Radio navigation provides the pilot with position information from ground stations located worldwide. There are several systems offering various levels of capability with features such as course correction information, automatic direction finder and distance measuring.

Most aircraft now are equipped with some type of radio navigation equipment. Almost all flights whether cross-country or "around the patch" use radio navigation equipment in some way as a primary or secondary navigation aid.

The fact that radio signals can travel all over the globe on the HF bands is widely used from radio hams to broadcasters and maritime applications to diplomatic services. Radio transmitters using relatively low powers can be used to communicate to the other side of the globe. Although this form of communication is not as reliable as satellites, radio hams enjoy the possibility of making these contacts when they occur. Other users need to be able to establish more reliable communications. In doing this they make extensive use of propagation programmes to predict the regions to which signals will travel, or the probability of them reaching a given area.

These propagation prediction programmes utilise a large amount of data, and many have been developed over many years. However it is still useful to gain a view of how signals travel on these frequencies, to understand why signal conditions change and how the signals propagate at these frequencies.

Radio signals in the medium and short wave bands travel by two basic means. The first is known as a ground wave, and the second a sky wave.

Ground Wave

Ground wave radio propagation is used mainly on the medium wave band. It might be expected that the signal would travel out in a straight line. However it is affected by the proximity of the earth it is found that the signal tends to follow the earth's curvature. This occurs because currents are induced in the surface of the earth and this slows down the wave front close to the ground. This results in the wave front tilting downward, enabling it to follow the curvature of the earth and travel beyond the horizon.

Ground wave propagation

Ground wave propagation becomes less effective as the frequency rises. The distances over which signals can be heard steadily reduce as the frequency rises, to the extent that even high power short wave stations may only be heard over a few kilometres via this mode of propagation. Accordingly it is only used for signals below about 2 or 3 MHz. In comparison medium wave stations are audible over much greater distances - typically the coverage area for a high power broadcast station may extend out a hundred kilometres or more. The actual coverage is affected by a variety of factors including the transmitter power, the type of antenna, and the terrain over which the signal is travelling.

Signals also leave the earth's surface and travel towards the ionosphere, some of these are returned to earth. These signals are termed sky waves for obvious reason.
D layer

When a sky wave leaves the earth's surface and travels upwards, the first layer of interest that it reaches in the ionosphere is called the D layer. This layer attenuates the signals as they pass through. The level of attenuation depends on the frequency. Low frequencies are attenuated more than higher ones. In fact it is found that the attenuation varies as the inverse square of the frequency, i.e. doubling the frequency reduces the level of attenuation by a factor of four. This means that low frequency signals are often prevented from reaching the higher layers, except at night when the layer disappears.

The D layer attenuates signals because the radio signals cause the free electrons in the layer to vibrate. As they vibrate the electrons collide with molecules, and at each collision there is a small loss of energy. With countless millions of electrons vibrating, the amount of energy loss becomes noticeable and manifests itself as a reduction in the overall signal level. The amount of signal loss is dependent upon a number of factors: One is the number of gas molecules that are present. The greater the number of gas molecules, the higher the number of collisions and hence the higher the attenuation. The level of ionisation is also very important. The higher the level of ionisation, the greater the number of electrons that vibrate and collide with molecules. The third main factor is the frequency of the signal. As the frequency increases, the wavelength of the vibration shortens, and the number of collisions between the free electrons and gas molecules decreases. As a result signals lower in the frequency spectrum are attenuated far more than those which are higher in frequency. Even so high frequency signals still suffer some reduction in signal strength.
E and F Layers

Once a signal passes through the D layer, it travels on and reaches first the E, and next the F layers. At the altitude where these layers are found the air density is very much less, and this means that when the free electrons are excited by radio signals and vibrate, far fewer collisions occur. As a result the way in which these layers act is somewhat different. The electrons are again set in motion by the radio signal, but they tend to re-radiate it. As the signal is travelling in an area where the density of electrons is increasing, the further it progresses into the layer, the signal is refracted away from the area of higher electron density. In the case of HF signals, this refraction is often sufficient to bend them back to earth. In effect it appears that the layer has "reflected" the signal.

The tendency for this reflection is dependent upon the frequency and the angle of incidence. As the frequency increases, it is found that the amount of refraction decreases until a frequency is reached where the signals pass through the layer and on to the next. Eventually a point is reached where the signal passes through all the layers and on into outer space.
 

