My teacher told me that todays smartphones wouldn't work if the theory of relativity hadn't been discovered, but he didn't explained why.
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Of course cellular phone systems could work without GPS.
In fact, cell phones DID work before they carried GPS chips!
Bottom Line (moved to the top): if there was never GPS or an understanding of General Relativity, the system would have been developed just fine without it.
edit: I'd like to add a few items:
Teaching is hard! Credit should be given to anyone who is willing to stand up every day and try to explain science concepts. It's necessary to adapt the information to the level of the group and even accommodate the range within the group. Give it a try sometime!
Teaching can also provoke thought and independent learning. If a teacher 'pushes the envelope' a bit and you catch them, especially by thinking and independent research, remember it wouldn't have happened if the teacher had't brought it up to begin with, and planted those seeds.
This PCworld article might be good for further reading. Some of it is speculative (e.g. 'your cell phone could...'), so take with a grain of salt.
Spend time in a city now with narrow roads and tall buildings (or inside those buildings) and you'll still have good cell performance even with NO GPS fix. Turn it off, take out the battery, then go somewhere else far away, into a building where you can't receive GPS, then put the battery back in an try to make a call, and it will still work. It may take a little longer to "find itself", but there are other ways to do this - like triangulation.
(note - useful GPS signals come mostly from above and you need at least 3 or 4, while cellular connections can happen from any direction or path, through walls or windows. Since in most cases the cellular signal is orders of magnitude stronger than a GPS signal, this also makes it easier.)
However, the triangulation requires careful timing from multiple cell towers or installations, and they also may use GPS to set their clocks. GPSDO GPS disciplined oscillator is a great way to have a low cost, automatically maintained clock. If we didn't have GPS, then the towers would implement something else, since it's only local timing that's important (to their interaction with the phones), any group of thee towers can just ping each other once in a while.
If GPS suddenly stopped without time to prepare, there could be substantial problems in some cellular systems, and some might stop working completely. However, if there was never GPS, the system would work fine using alternate methods, just like it used to!
Cell towers are connected to the larger network via cables, microwaves, and mostly these days by optical fibers. These networks also heavily rely on a broad distribution GPSDO clocks.
That's important also if you want to browse the internet on your "smart phone" instead of your "cell phone."
If GPS never existed, an alternate would have been found. If it suddenly stopped working, I'm going to hope that there are some backup methods there too. Maybe follow that up in Engineering Stackexchange.
uhoh
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According to general relativity, Time is not absolute . But in most of the electronic devices which we use, Time is taken as absolute (For easy calculations) in clock speeds.And when these devices interacts with satellites which are away from earth the difference in time would be a problem in accuracy. Hence the general relativity is used to correct the time difference and more accuracy is obtained. Please note that time passes slower near massive substances according to general relativity. Hope the doubt is clear.
Avi
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