[krisoft]

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.

[morcheeba]

Good explanation.

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).

more info: https://www.e-education.psu.edu/geog862/node/1715

[squarefoot]

> 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.

[toomuchtodo]

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?

https://geodesy.noaa.gov/CORS_Map/

https://www.e-education.psu.edu/geog862/node/1830

https://learn.sparkfun.com/tutorials/how-to-build-a-diy-gnss...

[colechristensen]

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.

[cm2187]

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?

[krisoft]

> 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.

Their location is calculated here: https://github.com/tylerni7/missile-tid/blob/main/tid/tec.py...

"Given a receiver and a satellite, where does the line between them intersect with the ionosphere?"

And then that is called here: https://github.com/tylerni7/missile-tid/blob/00c5fd25e2ab3c2...

"The locations where the signals associated with this connection penetrate the ionosphere."|

[layer8]

What are the stations/receivers? Is this crowdsourced data?

[williamscales]

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.

[0] https://en.wikipedia.org/wiki/GNSS_augmentation

[layer8]

The video is data from just a single receiver?

[krisoft]

No. These are the receivers from japan’s GEONET.[1]

“Geospatial Information Authority of Japan (GSI) operates GNSS CORSs that cover Japanese archipelago with over 1,300 stations at an average interval of about 20km for crustal deformation monitoring and GNSS surveys in Japan.” [2]

Basically the government of Japan pepered their country with GPS base stations and they let researchers use the data from them. This is just a novel use of that data.

1: https://mobile.twitter.com/tylerni7/status/15934664867144212...

2: https://www.gsi.go.jp/ENGLISH/geonet_english.html

[jwsteigerwalt]

This explanation helps understand the video.

[danbruc]

But what are the moving dots in the animation?

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.