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Tyneside, UK
2017 Sep 22
Friday, Day 265

Maintained by:

Omid Transmission Mode 3

On February 8, Omid was seen to switch from its alternating Mode1/Mode2 transmitting regime to Mode 3.

The signal structure is similar to Mode 1 but instead of a continuous transmission, the signal is heard at two minute intervals for 21 seconds. The scan looks strange because of the missing segments of transmission that would link everything into a smooth Doppler curve.

At first it was thought to be a battery conservation mode but a new suggestion has emerged that it is an identification beacon to indicate that Omid is above the horizon and available for use in its store-dump communications mode.

The image above is a 'close' up of a single burst of Mode 3 signal. The drop-outs caused by Omid's rotation are very obvious. It can be see that the carrier switches on first, followed by the sidebands about 2s later. Everything then switches off just about simultaneously.

This is a plot of the signal strength during a Mode 3 signal burst. It was produced by Dick Flagg from an audio recording made in the UK. Omid's rotation is very obvious. The timescale is annotated in hours, minutes and seconds but does not reflect the time of the actual event.


The first time Omid was detected as transmitting in this mode was 2009 February 8 when the first 21 second long burst of signal was detected just after 18:03:20 UTC. Omid was on a pass across the UK. Silence during the previous five minutes of the pass indicated that this was the moment when Mode 3 was first switched on.

Omid appears transmitted in this mode for about 48 hours from the middle of February 8 until the middle of February 10.

Interceptions from US and South American stations indicated that Omid spent further time transmitting continuously in Mode 3. Again it was proably for about 48 hours, between February 16 and February 18.
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