A POOR MAN'S PEN-RECORDER FOR PRINTING OUT PULSE DURATION TELEMETRY
+ A BASIC UNDERSTANDING OF PDM TELEMETRY AND [ ON-OFF BEACONS (^) ]
by: Aaron George Bailey
UNDERSTANDING CW - PDM TELEMETRY AND ON-OFF BEACONS
This is my homemade pen-recorder.  I wanted a better way to visualize Pulse Duration Modulation- (PDM)
after receiving 20.008Mhz Soyuz-25 telemetry on October 9, 1977.  This is what I threw together and believe
it or not, it worked, and even tracked the sync-pulse train in a consistent way.  This old type of Soyuz PDM
telemetry was broadcast on shortwave and is no longer in use today.  Soyuz PDM is more correctly called
CW-PDM since the transmitted radio signal was switched "on" and "off" at a rapid rate, much like (CW)
Morse Code.  The Russians used a similar CW-PDM telemetry throughout early manned related flights
starting with the Korabl Sputnik test series.  Most of these flights carried Dog(s) and other animal test
subjects.  The Korabl Sputnik test series matured into the manned Vostok and Voshkod% spaceships.  The
Voshkod spacecraft  was actually a modified Vostok craft.   Soyuz, a totally new spacecraft design, was
introduced in 1967 and it too used CW-PDM telemetry on shortwaves, as did some Kosmos satellites.

One frame of Soyuz PDM, like I received from Soyuz-25, is composed of 15 words,  plus a synchronizing  
series of rapid pluses.(14)  <SEE BOOK  Each word sounds like a "BEEP" over the radio and occupies one
second of time within the frame of telemetry.-(meaning "ON" time + "OFF" time = one word = 1 second total
time)  The  rapid synchronizing pulses make a "PURR" sound and mark the beginning of a new frame of 15
words.  The sync-pulses also reset ground recording equipment to zero in preparation for the new
telemetry frame.  Each word of the CW-PDM varies in length, or varies in "ON" time, up to, and approaching
one second in time.   A PDM word might be a (analog ?) measurement of some quantity on the spacecraft,
or it can represent some condition- some made up examples are- (short beep-OFF or long beep-ON) or
(short beep-DOCKED and long beep-UNDOCKED)  NOTE- The Russians used a variation of PDM that was
modulated, ON and OFF, by shifting frequency slightly.  This was called "Frequency-Shift-Keying" or
FSK-PDM and it sounded like a two tone PDM over the radio.  It was broadcast continuously on the
shortwave frequency 19.954 MHz by special space station modules- Cosmos 1267, 1443, and 1686.  Unlike
the HF Soyuz CW-PDM which was abandoned in the early 1980s, the Cosmos space tug series continued to
transmit FSK-PDM on shortwave until mid-December 1989, when I received Cosmos-1686 for the last time.

My PDM recorder works as follows - see circuit diagram below.  An ink tip* from a pen was attached to the
armature of a SENSITIVE  relay.  The ink tip operated up and down in step with the PDM and contacted the
teletype tape through a hole in the sheet medal tape guide.  The paper tape was advanced by the pinching
action of the motor and rubber wheel.  The PDM was played over a tape recorder whose audio output was
fed into the 8 ohm winding of an audio transformer and the stepped up voltage from this transformer
secondary was then rectified by a bridge and fed to the relay.  Also, the tape recorder volume setting was
used as a level adjust for proper relay action.  A stable, good quality recording was key for good results.  
The PDM printed out as lines of varying length on the paper tape.  I then could measure each word in
millimeters and over time I hoped to observe changes in the telemetry.  The problem was, the PDM was
turned off most of the time and consequently I didn't have enough data points to study.  Typically, after
launch, and immediately after orbital insertion, the shortwave antenna was deployed and then PDM was
switched on, but only remained on for several hours.  It was only during this brief time frame that I
received HF telemetry from Soyuz, very rarely, did I hear that spacecraft again.(#)  Capturing these signals,
without any prior knowledge of a launch, required thousands of hours of monitoring on 20.008 MHz.  In a
nutshell, this exemplifies the challenge of radio eavesdropping on the Soviet space effort.  I mean, my
shortwave receiver(s) ran continuously when I was home.  Cheap radio receivers from that era, like mine,
commonly drifted off frequency or got knocked off frequency.  So success meant, a measure of luck, lots of
patience, and all the while, maintaining the correct frequency by repeatedly using the crystal calibrator to
tweak the tuning.  In the South we have a saying, "Poor people have poor ways".  To me, this translated
into having to use cheap, toy radios.  I only had pictures of dream receivers like the Collins R-390 and
51S-1.  Then in the 1990s I got the AR-3000A , a real receiver.  I still have pictures of the R-390 and 51S-1, as
those receivers are almost museum pieces now, and radio collectors still pay high prices for them on ebay.

