Radio transmissions from Sputnik-1 could be heard on short wave radios around the world.


  1. A carrier signal only would not be particularly audible to an AM radio listener, was it modulated with an audible tone, or was a beet frequency oscillator (BFO) required to make the signals audible and get a good fix on the Doppler shift versus time?

  2. Was it a steady beep-beep-beep or was there a sequence of different tones? I ask because the first video suggests only a repeating tone but the second video sounds like its a sequence of four different tones, as if some telemetry were involved.

  • $\begingroup$ Russian version of Sputnik-1 wikipedia article gives three links to the sound of Sputnik, recorded in: 1)Chechoslovakia; 2) Washington; 3) Germany. All three sound quite steady on the tone. I couldn't find any information on change of the tone (by design) in the original technical report on design of the transmitter. $\endgroup$ Feb 4 '20 at 10:52
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    $\begingroup$ The electronics of Sputnik-1 was build using vacuum tubes only. To save batterie power for heating the cathodes of the tubes only very few tubes could be used. The simplest solution was an unmodulated carrier without different tones, Keying the transmitter saved power in the period between two beeps. $\endgroup$
    – Uwe
    Feb 4 '20 at 13:09

It appears to just be a carrier signal alternately transmitted and not transmitted. As the carrier is in the MHz range, the audio cannot be a direct mapping of that frequency. I don't know the specific mechanisms used to make the signal audible as a particular tone, or whether that tone would differ between different receivers.

There are multiple recordings of it on http://www.dd1us.de/spacesounds%201.html. While a few have varying pitch, they don't appear to repeat (and not over 4 beeps). To my ear any variation in pitch sounds more like the variances you hear when trying to tune in a signal. Most of the recordings have only a repeating tone. I suspect that your second video is just a repeated loop of four particular tones.

One of the audio files contains some contemporary analysis by someone at Caltech.

For the first three days, both signals were beeping -- that is the carrier was alternately switched on and off. And it appeared that when one channel was on the other was off.


It is our personal opinion that these signals contain no telemetering, or at most a low accuracy telemetering of simple information, and that the beeping was for purposes of easy identification.

Per wikipedia, the duration of the "on" signal could be modified by temperature and pressure information. I couldn't find any sources that countered the assertion that the carriers were fixed and not (intentionally) modulated within the craft other than the sub-second transmitting/not transmitting periods.

Another contemporary paper (Guier and Weiffenbach , "Theoretical Analysis of Doppler Radio Signals from Earth Satellites," Nature 181 , pp. 1525-1526 (1958). ) describes the signal capture this way:

We had one distinct advantage over other groups: one of us was completing a Ph.D. dissertation in microwave spectroscopy and had a high quality communication receiver and the necessary understanding of how to make precision frequency measurements. Because of the fortunate happenstance that the Beltsville, Md. WWV 20-MHz standard broadcast was at nearly the Sputnik frequency, the output from the receiver could be made to be the audible difference between the WWV standard and the Sputnik signal.


Our first reception of the Sputnik seemed only a momentary triumph when we realized that there was no telemetry on the 20 MHz signal, just a pure tone that at first appeared to wander in frequency by a surprisingly large amount. After about 5 minutes, it was unmistakably clear that the wandering was the moving Sputnik's Doppler shift, which we were by then recording with precise frequency and time information.

That said, if the reported frequencies were accurate, the difference would be .005MHz or 5kHz. While very audible, 5kHz is a pretty high tone. None of the recordings I listened to seemed to have tones that high. So I suspect a different method was used in general

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    $\begingroup$ The most relevant authorative data one could possibly find is contained in this D-200 manufacturer report, but it's in Russian. It contains all reasoning of design choices made, electrical scheme, list of electrical parts used, etc. I couldn't find in it though the choice of tone and it seems there's no mentioning of the tone variance by design. The length of beep-pause-beep fractions were meant to be varying depending on pressure and temperature in the spacecraft. $\endgroup$ Feb 4 '20 at 9:46

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