Unknown 4-Note Ascending Signal
 | |
|---|---|
| Frequencies | 253.821 MHz,260.675 MHz |
| Frequency Range | 253.821 MHz - 260.675 MHz |
| Mode | NFM, USB |
| Modulation | — |
| ACF | — |
| Bandwidth | 9.5 kHz |
| Location | FLTSATCOM / UHF Milsat |
| Short Description | Signal likely comes from a transponder of one of several older analog UHF Military communications satellites in the 250 MHz band, but specific origin is currently not known. Signal emits a set of 11 matching tones with a two second pause before emitting another set with a higher frequency. This is repeated four times before it begins again and seems to be continuous. |
| I/Q Raw Recording | — |
| Audio Sample | |
Received while I was in the middle of listening to other satellites and decided to look over at FLEETSATCOM.
My location in North America makes it a relatively weak and precise target, but I managed to record it. The signal seems to be a beacon or a channel marker of some sort that repeats a set of 4 ascending tones, each with about a second of space between. Interestingly, it seems to have totally wiped out voice communications on the satellite if there were any left - both of these things may be related to the satellite's decaying stationkeeping ability as it appears to be drifting further out of GEO over South America and further over the Pacific Ocean. Given where it is at the time of writing this (QTH: CI22wj), I find it unlikely it's coming from any kind of ground source - it was also never present in the background of any of the voice communications that I've heard or that I've seen others record from the satellite and appears to have shown up fairly recently. It is present on the upper end of the satellite's 12(?) UHFUltra High Frequency (300-3000 MHz) transponders, appearing to be next to the somewhat more known "waterfall" encrypted channel. Audible in both LSBLower Side Band Modulation and USBUpper Side Band Modulation (Radio, referring to reception and modulation mode)Universal Serial Bus (Computer, referring to USB Ports and cables) modes, but seems to be most audible in USBUpper Side Band Modulation (Radio, referring to reception and modulation mode)Universal Serial Bus (Computer, referring to USB Ports and cables).
Attempted observation on 8/15/2025 on same frequency when satellite was at apogee roughly above Columbia, did not seem to appear when transponder was active. Likely a beacon or idle channel marker.
Observed again on 12/06/25, frequency changed from 253.821 from observations in August to to 260.675. Satellite of origin is still unknown, likely FLEETSATCOM or other older analog UHFUltra High Frequency (300-3000 MHz) milsat, but antenna direction for strongest signal was pointed away from GEO. May be jamming or slow data(?). Observed sidebands from high signal strength on lower and higher transponders. Signal strength also made tones audible in NFMNarrowband Frequency Modulation demodulation, new recording provided is DSBDual Side Band Modulation to make the multiple tones audible. Image provided also showing that the signal becomes weak but does not stop when transponder is activated.
