Radio Navigation Aids and Position Fixing

A radio navigation aid, or navaid, turns a radio signal into navigation information: a bearing to or from a station, a distance from it, or guidance along a fixed path. Each type has its own principle, described in the articles on the NDB and ADF, VOR, DME and ILS. Much is common to all of them, however: they must be identified before use, they are trustworthy only inside a protected coverage, and they are flown with the same techniques of homing, tracking, intercepting and fixing.

These common principles run through the EASA PPL, IR and ATPL Radio Navigation syllabuses and the FAA written tests. They also remain the crew's defence when a flight management system or GNSS receiver goes wrong, since the raw data from ground aids is the independent cross-check on the magenta line.

What a radio navigation aid is

A single navaid measurement places the aircraft somewhere on a position line, a line on which it is known to lie. Its shape depends on what is measured.

The ILS gives guidance along a single approach path rather than a general position line, and GNSS gives a complete position directly.

Identification and integrity monitoring

Every ground aid transmits a Morse code identifier, and the rule is always tune, identify, then use.

During maintenance the identifier is removed, or a VOR may send T-E-S-T, as a warning that the aid is off the air and may be unreliable even though signals are received. The FAA's AIM adds that a procedure notified out of service must not be flown even if the identifier is heard, since it may be transmitted briefly during testing, and that any indication from a transmitter notified as unusable is to be disregarded however valid it looks. Many modern aircraft decode the Morse and display the letters: on the Boeing 737 the decoded identifier is shown on the PFD and navigation display, and the crew still monitors it because it is not compared with the FMC database.

Navaid integrity monitoring is done on the ground. A VOR's monitor switches to a standby transmitter or removes the identifier if the bearing error exceeds 1°. ILS monitors switch over or shut the facility down, so the crew gets no guidance rather than wrong guidance. A DME whose monitor detects an error goes to standby, sending its identification but no range replies. In the aircraft, VOR, ILS and DME receivers show a flag when the signal is unusable, but an ADF has no flag at all, so an NDB's identifier is monitored for as long as the beacon is in use.

Ground monitors cannot detect everything. The AIM asks pilots to report erratic course or bearing indications, flag alarms and garbled, missing or improper identifiers, because interference, new obstructions and changes in terrain near an aid can go unnoticed on the ground.

Designated coverage and interference

In ICAO practice, as applied in Europe, a navaid's published range is its designated operational coverage (DOC), the range and height within which its frequency is protected from interference. Frequencies are reused across a region, so outside the DOC a receiver may lock on to a different station on the same frequency and give plausible but false indications. A VOR's DOC applies by day and night; an NDB's applies by day only, because sky waves returned by the ionosphere after dark bring in distant stations.

The FAA uses standard service volumes instead: the airspace in which it assures adequate signal strength and course quality, free from interference by other aids on the same or adjacent frequencies. They do not account for blockage by terrain or obstructions, and published routes and procedures may use an aid beyond its standard volume where the signal has been checked.

Co-channel interference comes from another station on the same frequency. Frequency planning prevents it by distance and, for VHF aids, by the radio horizon between stations. The planned margin is expressed as a protection ratio, the minimum ratio of wanted to unwanted signal: ATPL texts give 3:1 for the DOC of an NDB. Propagation anomalies can still defeat it: night sky waves at LF and MF, and ducting or sporadic-E at VHF (see radio wave propagation).

Rho-theta and raw data navigation

Rho-theta navigation fixes position from a single station by distance (rho) and bearing (theta). A VOR with a co-located DME, or a VORTAC, is the standard ICAO short-range aid and the backbone of the conventional airway system, and simple two-dimensional area navigation equipment defines phantom waypoints by bearing and distance from such stations, each within the station's DOC (see area navigation).

Raw data is the unprocessed indication from a navaid: the CDI needle or deviation bar, a bearing pointer, a DME distance. A flight director's command bars are computed information, derived partly from the rate of change of that deviation; if the flight director fails, raw data can still be flown by hand. Raw data navigation means flying, or at least monitoring, from these indications. Airline procedures require raw data to be displayed on approach as a check on the FMS: if the two disagree beyond tolerance, the FMS position is treated as suspect, because map shifts from inertial drift, database errors or a wrongly selected procedure have led aircraft into terrain (see controlled flight into terrain).

