5 ms·

Yeah I'm not an expert in this stuff but I am an extra-class radio amateur and this sets off literally all my intellectual alarm bells. I'll take one for the te

Yeah I'm not an expert in this stuff but I am an extra-class radio amateur and this sets off literally all my intellectual alarm bells. I'll take one for the team and go read the report but I agree that it seems like pure unadulterated bullshit. The ionosphere is a very violent place for an HF radio signal. These radio waves do not travel in a straight line. They don't even come out of the ionosphere with the same polarization as they go in with. I cannot fathom how you could glean anything from a WSPR signal's RSSI. edit 1: I'm just gonna live-text my thoughts on this as I go through it into this comment. First off, the software that this person developed, GDTAAA, doesn't seem to be available for viewing. Shame because I'd love to see it, but I guess I'll have to go by the paper. edit 2: "An algorithm is run that calculates an expected received SNR based on the transmitter power and the short path distance" W O W, okay, not even taking into account an estimated noise for the receiving station. Nice. It also doesn't seem to take into account time of day, any of the different propagation modes like gray line, etc, or the wavelength of the transmission. Big yikes. edit 3: WSPR signals have a transmission length of about two minutes. A 777 travels about 20 miles during that amount of time, and you're trying to use the average SNR of the entire 2 minute period to estimate whether an aircraft crossed the path of the signal. Even if the WSPR signal occupied the entire 20 mile-wide swath of sky that the aircraft was in during the WSPR transmission, the aircraft can only block about 0.2% of that swath at an given moment on account of it being absolutely tiny.

dboreham 5y ago
I agree it seems far-fetched, but: possibly by integrating many data samples you can remove the ionosphere noise from an underlying signal.
- lambdasquirrel 5y ago
  If the ionosphere noise were merely noise, and not just distortion, this would be true. I'd suspect that even changes in solar radiation (from the rotation of the earth) would cause e.g. more ionization in some parts of the ionosphere compared to others.
jrockway 5y ago
I'm also an extra-class radio amateur, and I too am unconvinced by this article. The "tripwire" concept looks good in 2D, but I'm less convinced in 3D. Most HF signals propagate by bouncing the signal off the ionosphere (it's actually refraction, not reflection, but...), so the ideal path to the observer looks like shooting a signal straight at the horizon (that's the top of the ionosphere that's farthest from you and not shadowed by the earth). The angle of incidence equals the angle of reflection on the way down, to an ideal receive location where that point that the sender aimed at is where the receiver is aiming at. You can have multiple hops; the ground reflects signal back up towards the sky. (Tangential paper: https://www.physics.princeton.edu//pulsar/K1JT/HFTOA_1.pdf https://www.physics.princeton.edu//pulsar/K1JT/HFTOA_1.pdf You can measure how many hops a signal is taking at any given time, and it changes throughout the day.) So if we were in a "spherical cow" universe, you could see how this would sort of act like a tripwire. A radio transmitter illuminates a single point in the ionosphere, and a radio receiver looks at that point. If a plane flies along that path, the signal goes dark, and you know something crossed your tripwire. The sticky bit is that we aren't operating on points, we're operating on a continuum. If you aim your radio beam at the horizon, maybe 10% of the power is 10 degrees above that, and 5% is 20 degrees above that, and so on. The signal is "smeared" in the real world to the extent that it's not a plane-sized beam that is on or off, rather it's a big circle that gets brighter or dimmer as planes fly through it. And the circle is much, much bigger than any plane. I would be surprised if you'd even notice. (That is assuming an ideal one-hop HF setup. I have never heard of people routinely operating WSPR with an ideal setup, usually people use random wires that have very little directionality to them.) An analogy that you can try right now is getting a camera with a lens that has a large front element (I just tried this with a 90mm f/2.8 lens). Take a picture at the widest aperture. Then stick your big ugly finger in front of the lens, right on the front element, and take another picture. If you look at the second picture, you won't know that your finger was there. There is no obvious finger, or even a finger-shaped shadow. If you compare it to the other picture, maybe you will notice a difference, but maybe you won't. You have to cover a lot of the lens before you notice. (Added fun: consider the 2 minute exposure time of WSPR. Take a picture with a 2 minute exposure time, and walk through the frame midway. Can you see yourself in that picture?) I have to imagine that using WSPR transmitters is a lot like this. If you take lots of samples, maybe a single tiny airplane will make a difference. The data is technically there. But I don't think the random transmitters and receivers are consistent enough across cycles to be able to get this data from a single sample. (Remember that WSPR is 2 minutes transmit and 2 minutes receive. Does your radio heat up and cool down, shifting the frequency of the local oscillator in that interval? Mine certainly does!) As I was reading the article, I thought "neat, but I'm not fully convinced" until I read the part where the author thinks he can do this arbitrarily and detect any airplane at any time, and that it is conveniently done with proprietary software that doesn't show up anywhere on the Internet. That caused the alarm bells to go off. I'm going to need a lot more data to be convinced.

