Radio navigation

Radio navigation is the application of radio frequencies to determining a position on the earth.

In its most basic form is radio direction finding[?] (RDF). This could be as simple as using a loop antenna turned to find the maximum signal strength and therefore a line from station through one's position. Taking two such measurements and plotting the directions on a map will result in an intersection, your current location. Commercial AM radio stations can be used for this task due to their long range and high power, but strings of low-power radio beacons were also set up specifically for this task.

In the 1930s German radio engineers developed a new system, referred to simply as the beams. In this system beacons consisted of two signals broadcast from highly directional antennas with beams a few degrees wide. One was pointed slightly to the left of the other, and they overlapped in the middle. Aircraft could fly down the middle of the beam by listening to the signals, if they heard only one or the other signal in their headset, they knew they were "off the beam". The signals were chosen as the A and N letters form morse code, dot-dash and dash-dot respectively. This means that when the plane is "on the beam" the radio operator hears a steady tone.

The system was originally developed for landing at night, it could easily bring an aircraft down the centerline of the runway to within visual range. In the late 1930s they also started developing long range versions for night-bombing. In this case a second set of signals was broadcast at right angles to the first, and indicated the point at which to drop the bombs. The system was highly accurate and a battle of the beams broke out when the British intelligence services attempted, and then succeeded, in rendering the system useless.

The same system was commonly used by almost all countries in the post-war period. The United States installed a national system of airways using such beacons placed about 200 miles apart. However a new development soon rendered these systems obsolete.

If one direction could be eliminated the signal could be followed to the location of the radio station. Finding the direction to two stations of known position eliminated the directional ambiguity and resulted in a fix of the observers position at the point the two lines intersected. (See Radio fix)

Driven by World War II, radio navigation jumped into sophisticated systems using parts of the radio spectrum well outside broadcast ranges. Radio echoes, eventually in the microwave frequencies, were the basis for RADAR. Numerous "beam" systems were developed and the British engaged in a "war of the beams" with the Germans who were using them for bombing.

At the close of the war the development of systems still in operation began. One, using low frequencies, is Loran. The most recent are satellite navigation systems. From early Doppler (See Doppler effect) systems, where one satellite provided a fix of varying quality dependent on a number of factors (one being altitude of the observer), we now see the Global Positioning System's constellation of satellites providing high quality positions based on high frequency signals providing near constant highly accurate positions in three dimensions.