This jumble of letters was once very, very important to pilots navigating the skies of the U.S. at night. From the 1920s to the 1960s, air routes in the US at night were marked by a series of light installations called airway beacons. The idea was simple: a bright light on the surface is visible for a great distance from the air at night - so a line of such lights could be 'followed', one to the next, by a pilots to lead them to their destination. This system was more accurate than the complementary Adcock Range radio navigation system but required clear skies. It was the first infrastructural navigation system emplaced in the U.S., and the first airway was originally constructed by the Department of Commerce to support the first night-time air mail route between California and New York. Eventually, the system expanded into an entire network of routes with associated published maps, linking cities across the country. There were proposals to extend the system across the oceans using buoys, although this was never completed.
The letters WUVHRKDBGM were used to determine where on the route you were, in the following manner. Each station consisted of a flashing white beacon light (created by rotating a bright white lamp). This is known as white/white - by contrast, modern civilian airports (between twilight and sunrise) operate a rotating beacon that flashes green/white. Military airports flash green/white/white. In any case, the beacons needed not only to inform the pilots as to their location, but what direction they should be flying.
The navigation routes - called airways - can be imagined as a series of lines that connected the beacons, very much like connect-the-dots on a map. Pilots needed to be able to reorient themselves at night if they became lost and headed for a beacon which was visible. So, just to the side of the beacon and visible only from the approach/departure angles of the airway as it crossed the beacon, a red light would be placed which would flash Morse code signals to spell out a single letter. Along each airway, the beacons were set up to always report their identifying letters in the sequence given WUVHRKDBGM. Since beacons were ten miles or so apart, that meant that if a pilot spotted a beacon and could identify its code letter, he or she would know that there was no other beacon reporting that code within a hundred or more miles- which would make locating his or her position on a map of these beacons feasible. As an addendum - every third beacon had an emergency landing strip near to it (next to it if possible) and those beacons with landing strips had Morse code lights in green - and that's why modern civilian airports are marked with a green/white flash.
So the standard procedure, if one became lost, was to find the nearest white/white beacon and approach it, then begin to orbit it (see turns around a point). Eventually, a red light flashing Morse would become visible as the airplane crossed the airway; at that point, the pilot would turn directly towards the beacon and be able to determine, by the map, from which direction they were approaching it (if they had a working compass) and could then turn towards the 'next' beacon in sequence.
The sequence of letters may have been chosen to ensure that the Morse sequences for adjacent beacons were dissimilar, rather than for any significance of the letters themselves. Pilots used (what else?) a mnemonic to remember the beacon sequence; the most common one found in today's historical writings goes like this:
When Undertaking Very Hard Routes Keep Direction By Good Methods.
In 1973, the last Federal airway beacon was retired, and it now stands in the Smithsonian's National Air and Space Museum. The beacon light flashes every ten seconds, and the course lights spell out 'G' - but they are red, since there is no available landing strip nearby. You can see it in the 'America By Air' gallery in the museum in Washington, DC.
(IN 5 12/30)