft is on the Bimini 110° radial (remember, the ambiguity indicator shows that the selected course is to the station, radials are from the station, so when the ambiguity indicator points forward, you must look at the bottom of the OBS to determine the radial you're on).
3.The aircraft is on the Nassau 310° radial.
4.The aircraft is on the 080° bearing to the Chub Cay NDB.

VOR and NDB navigation takes a good amount of practice and study. This lesson was just an introduction to the subject. The references below give some resources for further study: 

1. Tim's Air Navigation Simulator∞ - A simulator using VORs and DME where you can see how the airplane, navaids, and indicators interrelate. Spend some time moving the airplane around (you can make it "fly"), then try moving the VOR stations around and note how the indications on the two OBSes change. 
2. United States' Airman's Information Manual∞ - Some fairly technical information about the various types of navaids.
3. Whitt's Flying electronic navigation page∞ - A very detailed article on how to use various types of electronic navigation; also see his home page∞ for other good information.
4. MSFS tutorials/lessons - MSFS comes with some fairly good tutorials on VOR navigation. There's only so much you can learn on the subject by reading about it; seeing it in action will help you understand much more clearly.
5. Private/Instrument pilot textbooks - There are several good textbooks out there with good information on electronic navigation.