Not that long ago, one of the RTL-SDR readers gave us some interesting input on a peculiar hobby that he discovered while using the RTL-SDR. He was using the RTL-SDR with his QFH antenna in order to find the outdated satellites that were decommissioned and working in the 136-138 MHz and 150-400 MHz frequencies. The thing with these satellites – their batteries have expired a pretty long time ago due to some form of chemical reaction caused by numerous recharge cycles. However, in time, these allowed the mentioned satellites to start charging directly via their solar panels which in turn enabled their transmitters. The following is the message we got from the reader:
I was receiving the NOAA/Meteor on the 137 MHz band and noticed some strange, unidentified signals that were interrupting one another. I had to go on Google and learned that these signals were coming from the dead satellites which were actively broadcasting under certain sunlight conditions. However, the signal was sufficient enough to appear on SDRSharp. Granted, most signals were not that strong and required a certain zoom up in the SDRSharp to be received properly.
I was surprised to find out that some of these satellites had a life of their own – they were coming from all sorts of decommissioned satellites – Weather, Navigation, Experimental, Military, I even spotted some coming from the Amateur ones. While they were unable to broadcast any actual telemetry or even weather images, they continued to use all the original frequencies that were sent to unmodulated carriers. Of course, this does create an interferences with the existing operational satellites functioning in the 136/138 MHz band. Generally speaking, when a satellite switches off and runs out of fuel, all its reserves are applied to move it into the so-called “graveyard orbit” that’s positioned above the usual orbits. This is when the satellite is switched off completely and left out in peace.
Yet, in some cases, the solar radiation or some other technical issues will make it stop responding to the ground station command. This leaves the satellite on the existing orbit. Some of those feature a special timer that will activate at the end of the lifecycle to terminate the power feed to the transmitter. This is supposed to stop the broadcast but it may fail as well.
Still, I found from here that some of the oldest satellites are still broadcasting. You will be surprised to learn about the Transit 5B-5 (Military Navigation) from 1964, as you can hear them clearly via the CW or USB mode. The noises they make may sound like songs, weird melodies that are somewhat scary to me.