Homing versus tracking

Homing means keeping the station on the nose: the ADF needle at zero relative bearing, or a succession of QDMs from a VDF station flown as headings. In a crosswind the aircraft drifts, the pilot keeps turning towards the station, and the path curves on the downwind side of the direct line. The aircraft still arrives, but by a longer route and from an unplanned direction.

Tracking holds a heading corrected for drift so that the aircraft follows the straight line. With starboard (right) drift the heading is the track minus the drift angle; with port (left) drift, the track plus the drift angle. On an ADF the needle then sits off the nose by the drift angle, on the downwind side, and stays there. A VOR needle is unaffected by heading, which shows only position, so the pilot finds the heading that keeps it centred.

Intercepting a radial or bearing

The intercept heading closes the desired course at a chosen intercept angle, commonly 20° to 30° when close to it. Inbound on the 240 radial, course 060°, an aircraft south of the course has it on its left and takes a 30° intercept heading of 030°; north of it, 090°. As the needle moves towards the centre, the pilot turns onto the course plus any wind correction. With the OBS on 360, TO showing, heading 360 and the needle two dots right, the course lies about 4° to the right, and a turn right of about 20° brings the needle back.

Under radar vectors the pilot turns to intercept only when cleared, normally with the intercept heading given in the approach clearance, since a self-initiated turn could conflict with other traffic. ICAO procedures limit the closing heading onto the localiser to 45° for a single runway, and 30° for some parallel runway operations. The aircraft is established on course when the localiser or VOR indication is within half full-scale deflection; until then an assigned altitude is maintained.

Station passage and time to station

Station passage over a VOR is the reversal of the TO/FROM indication; brief needle swings resembling station passage can occur away from the beacon, usually in mountains. Over an NDB the ADF needle swings towards the tail. Directly above either lies a cone in which signals are unusable; over a VOR, a coupled autopilot passes through it by holding the drift-corrected heading it had on entering. A DME never reads zero overhead: it shows the aircraft's height above the station as slant range.

The wingtip method estimates time and distance to a station. Turn until the station is abeam, on the wingtip, and time a set change of bearing:

• minutes to the station = seconds flown ÷ degrees of bearing change;
• distance in NM = TAS × minutes flown ÷ degrees of bearing change.

A 10° change in 120 seconds means 12 minutes to the station. At 150 kt, a 10° change in 3 minutes puts the station 45 NM away.

Position lines and fixes

Two position lines crossing give a position fix. The fix is most accurate when they cross at about 90°. The classic cross-fix uses two VORs: tune and identify each, centre the needle with a FROM indication, read the two radials and plot them. A VOR and a DME, two DMEs or an NDB bearing and a VOR radial work in the same way; flight management systems fix their position from two or more DMEs when GNSS is unavailable.

Three position lines rarely meet at one point. They form a small triangle, the cocked hat, and the aircraft is taken to be in its centre; a large one warns that at least one line is poor.

The errors differ. A bearing error is an angle, so by the 1-in-60 rule the ±5° commonly assumed for a VOR puts the position line 2.5 NM out at 30 NM and 5 NM out at 60 NM. The DME's own error is small and grows only slowly with distance, about ±0.25 NM plus 1.25% of the distance measured, while its slant-range error shrinks as the aircraft moves away. Radials are referenced to the variation at the VOR, while ADF bearings are converted using the variation at the aircraft. Radio waves follow great circles, so bearings plotted over long distances on a Mercator chart need a conversion angle correction.

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Hyperbolic navigation: LORAN-C

LORAN-C was a long-range hyperbolic navigation system. A master station and its secondary stations transmitted synchronised pulses, and the receiver measured the difference in their times of arrival. The points at which the time difference from one pair of stations is constant lie on a hyperbola, which is therefore a position line; a second pair of stations gives a second hyperbola, and the crossing is the fix.

The United States Coast Guard stopped all US LORAN-C transmissions on 8 February 2010, followed by the Russian-American signals on 1 August 2010 and the Canadian signals on 3 August 2010, and the FAA cancelled the technical standard order for airborne LORAN-C area navigation equipment. The ICAO flight plan equipment list still includes the letter C for LORAN-C.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.