Doppler Shift observed on Sputnik signal in 1957    
Sputnik 'BEEPS'coming up out of the noise, 
as the satellite comes into range 
of the club's antennas.  
See below to play the audio clip! St. Joe's High School Radio Club
Sputnik Beeps -- Doppler Shift!

Play the Sputnik Audio Clip and Hear the Doppler Shift!

This audio clip is about 94 seconds long with a pronounced downward change in pitch at 48 seconds demonstrating the "Doppler Shift" that occurs just as it passed over.  Make sure to be watching a clock with a "second hand" as you listen!

Have your timer ready?   OK,  Click here to play the Sputnik audio clip!

(NOTE: If you do not hear anything, you may have a flag somewhere
on your screen from your browser asking you to "allow blocked content".)



The signal itself did not actually have an audible "beep".  The pitch comes from using the "BFO" (Beat Frequency Oscillator) in the radio receiver, as is still done today, to hear Morse Code signals.  This is an oscillator in the receiver that is mixed with the incoming radio frequency which was being "keyed" on and off, and creates a "beat" or "difference" frequency that can be heard as an audible pitch. 

Different recordings have different pitches depending how the BFO was set by the operator.  In some recordings, the pitch varies wildly, merely indicating that the operator was still adjusting the the BFO while the recording was made. 

Once things were set up to obtain a "good listenable pitch", it was best never to touch that control again. Similarly, we left all our equipment on 24 hours per day for the entire tracking program, so as to achieve maximum temperature stability, because in those days, equipment performance changed drastically with temperature and oscillators had considerable warmup drift.  It was most important to have the equipment stabilized so that we could record the changes in frequency of the signal itself and confirm things like Doppler shift.

Therefore the pitch heard is that of the radio frequency relative to some fixed reference BFO frequency.  Thus the Doppler shift heard as a change in pitch in the audio difference frequency is actually a change in the radio frequency of the signal received from the satellite as a result of its velocity.

Most people, once made aware, have realized they regularly hear Doppler shift when a siren or horn on a vehicle quickly passes by. The effect is more pronounce with higher relative velocities. You can find plenty of references to learn more about this by doing a computer search about "Doppler shift", so we'll leave that for your homework assignment!  Similarly there are other Sputnik audio clips you can find on varous websites, but let us explain some of the "anomalies" you might hear there.

On the clip from the NASA web site, you will hear one section where there occurs a prolonged beep. This never happened with the real satellite signal.  It was probably caused by a poorly made splice on a copy of the original recording which was made into a tape "loop" to play indefinitely on a reel-to-reel machine, and someone did not take care to match up the ends sensibly.  Later, others who never heard the original simply made copies.

However, one factor remains fairly constant to the casual observer, and that was the approximately equal 0.3 second on-time of the "beeps" and the similar "off" time of the space between the them.  But, as suspected and later confirmed, the "on" and "off" times varied in subtle ways to convey information about conditions on the spacecraft, such as temperature.  This is called "telemetry" and is the reason why the FBI took our tapes to analyze!

A few samples out on the web have the time intervals very short.  I am surprised when I first heard some of these posted by people who claimed to have made the original recordings. But heck, it HAS been 50 years!   They may have forgotten that they might have recorded them at 3-3/4" per second and are now only able to play them back on some reel-to-reel machine that is 7" per second!

Incidentally, the phenomenon of "mixing" frequencies is also something the Russians exploited by choosing 20.005 MHz, because it was only 5 kHz away from the "U. S. National Bureau of Standards" time and frequency station, WWV located precisely at 20.000 MHz; so simply tuning in WWV, one could first hear the satellite with a nominal 5 kHz pitch, then tune up the band a couple kiloHertz from there and turn on the BFO to get a more comfortable listing pitch.

Of course in those days "cycles-per-second" had not yet been defined as "Hertz" (abbreviated Hz), named after the scientist Heinrich Hertz, so radio frequencies were referred to in terms of "Megacycles" (Mc) and "Kilocycles" (Kc), and people incorrectly omitted the "per second" part of the phrase.

To tune in the 40.002 MHz signal, we had to use a little trick.  We used an inexpensive Heathkit RF signal generator set to 20 MHz, and tweaked it until we got a "zero" beat with WWV on 20 MHz. Then we tuned the National Radio NC-109 receiver to the 40.000 MHz second harmonic of the generator signal.  Thus we would soon find the 40.002 MHz Sputnik signal as a 2 kHz audio pitch.

It is very easy to generate the second harmonic of a signal, so I always wondered why the Sputnik designers did NOT use the second harmonic of 20.005 MHz which is 40.010 MHz, --instead generating a separate 40.002 MHz signal?

Robert A. Leskovec, K8DTS, 2007


October, 1957 

Listening to Sputnik on the Concertone

Fred Imm and Mike Stimac with the Concertone 10" reel-to-reel tape machine used to record the Sputnik signals that the FBI "borrowed" for analysis. Each reel held 3,600 feet of "1-mil" tape.

This machine was professional full-track monaural, with 7.5 and 15 inches-per-second speed, and separate Record and Playback heads. Besides the synchronous capstan motor, it had a separate motor for each reel, so that Fast-Forward and Reverse were very quick, and the the tape was always properly tensioned during record and playback.

Because of the quick rewind to play back a Sputnik pass just recorded, the students noticed a pronounced pitch change between the beginning and end of the satellite pass. This, indeed was "Doppler Shift" of the carrier frequency, --and the observation was a big deal in the context of the burgeoning space science of the day!

Photo by Robert A. Leskovec, 1957

Fred Imm K8EVB at the controls to make a dub,
onto a 7-inch reel machine (not shown)
as Mike Stimac W8KTZ starts the tape.


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