Indian Mounds Park Airway Beacon by Robert Rydeen

An Airway beacon was a rotating light on a tower used for visual navigation by airplane pilots along a specified airway corridor. Approximately 1,500 Airway beacons were constructed, covering 18,000 miles (29,000 km) in the U.S. to guide pilots from city to city. Construction by the Post Office and the U.S. Department of Commerce occurred between 1923 and 1933. They were spaced 10 miles (16 km) apart and featured a 24 inch (610 mm) parabolic mirror and a 110-volt, 1000 watt lamp. In clear weather they could be seen for 40 miles (64 km). The beacons rotated at 6 rpm, creating a flash every 10 seconds for 1/10th second duration. The Saint Paul, Minnesota beacon sits on top of a 110 foot (34 m) steel tower in Indian Mounds Park. The Low Frequency Radio Range system began to replace this visual system in 1929.

Ref: Wikipedia