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One thing worth mentioning about the satellites is that since everything is based on very precise timing (a precision of 20-30 nanoseconds), for GPS to work at all there need to be two correction factors applied to the clocks. The first is due to special relativity: Since the satellites are moving so rapidly relative to an earthbound observer, the observer will see a GPS satellite's clock as ticking slower than his, by roughly 7 microseconds per day. The second is due to general relativity: Since the satellites are at a very high altitude, farther away from the Earth's gravity, spacetime is flatter there, and to an observer on the ground a GPS satellite's clock would tick faster than his by about 45 microseconds per day. So without corrections for this, the satellite clocks would run faster than clocks on the ground by 35 microseconds per day, which means about 10 kilometers in error, per day. The GR correction is built into the reference clocks on board the satellites, they're just designed at a slower rate here on the ground so that once you put them in orbit they tick at the correct frequency. The SR corrections are done at the receiver level, based on which satellites it's using for its calculation. HORSE-SLAUGHTERER posted:GPS devices are all manufactured with a hard limit on the altitude that they will work at so you can't use them for nukes Not all. The limit's based on speed (1000 knots) and altitude (18000 meters), not just altitude. Some manufacturers use OR and some use AND, and the former ones can cause problems for balloonists, and others don't honor either limit and are illegal to export from the US. Phanatic fucked around with this message at 21:38 on Feb 24, 2021 |
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Feb 24, 2021 21:20
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| # ¿ Apr 24, 2024 23:09 |
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Oh, let's talk about selective availability and differential GPS. Remember that the purpose of GPS, despite its enormous utility to civilian applications, was primarily military. Now you don't want to give this enormously useful tool for your military to the guys you're fighting with, so the idea is that there would be two types of signal sent: A coarse/acquisition code (C/A), which literally anyone could receive and use, but would be only accurate to within about 100 meters with existing receivers, and a precise code (P) useful for military purposes. Of course the P code is publicly known, so before the P code is broadcast it is XORed with an encrypted code called the W code which produces an encrypted bitstream called the Y code, so existing GPS satellites broadcast C/A and P(Y) signals. But turns out people started building better and better GPS receivers that improved accuracy by a factor of three. So it was decided to start deliberately tinkering with the C/A signal by putting an offset on the clocks; this was called Selective Availability, and the aim was to deny precision any greater than that of the original C/A intent to anyone but the military who had the encryption codes necessary to use the P(Y) signal. But GPS had the potential to completely obsolete a bunch of ground-based (and expensive to maintain) navigation aids, and this led to to people like airlines and coast guards and transportation specialists to figure out a workaround called differential GPS. The idea here is that you don't know where you are, so you ask the satellites, but the satellites are giving you an answer that's been deliberately hosed with to limit how precisely you can know where you are. But let's say we use some other method, like a really precise ground-based survey with dudes in safety vests and theodolites, to learn the exact location of some position on the ground. Then we put a really good GPS receiver on a permanent rigid mount on that location, and it asks the satellites where it is. And they tell it some bullshit answer like "You're at [where you actually are]+50 meters to the west." But you already know where [where you actually are] is since you've done your highly accurate survey, so when you see GPS tell you you're actually 50 meters to the west, you can just broadcast out at ground level "Hey dudes, wherever GPS tells you you are, add 50 meters to the east to the answer and that's where you are." So using these dGPS ground stations, you can provide a correction factor that can eliminate almost all the error for anyone in radio range who can see the same satellites as the dGPS station. Then the Gulf War came around in 1991, and there weren't enough military GPS receivers to go around, and a lot of troops wound up using civilian receivers that they'd purchased with their own dough (You go to war with the army you have, etc.). With that fact in mind, the military turned off selective availability for the duration of the war. And then a few years later DGPS was widespread enough that it was pretty much collectively realized that SA was useless and that dicking around with the C/A signal was a losing battle, and SA was turned off permanently in 2000. So in one sense, dGPS was somewhat overtaken by events, but since it was improving as well, it can still provide accuracy far higher than that possible by using even the unadulterated C/A signal. With a dGPS station nearby, you can determine location to within centimeters.
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Feb 24, 2021 22:42
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Bloody posted:Also why warm starts are faster than cold starts - ephemeris data and previous location can give it a better idea of where to start searching Once we disassembled a Chinook, loaded it onto a C-17, put it back together in India and started it up. I’ve never seen a GPS system take longer to lock on and figure out where it is.
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Mar 10, 2021 20:20
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