Refraction of a signal as it enters an ionised layer

Different frequencies

To gain a better idea of how the ionosphere acts on radio signals it is worth viewing what happens to a signal if the frequency is increased across the frequency spectrum. First it starts with a signal in the medium wave broadcast band. During the day signals on these frequencies only propagate using the ground wave. Any signals that reach the D layer are absorbed. However at night as the D layer disappears signals reach the other layers and may be heard over much greater distances.

If the frequency of the signal is increased, a point is reached where the signal starts to penetrate the D layer and signals reach the E layer. Here it is reflected and will pass back through the D layer and return to earth a considerable distance away from the transmitter.

As the frequency is increased further the signal is refracted less and less by the E layer and eventually it passes right through. It then reaches the F1 layer and here it may be reflected passing back through the D and E layers to reach the earth again. As the F1 layer is higher than the E layer the distance reached will be greater than that for an E layer reflection.
Finally as the frequency rises still further the signal will eventually pass through the F1 layer and onto the F2 layer. This is the highest of the layers reflecting layers in the ionosphere and the distances reached using this are the greatest. As a rough guide the maximum skip distance for the E layer is around 2500 km and 5000 km for the F2 layer.

Signals reflected by the E and F layers

Multiple hops
Whilst it is possible to reach considerable distances using the F layer as already described, on its own this does not explain the fact that signals are regularly heard from opposite sides of the globe. This occurs because the signals are able to undergo several reflections. Once the signals are returned to earth from the ionosphere, they are reflected back upwards by the earth's surface, and again they are able to undergo another reflection by the ionosphere. Naturally the signal is reduced in strength at each reflection, and it is also found that different areas of the earth reflect radio signals differently. As might be anticipated the surface of the sea is a very good reflector, whereas desert areas are very poor. This means that signals that are reflected back to the ionosphere by the Pacific or Atlantic oceans will be stronger than those that use the Sahara desert or the red centre of Australia.

Multiple reflections

It is not just the earth's surface that introduces losses into the signal path. In fact the major cause of loss is the D layer, even for frequencies high up into the HF portion of the spectrum. One of the reasons for this is that the signal has to pass through the D layer twice for every reflection by the ionosphere. This means that to get the best signal strengths it is necessary signal paths enable the minimum number of hops to be used. This is generally achieved using frequencies close to the maximum frequencies that can support ionospheric communications, and thereby using the highest layers in the ionosphere. In addition to this the level of attenuation introduced by the D layer is also reduced. This means that a signal on 20 MHz for example will be stronger than one on 10 MHz if propagation can be supported at both frequencies.

A VOR is a Very-high-frequency OmniRange radio transmitter. VORs constitute the backbone of current land-based aerial navigation in the U.S. and Western Europe. But first, let's start with the NDB because it's a simpler device.

An NDB (Non-Directional Beacon) is a radio beacon that broadcasts continuously on a specific frequency. Aircraft on-board radio equipment can determine in which direction from the aircraft an NDB signal is coming. The on-board aerial consists of a simple metal loop which is rotatable. The radio signal induces a current in the loop, as in a normal aerial, but this current is weaker or stronger depending on the orientation of the loop. When the loop is flat-on to the origin of the signal, the signal is strongest. (Think of the loop as the frame of a round mirror. The signal is detected to be strongest when the mirror is reflecting it back at itself.) The official definition is

A L/MF [low- or medium-frequency] or UHF [ultra-high-frequency] radio beacon transmitting non-directional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine his bearing to or from the radio beacon and "home" on or track from the station. When the radio beacon is installed in conjunction with the Instrument Landing System marker, it is normally called a Compass Locator.,

in which bearing means:

The horizontal direction to or from any point, usually measured clockwise from true north, magnetic north, or some other reference point through 360 degrees.

Because aircraft can determine from which direction the signal is coming, they can `home in on', fly in the direction of, the signal to arrive at the beacon.

NDBs broadcast in the frequency band of 190 to 535kHz (a `Hertz', Hz, is one cycle per second) and transmit a continuous carrier signal with either 400 or 1020 Hz modulation. An identification signal consisting of three letters in Morse code is also transmitted. The receiver equipment in the airplane is called an ADF (`Automatic Direction Finder'). The indicator consists of a round calibrated dial and a `needle' pointer which points in the direction that the signal is determined to be coming from.

There are two problems with NDBs. First, erroneous signals.

Radio beacons are subject to disturbances that may result in erroneous bearing information. Such disturbances result from such factors as lightning, precipitation static, etc. At night radio beacons are vulnerable to interference from distant stations. Noisy identification usually occurs when the ADF needle is erratic. Voice, music or erroneous identification may be heard when a steady false bearing is being displayed. Since ADF receivers do not have a "flag" to warn the pilot when erroneous bearing information is being displayed, the pilot should continuously monitor the NDB's identification.
 

