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!
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, for example!
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" named after the scientist
Heinrich Hertz, so radio frequencies were referred to in terms of "Megacycles" and "kilocycles" 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.
Comments by Bob Leskovec, 2007