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
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.