Second, you can only tell the relative bearing of your aircraft to the NDB - that is, the direction in which the NDB lies. Only by comparing this against the aircraft compass heading (as stably indicated by the directional gyroscope) and doing some trivial trigonometry in his/her head can a pilot determine at which (magnetic or true) bearing the NDB lies from the aircraft. This can be illustrated thus:

The circular black dial with indicator in the middle of the aircraft is what the pilot sees inside the aircraft.

Some aircraft have an instrument called an RMI (Radio Magnetic Indicator) which incorporates both a directional gyro and the ADF needle so that one can read the magnetic bearing to the beacon directly off the instrument without having to do mental trigonometry.

The service range of an NDB is the distance from the NDB within which a reliable signal is guaranteed. Service ranges are classified as 15, 25, 50 and 75 nautical miles.

NDB Navigation

NDB navigation is not necessarily easy. First, there is a course to be flown. Second, the aircraft may be on-course or slightly (or hugely) off-course. Thirdly, the heading of the aircraft may be different from track, to accommodate a crosswind. During an instrument approach, a pilot has to continuously determine all this information, and also calculate and fly corrections. Determining which heading to hold to accommodate a crosswind is an empirical matter. One guesses a heading and determines drift (range of divergence of track from course) and then corrects - first twice as much, to get back onto course, and then when back on course, enough to follow course. All this is quite tricky and one needs to be in practice. This is crucial when flying an instrument approach, since strict adherence to course and altitude restrictions are the only things that guarantee that the aircraft flies clear of obstacles. Anybody who has flown an NDB instrument approach to an airport runway knows how labour-intensive it is. One has to achieve course-following using the above procedure, correcting for probably-changing crosswinds as one descends in altitude, especially in non-level terrain, very accurately and all inside of 2 or 3 minutes.

Use of an ADF in NDB navigation can be illustrated thus:

The aircraft is flying on a course directly from the beacon. We don't know what azimuth this course has, but the dial on the ADF is set to straight-ahead=0° (on an RMI, there would be a directional gyro indicator here, not a settable dial). There is a crosswind coming from the left, so a heading correction to the left of 030° must be taken to maintain course in the crosswind. This heading correction shows up on the ADF, indicating that the beacon is relatively at a bearing of 210° behind the aircraft, which means with the heading correction for crosswind, that we are flying on a course with the beacon at a bearing of 180° to our course behind us.

The solution to NDB navigation problems is the VOR.

VORs

VHF Omni-directional Radio (VORs) are radio beacons that transmit an signal which contains precise azimuth information, so that upon reception of the signal, an aircraft can tell precisely what bearing with respect to magnetic north the station is from the aircraft (respectively, on what radial the aircraft lies from the station - this is just the reciprocal of the bearing to the station from the aircraft). Such a signal has the advantage that the bearing to the station is read directly off the indicator equipment. `The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1 degree'

Navigating on VOR information is akin to flying on a grid such as the following:

Here, the aircraft is positioned directly on the 315° radial from the VOR. (We do not know on what course the aircraft is flying, although its heading appears to be about 350°.)

Besides being very accurate, the VOR is much easier to use than an NDB. The indicator in the cockpit is a round dial with settable azimuth information, called the Omni Bearing Selector (OBS) and a vertical pendulum-like needle, called the Course Deviation Indicator (CDI) thus:

When the aircraft is on the radial set on the instrument (actually, one can set either bearing or radial, depending on which is more meaningful in the situation), the needle points vertically down. When the aircraft has deviated from course, the needle will deviate sideways. A series of dots is shown on the instrument dial so that one can tell how much deviation: `one dot', `two dots', `three dots', `four dots'. A dot is equal to about 2° of deviation. This means that at a distance of 60 nautical miles from the VOR, a one-dot deviation on the CDI indicates that the aircraft is about 2 nautical miles from the selected radial. At 30 nautical miles, one dot would indicate 1 NM of deviation, and so on. When the aircraft is left of course, the CDI needle will deviate to the right, showing the pilot which direction the desired course lies. Similarly, when right of course, the CDI needle will indicate to the left, as in this illustration:

In this diagram, the OBS is set to 0°=360°. An aircraft in position A, B, or C will show a deviation on the CDI as shown to the right (but somewhat exaggerated - the precise number of dots shows a 4° deviation, and the positions A, B, C are indicated much more than 4° to the right of course). Aircraft with 0° set in the OBS, to the south of the VOR in positions D and E, will show a similar deviation. It is important to realise that, unlike with an ADF display, the VOR indicator (OBS and CDI) shows position over the ground with respect to the VOR, and independent of heading or course.

