3 ms·

GPS is a pretty cool case. It's not the first navigation system out there, not even close. But the designers did something pretty forward-thinking and a bit ris

GPS is a pretty cool case. It's not the first navigation system out there, not even close. But the designers did something pretty forward-thinking and a bit risky: they made a big chart of all the different ways to implement GPS. A system where the ground based receivers has to talk back to the satellite. A system where every user needs their own atomic clock. A system where every user needs to communicate with a satellite. A system where every user needs to communicate with a separate ground station. Systems whose accuracy decreased at higher velocities. etc. The final solution was one that required more satellites, but allowed users to determine their own 4D position without needing any outside resources. The only downside was that it required portable atomic clocks, which didn't exist yet. Not sure if that's black swan territory or not, but IMO it was a great piece of forward-thinking that made GPS useful beyond just the original military applications.

jacquesm 3y ago
The most amazing thing to me about GPS is that it required compensation for relativistic effects to work. It is as far as I know the most direct impact relativity has on our every day life in a way that just sticking to Newton would have led to the project being abandoned or to the discovery of relativity if Einstein had not given it to us on a platter.
- defrost 3y ago
  The most interesting thing to me about relativistic corrections for GPS is that we didn't even have to know about or understand relativity in advance in order for GPS to work. That sounds strange .. but .. there are many small corrections that need to be applied to "straight forward" triangulated fixing off of moving monuments (term from surveying), relativitic time shift being just one. There are several recent HN threads about Kalman filters [e]. It's possible (and more or less roughly what already happens) to record GPS fixes against a fixed master station and compute the time series error twixt the naive computation and known location (or, indeed, mesh of locations across (say) Australia) and generate a Kalman filter to correct and return more precise positions for moving recievers in the mesh area. Had we not been aware of relativity we very likely would have discovered it via the time slip 'error' terms in the correction filter. In a similar manner we have improved our understanding for atmospheric wobble, continental drip [1], and other fine effects. [1] https://en.wikipedia.org/wiki/Continental_drip https://en.wikipedia.org/wiki/Continental_drip [e] https://hn.algolia.com/?dateRange=pastYear&page=0&prefix=false&query=Kalman&sort=byPopularity&type=story https://hn.algolia.com/?dateRange=pastYear&page=0&prefix=fal...

jacquesm 3y ago

The most amazing thing to me about GPS is that it required compensation for relativistic effects to work. It is as far as I know the most direct impact relativity has on our every day life in a way that just sticking to Newton would have led to the project being abandoned or to the discovery of relativity if Einstein had not given it to us on a platter.

defrost 3y ago
The most interesting thing to me about relativistic corrections for GPS is that we didn't even have to know about or understand relativity in advance in order for GPS to work. That sounds strange .. but .. there are many small corrections that need to be applied to "straight forward" triangulated fixing off of moving monuments (term from surveying), relativitic time shift being just one. There are several recent HN threads about Kalman filters [e]. It's possible (and more or less roughly what already happens) to record GPS fixes against a fixed master station and compute the time series error twixt the naive computation and known location (or, indeed, mesh of locations across (say) Australia) and generate a Kalman filter to correct and return more precise positions for moving recievers in the mesh area. Had we not been aware of relativity we very likely would have discovered it via the time slip 'error' terms in the correction filter. In a similar manner we have improved our understanding for atmospheric wobble, continental drip [1], and other fine effects. [1] https://en.wikipedia.org/wiki/Continental_drip https://en.wikipedia.org/wiki/Continental_drip [e] https://hn.algolia.com/?dateRange=pastYear&page=0&prefix=false&query=Kalman&sort=byPopularity&type=story https://hn.algolia.com/?dateRange=pastYear&page=0&prefix=fal...