4 ms·

What are the practical uses for this data specifically? I mean some examples, as I have completely no idea.

by gala8y 4y ago

myself248 4y ago
ISTR the genesis of the project was a days-long Galileo outage, back when it was in "testing but probably usable" phase, and a stark lack of transparency or updates from the folks running the network. But it's not exactly hidden, you can infer quite a lot of things about the satellite status, if you just listen to what they're telling you. The trouble is, they're all over and moving fast, so you need a lot of friends with receivers to feed you the data, then you can put together a picture of the constellation health. Ergo, Galmon. Since then, it's turned out that a bit of sunlight on the operation of all four GNSS constellations, not just Galileo, has revealed quite a lot about their inner workings, availability, and health of individual satellites. One bird in particular was launched into a wrong-but-not-useless orbit by a rocket failure that juuuuust barely got it up there but didn't finish the job. Trouble is, most receiver software assumes the orbits are very nearly circular, and a highly elliptical orbit is both problematic if used for general navigation, and a useful source for testing more robust receiver software that can compensate for it. (Maybe the software wasn't correcting for other satellites' slight ellipsity as well as it could either.) So Galmon provides a view of that satellite's status as observed by receivers all over.
VLM 4y ago
On a staggeringly smaller scale I had VERY mixed results with storing data points that look like (satellite ID, timestamp, el, az, signal strength) and then doing a data analysis to map out "anomalous attenuation" by el,az (kind of a bistatic radar display). "Everyone knows" since day 1 of GPS back in the 80s that tree leaves and rain storms ruin GPS reception, but rather than complaining about it and/or avoiding it, why not use it as a data source? Turns out I had trouble pulling a useful signal out of the noise. Maybe with a lot more data or a lot more RX running in parallel or ... There are of course inherent limits as discovered by Air Forces over the decades where bistatic radar works best directly overhead whereas I was interested in "storm clouds on the horizon" exactly where it didn't work. I had some optimistic imagination in that if heavy rain causes 30 dB of attenuation, then extreme data analysis might indicate something as trivial as cloud cover, or even fog, might cause 0.01 dB of attenuation and I could map out regular non-rain clouds or something. That data could not be pulled out of the noise. Very optimistically, if I placed antennas on corners of my property, I could "edge detect" even if the signal level varied randomly-ish by comparing the multiple RX and doing some trig. I live close enough to an airport that occasionally a plane should be between me and a satellite. That did not work either. Anyway, the linked distributed project is my merely a thousand times larger than my playing around experiment.
- myself248 4y ago
  Galmon reveals fun stuff including the horizon around each receiver. I don't have the link handy, but one of the contributors was plotting "where in the sky a given receiver has ever seen a satellite", and it's plain to see which ones are next to mountains and stuff, because the outline of the mountains is vividly traced on the horizon. However, that's all long-term stuff. Moment to moment, GNSS receivers try very hard to mask momentary blips in data. I think you'd need to be down at the RF/baseband level to see the stuff you're trying to see, rather than taking "cooked" data from the receivers. The SDR GPS receiver in the KiwiSDR may be a good start.
mandevil 4y ago
So, not this data exactly but broadly similar data (Japanese publications of differential GPS corrections) have been used to spot North Korean missile launches in the open source world. https://www.armscontrolwonk.com/archive/1216884/detecting-missile-launches-with-ionospheric-disturbances/ https://www.armscontrolwonk.com/archive/1216884/detecting-mi... has more info on that. Essentially the corrections for each satellite can give you a map of what the ionosphere looks like, and so having more satellites is better. If there were publicly available dGPS- or dGLONASS/dBeiDou etc. archives available in other countries it would make this even more useful. If China or Russia published these it would be very useful indeed for triangulating the flight path through the ionosphere.

What are the practical uses for this data specifically? I mean some examples, as I have completely no idea.

by gala8y 4y ago

What are the practical uses for this data specifically? I mean some examples, as I have completely no idea.

ISTR the genesis of the project was a days-long Galileo outage, back when it was in "testing but probably usable" phase, and a stark lack of transparency or updates from the folks running the network. But it's not exactly hidden, you can infer quite a lot of things about the satellite status, if you just listen to what they're telling you. The trouble is, they're all over and moving fast, so you need a lot of friends with receivers to feed you the data, then you can put together a picture of the constellation health. Ergo, Galmon. Since then, it's turned out that a bit of sunlight on the operation of all four GNSS constellations, not just Galileo, has revealed quite a lot about their inner workings, availability, and health of individual satellites. One bird in particular was launched into a wrong-but-not-useless orbit by a rocket failure that juuuuust barely got it up there but didn't finish the job. Trouble is, most receiver software assumes the orbits are very nearly circular, and a highly elliptical orbit is both problematic if used for general navigation, and a useful source for testing more robust receiver software that can compensate for it. (Maybe the software wasn't correcting for other satellites' slight ellipsity as well as it could either.) So Galmon provides a view of that satellite's status as observed by receivers all over.

VLM 4y ago

On a staggeringly smaller scale I had VERY mixed results with storing data points that look like (satellite ID, timestamp, el, az, signal strength) and then doing a data analysis to map out "anomalous attenuation" by el,az (kind of a bistatic radar display). "Everyone knows" since day 1 of GPS back in the 80s that tree leaves and rain storms ruin GPS reception, but rather than complaining about it and/or avoiding it, why not use it as a data source? Turns out I had trouble pulling a useful signal out of the noise. Maybe with a lot more data or a lot more RX running in parallel or ... There are of course inherent limits as discovered by Air Forces over the decades where bistatic radar works best directly overhead whereas I was interested in "storm clouds on the horizon" exactly where it didn't work. I had some optimistic imagination in that if heavy rain causes 30 dB of attenuation, then extreme data analysis might indicate something as trivial as cloud cover, or even fog, might cause 0.01 dB of attenuation and I could map out regular non-rain clouds or something. That data could not be pulled out of the noise. Very optimistically, if I placed antennas on corners of my property, I could "edge detect" even if the signal level varied randomly-ish by comparing the multiple RX and doing some trig. I live close enough to an airport that occasionally a plane should be between me and a satellite. That did not work either. Anyway, the linked distributed project is my merely a thousand times larger than my playing around experiment.

myself248 4y ago
Galmon reveals fun stuff including the horizon around each receiver. I don't have the link handy, but one of the contributors was plotting "where in the sky a given receiver has ever seen a satellite", and it's plain to see which ones are next to mountains and stuff, because the outline of the mountains is vividly traced on the horizon. However, that's all long-term stuff. Moment to moment, GNSS receivers try very hard to mask momentary blips in data. I think you'd need to be down at the RF/baseband level to see the stuff you're trying to see, rather than taking "cooked" data from the receivers. The SDR GPS receiver in the KiwiSDR may be a good start.

mandevil 4y ago

So, not this data exactly but broadly similar data (Japanese publications of differential GPS corrections) have been used to spot North Korean missile launches in the open source world. https://www.armscontrolwonk.com/archive/1216884/detecting-missile-launches-with-ionospheric-disturbances/ https://www.armscontrolwonk.com/archive/1216884/detecting-mi... has more info on that. Essentially the corrections for each satellite can give you a map of what the ionosphere looks like, and so having more satellites is better. If there were publicly available dGPS- or dGLONASS/dBeiDou etc. archives available in other countries it would make this even more useful. If China or Russia published these it would be very useful indeed for triangulating the flight path through the ionosphere.