dboreham 5y ago

I agree it seems far-fetched, but: possibly by integrating many data samples you can remove the ionosphere noise from an underlying signal.

lambdasquirrel 5y ago
If the ionosphere noise were merely noise, and not just distortion, this would be true. I'd suspect that even changes in solar radiation (from the rotation of the earth) would cause e.g. more ionization in some parts of the ionosphere compared to others.

jrockway 5y ago

I'm also an extra-class radio amateur, and I too am unconvinced by this article. The "tripwire" concept looks good in 2D, but I'm less convinced in 3D. Most HF signals propagate by bouncing the signal off the ionosphere (it's actually refraction, not reflection, but...), so the ideal path to the observer looks like shooting a signal straight at the horizon (that's the top of the ionosphere that's farthest from you and not shadowed by the earth). The angle of incidence equals the angle of reflection on the way down, to an ideal receive location where that point that the sender aimed at is where the receiver is aiming at. You can have multiple hops; the ground reflects signal back up towards the sky. (Tangential paper: https://www.physics.princeton.edu//pulsar/K1JT/HFTOA_1.pdf https://www.physics.princeton.edu//pulsar/K1JT/HFTOA_1.pdf You can measure how many hops a signal is taking at any given time, and it changes throughout the day.) So if we were in a "spherical cow" universe, you could see how this would sort of act like a tripwire. A radio transmitter illuminates a single point in the ionosphere, and a radio receiver looks at that point. If a plane flies along that path, the signal goes dark, and you know something crossed your tripwire. The sticky bit is that we aren't operating on points, we're operating on a continuum. If you aim your radio beam at the horizon, maybe 10% of the power is 10 degrees above that, and 5% is 20 degrees above that, and so on. The signal is "smeared" in the real world to the extent that it's not a plane-sized beam that is on or off, rather it's a big circle that gets brighter or dimmer as planes fly through it. And the circle is much, much bigger than any plane. I would be surprised if you'd even notice. (That is assuming an ideal one-hop HF setup. I have never heard of people routinely operating WSPR with an ideal setup, usually people use random wires that have very little directionality to them.) An analogy that you can try right now is getting a camera with a lens that has a large front element (I just tried this with a 90mm f/2.8 lens). Take a picture at the widest aperture. Then stick your big ugly finger in front of the lens, right on the front element, and take another picture. If you look at the second picture, you won't know that your finger was there. There is no obvious finger, or even a finger-shaped shadow. If you compare it to the other picture, maybe you will notice a difference, but maybe you won't. You have to cover a lot of the lens before you notice. (Added fun: consider the 2 minute exposure time of WSPR. Take a picture with a 2 minute exposure time, and walk through the frame midway. Can you see yourself in that picture?) I have to imagine that using WSPR transmitters is a lot like this. If you take lots of samples, maybe a single tiny airplane will make a difference. The data is technically there. But I don't think the random transmitters and receivers are consistent enough across cycles to be able to get this data from a single sample. (Remember that WSPR is 2 minutes transmit and 2 minutes receive. Does your radio heat up and cool down, shifting the frequency of the local oscillator in that interval? Mine certainly does!) As I was reading the article, I thought "neat, but I'm not fully convinced" until I read the part where the author thinks he can do this arbitrarily and detect any airplane at any time, and that it is conveniently done with proprietary software that doesn't show up anywhere on the Internet. That caused the alarm bells to go off. I'm going to need a lot more data to be convinced.