Friday, August 23, 2013

Radio Navigation

In several previous posts about radio navigation I've mentioned beacons, and that airplanes use them as points of reference. In one detailed post about Amelia Earhart I even roughly described the "boxing" maneuver aircraft use to orient their course.  But the particulars of this technique eluded me. Recently I found a simple 8 page student document from 1942 that comes complete with diagrams that describe four different ways to do so with a simple RDF (Radio Direction Finder) system. Later ADF (Automatic Direction Finder) systems had motorized antennas.
The first technique is called "Loop Orientation."  In using a loop antenna the loop "faces" bi-directionally. So after reaching a proper airspeed the first instruction is to set the loop indicator parallel to lateral axis of the airplane. Then the airplane is gently banked to find the first null. By holding this position, the azimuth dial can now be set. After holding that course the loop can now be rotated to find a second null. the angular difference between the 1st and second nulls determines the distance from the radio transmitter using the following formula:

Ground speed x time between nulls = distance out  / angular difference between the nulls.  For example that could be 130 mph x 3 minutes / 45 degrees. That's 8.6 miles. The use of "angular difference" is merely triangulating a location.  The document goes on to describe Radio Boxing, which further orients the path starting with the above calculations.  You can download this document at the link below