Thus position information along a radial from a VOR is shown directly and accurately on the VOR indicator and there is no need to follow the complicated procedures involved in NDB navigation. One can determine absolute position by tuning two VOR receivers to two different VORs, determining (by centring both DCIs) on what radial from each VOR the aircraft currently lies, and then drawing these two extended radials on the chart on one's knee (the radials are shown on the chart for each VOR, but they don't extend very far from the VOR) to see where they intersect, and that's where the aircraft is!

There are some devices called RNAV which integrate such information from two or more VORs to construct a `virtual VOR' on any chosen course, so that to fly this course, one just centres the CDI needle on the indicator, as though there were actually a VOR one is flying towards. RNAVs are nice.

VOR navigation is the major navigation technique throughout much of the world, and certainly in developed countries such as the U.S. and Western Europe. It was developed a half-century ago, after the Second World War.

Technically, VOR operation is achieved by transmitting two signals (actually, there is a continuous carrier with modulation, but let us for the moment consider it as a discrete signal). The base signal is transmitted at regular intervals, let us say time interval T, and in between base signals occurs an azimuth signal. The difference in time between base signal and azimuth signal determines the radial azimuth (from magnetic north) from the VOR that the aircraft is on. When the aircraft is due (magnetic) north of the VOR, the two signals coincide. As the radial angle increases clockwise, the signals become further and further apart. For example, at 90°, the azimuth signal will be at 0.25T; at 180°, at 0.5T; at 270°, 0.75T, until when at magnetic north again, the two signals again coincide.

VOR reception is line-of-sight. `VORs operate within the 108.0 to 117.95 MHz frequency band, and have a power output necessary to provide coverage within their assigned operational service volume'. The service volumes are given by the class of VOR:

• T (Terminal): From 1000 feet above ground level (AGL) up to and including 12,000 feet AGL at radial distances out to 25 NM;

• L (Low Altitude): From 1000 ft AGl up to and including 18,000 feet AGL at radial distances out to 40 NM;

• H (High Altitude): From 1000 feet AGL up to and including 14,500 feet AGL at radial distances out to 40 NM. From 14,500 AGL up to and including 60,000 feet at radial distances out to 100 NM. From 18,000 feet AGL up to and including 45,000 feet AGL at radial distances out to 130 NM.

Finally, many aircraft (and most commercial transports) have an instrument that combines a directional gyroscope with a VOR receiver, called a Horizontal Situation Indicator (HSI) and which looks like this:

Distance Measuring Equipment (DME)

DME as its name states is an electronic device that measures "slant range" from the DME station. Slant range is a measure of an aircraft's position relative to the DME station that incorporates the height of the aircraft, its angle from the ground station and its unknown ground range based upon a 90° angle. The farther the aircraft is from the station and the lower the aircraft's altitude, the more accurate the distance reading. An aircraft could be directly over the DME station at an altitude of 10,500 feet above ground level (AGL) and the DME would correctly indicate the aircraft is two miles from the station.

Marker Beacons

1. ILS marker beacons have a rated power output of 3 watts or less and an antenna array designed to produce an elliptical pattern with dimensions, at 1,000 feet above the antenna, of approximately 2,400 feet in width and 4,200 feet in length. Airborne marker beacon receivers with a selective sensitivity feature should always be operated in the "low" sensitivity position for proper reception of ILS marker beacons.

2. Ordinarily, there are two marker beacons associated with an ILS, the OM and MM. Locations with a Category II and III ILS also have an Inner Marker (IM). When an aircraft passes over a marker, the pilot will receive the following indications: (See Table 1-10[1]).

MARKER	CODE	LIGHT
OM	---	BLUE
MM	.-.-	AMBER
IM	....	WHITE
BC	.. ..	WHITE

Table 1-10[1]

• (a) The OM normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glide path.

• (b) The MM indicates a position approximately 3,500 feet from the landing threshold. This is also the position where an aircraft on the glide path will be at an altitude of approximately 200 feet above the elevation of the touchdown zone.

• (c) The inner marker (IM) wll indicate a point at which an aircraft is at a designated decision height (DH) n the glide path between the MM and landing threshold.

Differential GPS or DGPS

DGPS uses a ground station to correct the code received from the satellites for 5 meter accuracy. DGPS could be used for Precision approaches to any airport.