I may start a complete subcategory on this website for unidentified signals I find.
Background
For those of you who don’t know, an RTL-SDR (Realtek Software-Defined Radio) is a small USB that you can plug into a computer and listen to almost any common radio frequency (roughly between 1 MHz and 2 GHz).
So sometimes, I wander around the radio spectrum, straying far from the police scanners or common music channels. Today, I went as far as the GPS signals in the L-Band, around 1.57 GHz. Just above GPS, I found something… out of the ordinary.
The Signal
Now I’d like to point out that, at this point, I have strictly Q-Branch sampling enabled, as opposed to I/Q sampling. So this signal may not actually be in the L-Band, it could just be something heterodyned up. But regardless, I was able to consistently find it there, even after moving up and down the spectrum and completely reconfiguring the dongle.
Frequency: 1,576,923,333 Hz
Bandwidth: 2700 Hz
Demodulator: USB
Sampling: Q-branch
Pattern: Transmits for 13 seconds, silent for 2 seconds
Receiver Location: Northwestern Illinois
For 13 seconds, it transmits any combination of several tones, typically shifting out of phase slightly. I have noticed that, at some points, it alternates: for 13 seconds it’ll send up to 6 or 7 tones, silence for 2 seconds, then only transmit 1 or 2 quieter tones for the next 13 second interval.
I’ve never seen any type of transmission like this. If it’s a data transmission, it’s the slowest baud rate I’ve ever seen. I can’t help but think about the countless hours I’ve read about Cold War number stations, and wonder if this might be some sort of encoded variation.
EDIT: So it’s the morning after now, about 6:26 AM local time, 12:26 UTC. The signal has not stopped all night, but followed its regular pattern. Can anybody else hear this thing?