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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. |