PDM signal format
This signal format was
for almost 30 years by many Soviet space vehicles. It was employed on
transmitters operating close to 20 MHz. Especially in the CW-PDM mode
below), this transmission method used very little bandwidth and could
be heard far beyond the radio horizon. In fact, on early Soyuz missions
around-the-world propagation often occured. This was caused by the
gallery effect", i.e. the propagation of the signal through a duct
layers in the ionosphere. Often the signal strength was constant for a
long time and then suddenly peaked when the satellite reached the
This "antipode-effect" could be seen as a focussing affect. All radio
leaving the spacecraft in the horizontal plane had to meet at the other
side of the earth!
with the "off" and "on" periods transmitted on two adjacent
approximately 1000 Hz apart. The telemetry frame consists of a train of
rapid pulses followed by 15 words transmitted at a rate of
one word per second. These words are pulse-duration modulated (PDM). Listen
to Kosmos 929, FSK-PDM, 19.954
September 20, 1977 (62 kB, mp3)
A carrier (CW) keyed "off"
"on". The telemetry frame consists of a train of rapid pulses followed
by 15 words transmitted at a rate of approximately one word per second.
These words are pulse-duration modulated (PDM) by keying the carrier. Listen
to Cosmos 186 on 20.008 MHz,
at 1420 UT, October 30, 1967 (57
When displayed on a
these two signal formats look extremely similar. Below, an example of
telemetry is shown.
Cosmos 140 (a Soyuz test flight) on 20.008 MHz recorded on rev.5,
Copyright © 1996 Sven Grahn