Many thanks to SWLing Post contributor, Richard Langley, who writes:

Yesterday, 4 October, was the anniversary of the Soviet Union’s launch of Sputnik I, the first artificial Earth satellite. The launch heralded the beginning of the space age. Sputnik I’s Doppler-shifted radio transmissions on 20.005 and 40.002 MHz led to the development of the U.S. Navy Navigation Satellite System (Transit) and the equivalent Soviet system (Tsikada) and, eventually, to GPS and GLONASS and the other modern global navigation satellite systems.

The Sputnik I radio signals were picked up by many shortwave listeners. The 20 MHz signal was close to that of WWV and so was easy to find. And, apparently, WWV turned off its 20 MHz transmitter during some of Sputnik I’s passes over the U.S. so as not to interfere with reception.

There are several good sites on the Web with information about Sputnik I and its radio signals including: