Navigation

VII

Electronic Navigation

In this method of navigation, electronic devices use the information provided by radio or radar to chart the position and route of a craft. Electronic and precision aids have increased the safety of navigation by supplying important information rapidly during periods of poor visibility, particularly in dangerous and congested waters. The modern navigator makes wide use of these devices, both in pilotage waters and in the open sea. Radio provides the navigator with auxiliary information, including time signals, regular weather reports, storm warnings, and general navigational warnings concerning such hazards as derelict ships, extinguished navigational lights, and buoys adrift.

Radio was first used as an aid to navigation in the early 1900s, and in the 1930s aircraft were fitted with communications equipment to enable them to receive navigational direction from the ground; direction-finding loops enabled them to take bearings on ground transmitters. One basis for modern navigational aids is radio direction-finding, used in one of two ways: an aircraft or ship takes bearings on fixed transmitters on the ground and fixes its position relative to two or more of them; or bearings taken by ground stations on a transmission from an aircraft or a ship are centrally correlated and a position is passed on to the craft. The principal electronic devices and systems are described below.

A

Direction-Finder (D/F)

The radio direction-finder was the first navigational aid to come into general use. If the bearings of two transmitters with known locations can be measured, the position of the receiver may be determined. In its simplest form, a modern D/F consists of a conventional radio receiver with an antenna in the form of a coil of wire called a loop. Such a loop antenna has strongly directional properties; if it is mounted so that its axis points directly to a radio station, it will receive no signal whatsoever from that station; if it is mounted so that the plane of the loop passes through the radio station, it receives a strong signal. At intermediate positions the signal is intermediate in strength. In practice, a known station is tuned in, and then the loop is rotated until no signal is heard; this position is called the aural null. The axis of the loop must then point directly towards (and away from) the station; this direction is plotted by the navigator as a line of position.

An automatic direction-finder (ADF) has a motor that rotates the loop antenna, keeping the loop always in the null position. The motor also actuates a needle, similar in appearance to a compass needle, that indicates the position of the loop. This so-called radio compass points not towards north but towards whichever station is tuned in on the loop antenna. Such direction-finders can operate on any radio station broadcasting a continuous carrier on a frequency that the radio set can receive. Virtually all aircraft and ships are equipped with D/F equipment. Ground D/F stations have also been installed to aid lost aircraft. Radio D/F equipment is also used in police work and counter-espionage to locate hidden radio stations.

VIII

Radio Ranges

Radio ranges and D/F were the principal radio navigation aids in general use before World War II. They operate on low frequencies (200 to 415 kilohertz) and so are subject to bending, night effect, and other anomalies.

A radio range consists of two pairs of antennas broadcasting in Morse code, one broadcasting the letter A (dot, dash), and the other broadcasting the letter N (dash, dot). The timing of the two letters is such that the space between letters just equals the time of a dash, while the space between the two parts of a letter just equals the time of a dot. The patterns thus interlock so that if both are heard at once, the sound is continuous. The transmission pattern from each pair of antennas is directional, and is projected into two opposite “quadrants”, each covering about 90°. An aircraft in one of the quadrants will hear only a single letter, either A or N; however, if it is on the borderline between the two quadrants, the navigator will hear the continuous tone, which is called the on-course signal. This borderline is called the beam, and is generally about 3° wide. Directly above the range is an area in which no signal is heard. This area is called the cone of silence and is small at low altitudes, but increases in size at higher altitudes.

A

Radio Beacons

A beacon is a radio station that is equipped with a nondirectional antenna; it is used principally for homing. Low-powered beacons are called locators and are used in conjunction with radio compasses.

B

Omnirange or Omnidirectional Range (MOR or VOR)

Omnirange is, in effect, a radio range with an infinite number of beams (or, in practice, 360 beams). Omnirange stations are operated on both VHF (very high frequency) and LF (low frequency): VHF omnirange is called VOR; the designation of low-frequency omnirange, originally LOR, was changed to MOR to avoid confusion with loran (see below). VOR is useful at ranges up to 160 km (100 mi).

The omnirange station has four antennas similar to the antennas of a range station, plus one central antenna. The central antenna broadcasts a continuous reference signal; the other antennas broadcast a variable signal in a beam that is rotated at 1,800 rpm. At the instant when the rotating signal points due north, it is in phase with the reference signal; at all other times it is out of phase with the reference signal by an amount that depends on its direction. The receiver, by measuring this phase difference, can determine its bearing from the station. In practice the omnirange receiver has three dials, one of which can be set manually to any desired course, the second of which tells whether the plane is to the left or right of the course, and the third of which resolves 180° ambiguity by indicating either “from” or “to”. Omnirange can be used for homing and for determining a line of position.