Since before World War II ADF equipment has been used on aircraft as an aid to navigation. The function of an ADF system is to enable the pilot to determine the headings, or direction, of the radio stations being received. The ADF system operates on a frequency range of 90 to 1800 kHz, a range that makes it possible for the system to receive radio-range stations in the LF band and standard broadcast stations. By use of the ADF system, a pilot can determine the aircraft’s position, or the pilot can “home” on a radio broadcast station or a radio beacon station by flying directly toward that station using the indication of the radio compass or radio magnetic indicator. To find the aircraft’s position, the pilot or the navigator determines the headings of two different radio stations and then plots the headings on a navigation chart. The point at which the heading lines cross will be the location of the aircraft.
ADF systems utilize the directional characteristics of a loop antenna to determine the direction of a radio station.
A simple direction finder that is not automatic may be made by using a loop antenna with an ordinary radio receiver. By rotating the antenna, the strongest reception can be determined and also the point at which the signal
fades out. This point is called the null position, and from it, a fairly accurate indication of the station direction can be determined.
On modern airliners, it is common practice to utilize two sets of ADF equipment. The two units may be tuned in to two different radio stations and an immediate fix is determined by plotting the lines of position for the two radio stations on the navigation chart. A further value of having an airplane equipped with two ADF systems is that if one system fails, the other is still available for direction finding.
ADF equipment is especially valuable in areas of the world where special navigational aids are not available but where the pilot may tune in on a standard broadcast station.
RADIO MAGNETIC INDICATOR
The RMI is the instrument designed to provide visual information to the pilot and copilot concerning the data received by the ADF equipment. This instrument makes it possible for the pilot to navigate the
aircraft without the necessity of numerical or graphical calculations. The instrument displays the magnetic heading of the aircraft and the magnetic bearing of two radio stations. Magnetic-heading information for the instrument is provided by a fluxgate compass or other system. The bearings of the two radio stations are provided by the two separate ADF receivers operating with the loop antenna.
The magnetic heading of the aircraft is indicated on a rotating disk-type dial, and the magnetic bearings of the two radio stations are shown under the two-pointers.
The face of the RMI consists of a fixed outer dial with 45″ markings through 360°, an inner rotating compass dial graduated from 0 to 360″ clockwise in 2″ increments, a wide pointer with parallel grids at the outer edge, and a narrow pointer mounted concentrically with the wide pointer and compass dial. The two pointers provide radio-bearing indications, that is, the bearings of two radio stations being received. The indications of the pointers are read in reference to the compass dial, or card, and thus provide the magnetic bearings of the radio stations on the navigation chart and immediately locate the airplane at the position where the two lines cross.
