Another Mystery Signal

Hey guys, here’s another mystery signal I’m seeing in my RS1D shake: a spike at all frequencies every 30 minutes (but not exactly 30 minutes, there’s some drift):

Spectrogram of a single spike looks like this:

The pattern seems to only occur between the local hours of 8PM - 9AM, both weekdays and weekends. Any thoughts on what this could be?

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I finally looked up where you are … I can see why you have helicopters circling around day and night with Miramar air base that close.

That also makes me wonder if some of the spikes you are seeing might not be due to the odd radar or radio altimeter pulses as the aircraft fly in and out of there?? I would guess that they do a lot of instrument only training.

One way to find out (might be useful anyway) would be to put the shake inside a Faraday screen. Just placing something metallic over it won’t do it has to surround it. A quick and easy solution would be something like a biscuit tin.

I wouldn’t just put it in the tin, I would drill holes in the bottom for the screw/feet of the shake to go through so that it still makes direct contact with the ground.

Clean any paint off the lid/lip so that the lid makes good electrical contact.

Add clamp-on ferrite chokes, one on the inside and one on the outside of the tin for the ethernet and power cables.

I was thinking of doing this for mine, but it’s a SB, so a sealed container would not help the infrasound side of things. I think I would have to fabricate a “box” out of metal mesh.

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I’m also having this same issue with the spikes. Mine look identical to yours and i’m wondering if they are indeed caused by aircraft flying overhead. We do have a fair amount of aircraft that fly overhead from LAX. I’m going to see if there’s any correlation between the two and let you all know. (See the screenshots of my blips)


For me the blips seem to come primarily at night. Would aircraft only use whatever is causing the blips at night?

@PhilipPeake I bought a cookie tin/ferrite chokes and will be encasing my shake in a proper Faraday cage on Friday, I’ll see if it makes a difference. I already have it in a “partial” Faraday cage but the bottom is indeed exposed so it’s probably not blocking all of the signal.

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If it is related to aircraft … the most likely signal would be radio altimetery. I don’t know if these are used all the time. I would guess not by most aircraft. However, at night, especially when flying relatively low I would guess that it might be a good idea to know how far you are off the ground. There are regular-ish flight by helicopters around here. I think it’s partially training, and partially looking for people growing illegal crops.

Radio altimeters are about 2W @ 4.5GHz, and have a quite narrow beam.

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So I’ve been watching the past few days and I believe that a fair amount of these spikes are caused by planes and the signals they give off. I’ve noticed that larger airliners (737s) do cause a blip in the signal when flying directly overhead at around 10k ft, however planes higher then about 20k ft don’t cause the blips. I don’t know much about planes but as @PhilipPeake said it’s probably caused by Radio Altimetry. However, I have seen planes flying overhead during the day and still see a blip. So whatever it is it must be used somewhat regularly during the day as well.

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It’s possible the flight paths at night are more typically over my house. I’ll try to pay more attention to it.

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I have been playing with Flightradar24, which as well as giving live flight tracking information, has the ability to replay past tracking history.

Looking around the times of the last couple of events my Shake recorded, all I see are a couple of commercial airliners at 37,000’ - cruising altitude, so unlikely that they would have radio altimeters active, and it probably is not effective at that hight anyway.

There are a couple of small piston engine planes, from a training school. They don’t go directly over my house, and the times are off by a few minutes.

So I am going off the idea of this being radio altimetry - although, I don’t think Flightradar24 tracks military flights. So I am not going to entirely write off the idea.

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Although … having said that:

Not sure how accurate the time is on Flightradar24.

That is right over my house.

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This one is not so mysterious. Neighbors fireworks party.

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Summary: I gave the cookie tin a try and had surprisingly poor results. I think the issue is that the tin is far too cramped for the Shake enclosure, putting a lot of tension on the ethernet/power cords. It’s also extremely difficult to fit the feet into holes I drilled at the bottom without having the tin somehow touching the Shake’s body. Lastly, there’s no room for sound insulation in the cookie tin, so mechanical noise is a big issue.

Here’s what the modified tin looks like before I put the Shake into it:

Here’s what the “background” level looks like in the middle of the night. Amplitude of a micron/s was pretty typical. Huge undulating signal present.

And what’s worse is that it seemed to do nothing to dampen those huge spikes I was getting every 30 minutes or so, which was the whole point of this exercise:

To compare here’s my prior setup which I have since reverted to: a plastic storage container that I covered in aluminum foil, filled with sound-deadening foam, then flipped upside-down. You can see a small hole I drilled into one side that I put the two cables through.

Typical background level looks like this for this setup, about 0.3 - 0.4 microns/s, mostly at a few typical frequencies. I’ve tried knocking out every source of vibration in my home and these signals persist, so I suspect it’s from some neighbor’s appliances and/or AC unit.

The upside of this experiment was that I got to eat some butter cookies! :slightly_smiling_face:

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Did you run the cookie tin setup long enough to see if it helped with the big spikes?
Otherwise, it does look like your problem is more with airborne noise.

This is my “quiet time of night” looks like. I suppose I might be able to get it better…

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Yeah I didn’t see any change in the large spikes with the cookie tin setup, they’re still there.

I’m going to try more sound deadening with another enclose that I’ll place around this enclosure and see if that makes a difference. It’s definitely good enough to see most seismic events as it is.

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