Air Navigation - Navigation Aids

Navigation Aids

Good pilots use all means available to help navigate. Many GA aircraft are fitted with a variety of navigation aids, such as Automatic direction finder (ADF), inertial navigation, compasses, radar navigation, VHF omnidirectional range (VOR) and GNSS.

ADF uses non-directional beacons (NDBs) on the ground to drive a display which shows the direction of the beacon from the aircraft. The pilot may use this bearing to draw a line on the map to show the bearing from the beacon. By using a second beacon, two lines may be drawn to locate the aircraft at the intersection of the lines. This is called a cross-cut. Alternatively, if the track takes the flight directly overhead a beacon, the pilot can use the ADF instrument to maintain heading relative to the beacon, though "following the needle" is bad practice, especially in the presence of a strong cross wind - the pilot's actual track will spiral in towards the beacon, not what was intended. NDBs also can give erroneous readings because they use very long wavelengths, which are easily bent and reflected by ground features and the atmosphere. NDBs continue to be used as a common form of navigation in some countries with relatively few navigational aids.

VOR is a more sophisticated system, and is still the primary air navigation system established for aircraft flying under IFR in those countries with many navigational aids. In this system, a beacon emits a specially modulated signal which consists of two sine waves which are out of phase. The phase difference corresponds to the actual bearing relative to magnetic north (in some cases true north) that the receiver is from the station. The upshot is that the receiver can determine with certainty the exact bearing from the station. Again, a cross-cut is used to pinpoint the location. Many VOR stations also have additional equipment called DME (distance measuring equipment) which will allow a suitable receiver to determine the exact distance from the station. Together with the bearing, this allows an exact position to be determined from a single beacon alone. For convenience, some VOR stations also transmit local weather information which the pilot can listen in to, perhaps generated by an Automated Surface Observing System.

Prior to the advent of GNSS, Celestial Navigation was also used by trained navigators on military bombers and transport aircraft in the event of all electronic navigational aids being turned off in time of war. Originally navigators used an astrodome and regular sextant but the more streamlined periscopic sextant was used from the 1940s to the 1990s. From the 1970s airliners used inertial navigation systems, especially on inter-continental routes, until the shooting down of Korean Air Lines Flight 007 in 1983 prompted the US government to make GPS available for civilian use.

Finally, an aircraft may be supervised from the ground using surveillance information from e.g. radar or multilateration. ATC can then feed back information to the pilot to help establish position, or can actually tell the pilot the position of the aircraft, depending on the level of ATC service the pilot is receiving.

The use of GNSS in aircraft is becoming increasingly common. GNSS provides very precise aircraft position, altitude, heading and ground speed information. GNSS makes navigation precision once reserved to large RNAV-equipped aircraft available to the GA pilot. Recently, more and more airports include GNSS instrument approaches. GNSS approaches consist of either overlays to existing non-precision approaches or stand-alone GNSS non-precision approaches.