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Sputnik's radio signal reveals navigation by Doppler shift
From How GPS Became Invisible Infrastructure: Satellites, Smartphones and Precise Time
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Precursor
Sputnik's radio signal reveals navigation by Doppler shift
Scientists tracking Sputnik found that its changing radio frequency could reveal the satellite's orbit. Reversing the calculation suggested that a known satellite orbit could reveal a receiver's position.
As Sputnik passed overhead, scientists at Johns Hopkins noticed its radio signal shifted pitch the same way a passing ambulance siren does, higher as it approaches, lower as it recedes. This effect is called Doppler shift. By tracking exactly how the pitch changed over time, they could calculate the satellite's orbit precisely. Then they realized the idea could run in reverse: if a satellite's orbit is already known precisely, the Doppler shift of its signal could instead reveal the location of whoever is listening on the ground. That reversal, tracking a receiver instead of a satellite, became the founding idea behind every satellite navigation system that followed.
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