GPS signals go through the ionosphere on their way from the satelites. The ionosphere causes distortions in the signal. If you want to achieve the best navigational accuracy you need to account for these distortions.
These distortions are not constant. They change from time to time. There are many different ways to account for them. One of the most accurate solution is to keep a GPS receiver on a well known location. Since you know that this receiver haven’t moved you can use the signal measured to estimate the parameters of the ionosphere between that station and the satelite.
Normally these signals are used to correct GPS navigational solutions. You take the closest station to your moving receiver and assume that whatever way the ionosphere was distorting for that station will do the same for your receiver too. This is valuable so there are network of such GPS stations in a lot of places.
Here they use the data collected by these stations differently. Instead of correcting a navigational solution they visualise the measured state of the ionosphere as seen by a bunch of these stations.
A simple GPS receiver will have a generic mathematical model for the ionosphere and use that as a good guess. More advanced ones can measure the delay directly.
The ionosphere affects different frequencies differently, so the GPS satellites transmits additional signals at different frequencies. By measuring the phase of these signals (L1 and L2), the math can be done to get a better estimation of the delay caused by the ionosphere between each satellite and the receiver. Those are the dots we're seeing on this animation. (GPS also uses the L2 signal to transmit encrypted information that lets military receivers get a better fix than civilian receivers).
> One of the most accurate solution is to keep a GPS receiver on a well known location.
I wonder if a network of connected devices with a GPS-disciplined SDR receiver and a regular GPS one could work both as this project does plus as passive radar like the software that was recently taken down. The purpose would be to have much wider coverage along with redundancy and error correction.
Such networks exist and make their data public. I think the equivalent you’re looking for is like LightningMaps, where there is real time reporting of observations instead of having to process recorded data to look back in time?
I worked on something like this in university. GPS bistatic radar. Two SDR frontends with directional antennas pointed in different directions to do various remote sensing, ranging, and other things.
The GPS network is essentially kept up to date with a few ground stations. The ground station is a source of truth that is used to send correction updates to the constellation periodically which are sent to all receivers.
But what are the moving dots in the animation? Planes, satellites? (seems to move like neither).
GPS being a military technology, I presume those fixed gps stations are only located in US-friendly countries. You wouldn't get that adjustment if you are flying over Russia or China, or any ocean. How much of an error in absolute distance are we talking about here? A few cm or meters or a km?
> But what are the moving dots in the animation? Planes, satellites?
Neither. The stations are in Japan. Imagine a line going from each of those stations to the satellite. Where this line crosses the ionoshpere that spot is what is measured. That is what you have information about. Those spots are the dots.
So you basically see the arc of the Japanese islands projected up towards each satelite which is visible from these stations. When the satelite is low on the horizon this projection seems to move fast, and when it is near the zenit it seems to move slow. This is what you are seeing with the dots.
They have a GPS receiver in a fixed, known location. They measure the received signal and from the variations infer corrections for ionospheric effects. They are part of the GPS network.
I would guess the moving dots are fixed GPS receivers, or more precisely the intersections of lines between fixed GPS receivers and moving GPS satellites with a sphere around Earth representing the ionosphere. If you look at the shape of the moving clusters, some look like Japan.
GPS receivers work by figuring out how far away they are from a number (>3) of GPS satellites. The receiver knows where the GPS satellites are (since the satellites broadcast their orbit parameters) so if a receiver knows how far it is from several satellites it can work out where it is itself.
Now, as the GPS satellite signals travel through Earth's atmosphere, they can be slowed down by different atmospheric effects. A slower signal will cause the receiver to think it's farther away from a satellite than it really is, so the receiver might estimate that it's position has changed a little bit. However, if you know the receiver's position hasn't change (maybe it's fixed in place to a big rock), then you can attribute the receiver's measured "change in position" to a change in atmosphere characteristics.
In this paper, they seem to have lots of fixed GPS receivers all over the place. By looking at all of them together, they can make a sort of map of the atmosphere characters in a part of the sky that's affected by rocket launches. The authors see these big ripples emanating from a Falcon Heavy launch in the US and this tweet shows those same ripples emanating from a launch site in North Korea.
You can go one better and filter your position data and store the data from the "worst fit" satellite. Which will usually be on the horizon or behind a thunderstorm, but not always, maybe it'll be behind a ICBM...