Of coarse my PDM pen-recorder was a crude prototype and it was not suitable for serious scientific work
but it was fun to tinker with and it did make a rough hard copy of PDM that you could hold in your hands
and study.  It would have made a nice science project had I known about CW-PDM telemetry when I was in
school.  Certainly it could have been improved, like adding a better motor with a heavy flywheel.  It was
made of bits and pieces from my junk box, as with my other projects.  An experimenter / builder without a
big junk box, is like a man who is brain dead.  Having a piece of junk, in hand, where you can study it,
actually stimulates thought on how it can be applied to a project or experiment. - example - Having a motor
in my junk box was a starting point for this PDM printer.

* The armature, of the small relay, had to have low mass in order to operate fast.-(above)  Firstly, I pulled
out the felt tip from a pen.  It was one inch long and porous.  A short piece of 20 gauge wire was bent at a
right angle and soldered to the relay armature.  Then the felt tip was attached to the armature wire with a
piece of plastic sleeving.  The snug fit of the sleeving permitted up and down adjustment of the felt tip for
best contact with the paper tape.  Finally, before printing, the felt tip was saturated with a drop of ink.
(+) UPDATED - SEE FOOTNOTE BELOW - April 1, 2010 email to me from Soviet Space Tracker - Richard Flagg

(^) Sputnik 1 and 2 broadcast a beeping signal on 20.005 megacycles.  At first glance it is difficult to  
visualize how any useful information could be conveyed by a simple on/off beep.  The beeping signal is
made of two components, beep ON TIME and beep OFF TIME, and both of these can be varied in length just
like the PDM described above.  In fact, the Sputnik 1 and 2 beacons were a rudimentary form of PDM as
they did send back spacecraft temperature and pressure.  

(#) This was my observation from October 9, 1977 till the early 1980's when the HF CW-PDM was
discontinued as more upgraded Soyuz-T models came on line.  The point is, the HF CW-PDM might have
been switched on at other times but I didn't hear it.  Of coarse, from the beginning, I couldn't man my
radios 24 hours a day as life goes on with the daily routine and other obligations, school, work, military
service, family, etc.  The Midwest United States is known for strong thunderstorms and many times my
radio gear was disconnected for this, or even the hint of an approaching storm front.  Listen, you wanta
fast fry a radio receiver, just leave it hooked to a sky wire antenna during a lightning storm, and presto,
you got yourself a boat anchor, or door stop, take your pick?  WARNING - UNPLUG the AC power too !

RETURN TO HOME PAGE

RADIO SIGNALS FROM SPACE
THE JOY OF BEING AN UNBRIDLED EXPERIMENTER / BUILDER
COMING SOMEDAY, PICTURES OF THE ORIGINAL TAPE MADE BACK IN 1977, WHEN I FIND IT.
CLICK IMAGE
TO ENLARGE
(+) April 1, 2010 - email from space tracker Richard Flagg.  Dick sent me a picture link, showing the
Kettering Group listening station in England.  Reference to my PDM Pen-Recorder, Dick Flagg wrote, "It
reminds me of the strip chart device used at the Kettering Grammar School - the Great Northern Telegraph
Undulator."  SEE- "GNT Undulator" in the Kettering station photo
TASS NEWS AGENCY- March 16, 1965.  Soviet Scientists appear to be using an electromechanical device to
record pulse and breathing  by printing PDM telemetry onto paper tape. This photo was purchased on ebay.
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