Each radio navigation aid transmits an aural Morse code 'ident' which enables the pilot to identify the radio aid.
Each radio navigation aid will have a designated operational coverage (DOC) outside of which there may be interference from other radio aids using the same frequency. The DOC and other details including the frequency, callsign or identification, hours of service, location and remarks are published in the UK AIP ENR 4.1.
A non-directional radio beacon (NDB) is a ground-based transmitter that transmits radio energy in all directions. The automatic direction finder (ADF), fitted in an aeroplane has a needle that indicates the direction from which the signals of the selected NDB are being received which allows the pilot to fly specific tracks, taking into account wind drift, to (QDM) or from (QDR) an NDB.
NBDs transmit in the low-frequency or medium-frequency band. Low powered NDBs (locators) are used for approaches and generally have a limited range of about 10-20 nm whereas en-route NDBs can have a much longer range. Within the promulgated range, the NDB should be accurate within ± 5°, however the accuracy can be affected by a number of factors:
· Thunderstorm effect - ADF needle may be deflected towards a nearby electrical storm
· Night effect - skywaves from the NDB returning to earth from the ionosphere cause interference with the surface waves from the NDB, resulting in a fading signal and wandering ADF needle.
· Interference from other NDBs transmitting on similar frequencies
· Mountain effect - reflections of the NDB signals from mountains
· Coastal effect - caused by the NDB signal bending slightly towards the coastline when crossing it at an angle
· Turning error - the needle will dip towards the lower wing
When an aircraft passes overhead the NDB, it will enter a cone of silence where no signals are received for a few seconds.
A VHF omnidirectional radio range (VOR) is a very high frequency radio navigation aid and is the ICAO standard short -range radio navigation aid. The VOR offers many advantages over the ADF/NDB combination, including reduced susceptibility to electrical and atmospheric interference and elimination of the night effect, since VFR signals are line-of-sight and not reflected by the ionosphere.
The VOR allows the pilot to select the desired magnetic track, to fly to or from the VOR station and determine how far left or right of track they are. There are 360 individual tracks (1 per degree) which radiate from a VOR (like the spokes of a wheel). Each of these tracks is called a radial.
Within the DOC, VOR reception should be reliable and accuracy of ± 2° (or better) can normally be achieved.
As with an NDB, when an aircraft passes overhead the VOR, it will enter a cone of silence where no signals are received for a few seconds.
Distance-measuring equipment (DME) is a transponder-based radio navigation aid which provides slant range distance by measuring the propagation delay of radio signals. The aircraft use DME to determine their distance from a land-based transponder by sending and receiving pulse pairs. DME frequencies are normally paired with VOR or localiser frequencies and therefore can be selected using a VHF-NAV radio. When directly overhead a DME ground station, the DME indicator in the cockpit will show the height of the aeroplane in nautical miles.
A DME arc may form part of an instrument approach and is flown by maintaining an approximately constant DME reading.
An Instrument Landing System (ILS) is a ground-based system, comprising of three elements, which enables aircraft to carry out a precision approach to a particular runway.
The localizer (LLZ) provides tracking guidance along the extended centreline (guidance in azimuth left or right of the extended centreline).
The glideslope provides vertical guidance towards the runway touchdown point, usually at a slope 3° to the horizontal.
Marker beacons provide accurate range fixes along the approach (usually outer marker and middle marker) although on some approaches locator beacons or DME may be used instead.
When the glideslope is not available, or if required, aircraft may carry out a non-precision localizer- only approach.
ILS Marker Beacons:
· Outer Marker (OM) is located between 3 and 7nm from the runway threshold and may be co-located with a locator beacon (NDB) and labelled 'LOM' - locator outer marker and may be used for tracking and holding.
· Middle Marker (MM) is located approximately 0.6 nm from the runway threshold.
· Inner Marker (IM) a few ILSs have inner markers situated very close to the threshold.
Normally the initial approach fix will be a radio beacon such as an NDB or a VOR which will enable the pilot to home towards the IAF and track outbound to establish on the ILS localiser.
VHF Direction Finding (VDF)
Some aerodromes are equipped with dipole or Doppler radio aerials that can sense the direction of VHF communications (voice) signals received from an aeroplane. This enables controllers to give the pilot a bearing of the aircraft relative to the aerodrome either as a QDM (magnetic bearing to the ground station) or a QTE/True bearing (true bearing from the ground station) which enables the pilot to home towards the VDF station and track outbound if required. The accuracy of bearings is classified by the controller as:
Normally, accuracy no higher than Class B will be available. Sources of error are site errors, such as reflection from nearby obstacles and propagation error caused by differing terrain.
VDF approaches are available at some airports and are similar to NDB approaches, except that the bearing information is obtained from the controller instead of the ADF. Aircraft will be cleared to the VDF overhead and will request bearings from the controller throughout the procedure.
RNAV and GPS
Area navigation (RNAV) allows aircraft to fly waypoint-to-waypoint on a direct course without having to overfly ground-based radio aids. A waypoint is a geographical position usually specified by latitude and longitude, or by radial and distance from a VOR/DME. RNAV routes can be flown using a combination of VOR/DME equipment and RNAV equipment, which can store waypoints and display position information, or approved GPS equipment.
The global positioning system (GPS) is a space-based satellite navigation system which can be used for non-precision RNAV (GPS) approaches. The missed approach procedure on RNAV approaches is based on a conventional radio navigation aid in case of the GPS system becoming unserviceable during an approach.