On August 20, 1920, the United States opened its first coast-to-coast
airmail delivery route, just 60 years after the Pony Express closed up shop.
There were no good aviation charts in those days,
so pilots had to eyeball their way across the country using landmarks.
This meant that flying in bad weather was difficult,
and night flying was just about impossible.
The Postal Service solved the problem with the world’s first ground-based
civilian navigation system: a series of lit beacons that would extend from
New York to San Francisco. Every ten miles, pilots would pass a bright yellow
concrete arrow. Each arrow would be surmounted by a 51-foot steel tower
and lit by a million-candlepower rotating beacon.
(A generator shed at the tail of each arrow powered the beacon.)