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!

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 speeds, 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.

Play the Sputnik Audio Clip and Hear the Doppler Shift!

This audio clip is about 93 seconds (1:33) long with a pronounced downward change in pitch at 47 seconds demonstrating the "Doppler Shift" that occurs just as it passes over.  Make sure to be watching a clock with a "second hand" as you listen!

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

You can also manually left click on the "Dot"
and slide it to different sections to hear the Doppler shift.

(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 Sputnik transmitter carrier signal was being "keyed" --meaning turned on and off, in a repetitive sequence. 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 of Sputnik 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.   To this end, 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 of this, have realized they regularly hear Doppler shift when a siren or horn on a vehicle quickly passes by.   The effect is more pronounced with higher relative velocities.   To learn more about this, you can find plenty of references 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, if they have never fixed it yet, you will hear one section where there occurs a prolonged beep.  This is probably due to a poorly made splice on a copy of the original recording which was crafted into a tape "loop" to play indefinitely on a reel-to-reel machine, and someone did not take care to match up the ends at the splice sensibly.  Later, others who never heard the original simply made copies of the NASA audio, and if you hear that periodically repeating "longer" beep, you can tell where they got their copy from. --and where a longer tape is made up of that repeating loop, neither will you hear the Doppler shift.

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, when comparing many successive passes, and was suspected 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 for analysis!

A few samples out on the web have both 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.   Thus, simply tuning in WWV, one might hear the Sputnik signal beating with WWV producing a 5KHz pitch, or one could tune "up the band" a couple of kilohertz from there, and turn on the BFO to hear the satellite at a more comfortable 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/second" (Mc/sec) and "kilocycles/second" (kc/sec), and people incorrectly omitted the "per second" part of the phrase. (Incidentally even the capital "M" has significance, since that stands for the abbreviation "Mega" --meaning "Million", whereas the lower case "m" is used to denote "milli" or "0.001".)

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 "beating" with that generator signal.

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 which would have required addtional components and increased the payload. But, with due credit I suspect there was a good reason!

Comments above are by Robert A. Leskovec, K8DTS, in 2007, 50 years after the experience.

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