Not Lost in Space, Just Wandering?
On the afternoon of Sunday, September 8, 2019, I was leisurely monitoring deep space when I noted an S-band signal that shouldn’t have been where it was if all was normal with the GEO belt world… GOES-15 presently lives at 128W and GOES-17 at 137W, so what was a GOES N,O,P series spacecraft doing in between them? A quick check of Space-Track.org data revealed it was GOES-13. But why was it here and still drifting westward? GOES-13 isn’t lost, just wandering? Did it break out of its retirement home?
The plot above shows the signals from GOES-15 and GOES-13 in close proximity to each other. They share the same TT&C frequency as they are from the same GOES series.
At first, I assumed that GOES-13 may have been quietly retired and injected into a graveyard orbit above the GEO belt. Retired GEO satellites are usually maneuvered up in their orbit to get them outside the 300km no satellite retirement zone. In this case the Space-track.org data indicated a perigee of ~35840km and apogee of 35900km, still within the 300km bounds of the nominal 37786km GEO orbit.
The plot above shows dynamic data side-band behaviour from the GOES-13 spacecraft from September 8, 2019.
You may ask why I would assume a retired satellite may still be emitting S-band radio signals even after retirement. In some cases it has been observed that actively ‘switched off’ missions have continued to emit radio signals even after they have been retired. While not very common it does happen. Many DSP and POLAR missions are good examples. See Space Strikes Back, IMAGE Returns to Silence… for some discussion on this topic.
After awhile signal dynamics where observed which closely resembled a spacecraft locking and unlocking from a ground station.
The plot above shows dynamic carrier behaviour similar to other missions where ground station locking has been observed. The carrier frequency was noted to change during these events as well, strongly supporting the ground locking theory.
One other characteristic of the signal that was interesting is that it was not stable in intensity. The signal rose to levels well above GOES-15 and right into the noise at times. This implies some sort of rotational motion in the spacecraft. Given the platform is known to be 3 axis stabilized it did give me pause that it may not be functioning correctly as other GEO satellites observed in the past had stable signals. However, I have also seen operational satellites like SDO with very dynamic signal intensity characteristics.
The plot above shows dynamic signal intensity behaviour exhibiting a period of around 30 minutes.
A closer look at the Space-track.org data reveals that between July 2nd and July 3rd, 2019 GOES-13 started its drift westward. No public notices, status reports or other indication aside from the Space-track.org data suggested the satellite was on the move that I could find online. So retired or otherwise there was no public reason to be drifting past 135W and disturbing my peaceful Sunday afternoon…
The plot animation shows GOES-13’s position on July 3, 2019 and September 8, 2019 respectively.
Things That Go Bump in the Night
GOES-13 seemed very much alive given my observations noted above. But was it really operational? The litmus test is for the satellite to pass through eclipse and see if the radio emissions cease. This would imply the battery had either failed or had been intentionally disconnected as part of passivization procedure during the retirement process. The idea here is to ensure the battery doesn’t explode or do something else unwelcome after the system is shutdown and create unintentional space debris.
It turns out that GOES-13 seems to have a good battery still as it passed through almost an hour of eclipse and emitted a strong radio signal. The change is frequency is normal and seen on most GEO missions that experience a solar eclipse. This sealed the notion that GOES-13 is operational in my mind.
The plot above shows GOES-13’s response to a solar eclipse. Indicating what appears to be nominal battery operation.
After posting this info to Twitter someone who later deleted their twit shared a link to an interesting article by Jeff Foust indicating the USAF and NOAA had been discussing the use of under utilized GOES N,O,P series spacecraft for military purposes. Namely, providing weather monitoring services to under served areas of high interest to the US military (areas around the Indian Ocean).
Why send the oldest of the operational GOES on this mission? I can only speculate that NOAA wants to keep the largely fully functional GOES-14 and 15 near home in case of troubles with the new GOES-16 and 17. As GOES-17 has had some issues and GOES-15 is still active, it makes sense they would be conservative and send the least useful asset. GOES-13’s visible imaging payload is fully functional but its sounder is not so it can still product useful weather data. Thus leaving the fully operational spare GOES-14 over the Continental US. Imagine the politics if they sent it and GOES-16 or 17 failed…
Only time will tell where GOES-13 ends up. But given the nature of these observations there is no doubt in my mind the spacecraft is alive and under intelligent control. The question who is the intelligence?
Good luck with your new career what ever that may be Airman GOES-13 as it seems your Selective Service number came up.