PDM signal format

This signal format was used for almost 30 years by many Soviet space vehicles. It was employed on shortwave transmitters operating close to 20 MHz. Especially in the CW-PDM mode (see below), this transmission method used very little bandwidth and could easily be heard far beyond the radio horizon. In fact, on early Soyuz missions around-the-world propagation often occured. This was caused by the "whispering gallery effect", i.e. the propagation of the signal through a duct between layers in the ionosphere. Often the signal strength was constant for a long time and then suddenly peaked when the satellite reached the antipode. This "antipode-effect" could be seen as a focussing affect. All radio waves leaving the spacecraft in the horizontal plane had to meet at the other side of the earth!

FSK-PDM

Frequency-shift keyed (FSK) with the "off" and "on" periods transmitted on two adjacent frequencies, approximately 1000 Hz apart. 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). Listen to Kosmos 929, FSK-PDM, 19.954 MHz, September 20, 1977 (62 kB, mp3)

CW-PDM

A carrier (CW) keyed "off" and "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, command-off at 1420 UT, October 30, 1967 (57 kB,mp3)

When displayed on a pen-recorder these two signal formats look extremely similar. Below, an example of PDM telemetry is shown.

CW-PDM signals from Cosmos 140 (a Soyuz test flight) on 20.008 MHz recorded on rev.5, February 7, 1967.

[Sven's Space Place]

[Space Radio Notes]