For made up simplified example, assume you have a moving GPS in an airplane or something, and there's exactly four sats N S W E. All four sats relatively agree you're flying the plane normally in a straight ish line as usual. Suddenly, the data from the S sat gets wildly distorted, but the other three sats remain normal. I suppose the S sat could have malfunctioned but more likely something is in between your plane and the S sat. So your navigation chip tosses the data from the S sat and marks its SNR way down and generally ignores that sat... However, if you were to log that "bad" data from the sat to the south... then compare to someone flying a plane a hundred miles to your south, and their GPS reports data suddenly was trash to their north, then you know something flew between your two planes. Maybe an ICBM, maybe a thunderstorm, maybe a GPS jammer weapon, a lot of "it depends". Sometimes its the data thats tossed out thats the most interesting.
I've been fooling around with something a LOT cruder at home WRT tracking thunderstorms. Its not rocket surgery to know that severe rain attenuation can impair GPS signals. One of the standard NMEA output lines contains each sats SNR, so if I know from my "vast" database that in normal weather satellite #43 at az 45 degrees elevation 45 degrees reports a SNR of made up number -10 plus or minus 2 over the past few years, then if it reports -20 today that would imply either the sat just burned out (unlikely) or there's a rain cloud causing "about 10 dB attenuation" at az 45 degrees elevation 45 degrees relative to my house. The linked project in the article is enormously fancier of course than merely logging SNR fluctuations.
I'm amused at the idea of crowdsourcing a "large amount" of forest hiking data over time to evaluate the health of the tree canopy in forests. Where I live the leaves are all down now so GPS signals should be very strong for hikers in forests.
As the Air Force discovered decades ago, for various EE and trigonometry reasons, bistatic radar works best overhead its kind of the opposite of what you want for an early warning radar so you can see why bistatic radar never went much of anywhere compared to traditional radar for the usual Air Force mission purposes. Although there are interesting modern "IoT" distributed sensor applications, at least if you have unimpeded fast communications systems, etc.
Fun house mirrors are curved and distort your reflection.
Some fun house mirrors are flexible so they can get pushed or pulled which will make you look taller or shorter or fatter or skinnier than you know you are.
By observing the difference between how you appear compared with how you are;
you can learn something about how the flexible mirror is being curved.
How this works in the fun house is there is you the mirror and LIGHT.
Both you and the light are well known and easy to predict; light will travel straight(ish) and you will not suddenly become very very short, so the thing that is changing your appearance is the flexible mirror.
In the GPS rocket case,
the GPS satellites are the illuminating source corresponding to the light in the room sending out radio (electromagnetic radiation same as light just at longer wavelengths)
The ground station GPS receivers correspond to your eyes (they know what they *should* see).
The earth's ionosphere corresponds to the flexible fun house mirror.
taken together, the same way you could tell if something we can't see behind the mirror flexed it, the author of the post showed they can tell if, when and where
an unannounced rocket goes through the ionosphere.
I don't think I could explain it to a small child, given that I don't have a great understanding of it myself. But here's what I could scrape together based on a linked paper[1]:
You use GPS receivers to detect ionospheric disturbances. Ionosphere, coming from the word "ionized", means it consists of charged particles, positive or negative. (Missile) exhaust is mainly neutral molecules, creating a "hole". These ionospheric holes can be detected through the Faraday Effect[2]. By measuring the Faraday rotation of radio signals (like GPS), you can detect these holes. I think this is similar to how polarized light 3d cinema systems work, except it's the radio spectrum instead of light.
As you walk past a school you yell with your voice, your friend nearby knows what you sound like but he hears you differently because your voice also bounces off the school wall. He adjusts the sound based on what you should sound like, the leftover bit is the shape of the wall.
You can figure out if someone moved a brick (or launched a missile) because your voice changes when it reaches your friend and he needs to apply a new change to get your voice back.
We can see the signal changes the rocket causes to the ionosphere and know that it's happened.
These distortions are not constant. They change from time to time. There are many different ways to account for them. One of the most accurate solution is to keep a GPS receiver on a well known location. Since you know that this receiver haven’t moved you can use the signal measured to estimate the parameters of the ionosphere between that station and the satelite.
Normally these signals are used to correct GPS navigational solutions. You take the closest station to your moving receiver and assume that whatever way the ionosphere was distorting for that station will do the same for your receiver too. This is valuable so there are network of such GPS stations in a lot of places.
Here they use the data collected by these stations differently. Instead of correcting a navigational solution they visualise the measured state of the ionosphere as seen by a bunch of these stations.