Timeline for Why does GPS need the fourth satellite?
Current License: CC BY-SA 4.0
11 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Feb 16, 2022 at 9:45 | comment | added | TooTea | @YahyaAouledAmer There are no spheres with GPS. Anyone talking about spheres in the context of GPS is oversimplifying it in an attempt to explain to laypeople who have never heard of a hyperboloid but can imagine a sphere. | |
| Feb 16, 2022 at 8:08 | comment | added | Yahya Aouled Amer | to get an "accurate local time" I need a fourth satellite, so it means I don't need that satellite if I will use the difference between two satellites ? | |
| Feb 15, 2022 at 18:02 | comment | added | Eugene | @YahyaAouledAmer It's easier to grok in 2D 1st. If you have an accurate local time, then you can measure the "time of flight" and therefore the exact distance to a satellite A, the shape of where you could be with relation to A is a circle. If you don't have an accurate local time, but you can measure the difference between A and B times, the shape of where you could be with relation to A and B is a hyperbola. If you rotate those shapes to get a 3d object, you get a sphere and a hyperboloid respectively. | |
| Feb 15, 2022 at 10:08 | comment | added | Yahya Aouled Amer | I didn't exactly why we are talking here about hyperboloids, and in other documents about spheres intersection? Does it mean that there are two methods of computing the position, one by using spheres calculation and one by using hyperboloids from differences between satellites transmitting time ? if you have drawings or illustrations it will be fantastic | |
| Feb 15, 2022 at 8:09 | comment | added | Eugene | I've been thinking about this, and I think there's something missing. You don't have just the delta, you have accurate readings of A and B, which are synchronized with atomic clocks, then you can tell which "half" of the hyperbola you're on: if A<B, you are on the one that's closer to A and visa versa. Three of these half hyperboloids have only a single intersect. | |
| Feb 15, 2022 at 0:53 | comment | added | Eugene | Ah I see, wrote the second comment before seeing your reply, sorry. | |
| Feb 15, 2022 at 0:46 | comment | added | Mark | @Eugene, there are two points directly on the line from A to B: one close to A, and one close to B. | |
| Feb 15, 2022 at 0:44 | comment | added | Eugene | I mean: given the shape of a hyperboloid, where would the point directly on the line between A and B be? | |
| Feb 15, 2022 at 0:42 | comment | added | Mark | @Eugene, because that's the definition of a hyperbola/hyperboloid: the set of all points such that the difference between the distances to the two foci is constant. | |
| Feb 15, 2022 at 0:31 | comment | added | Eugene | Why is the shape of all the places that have some delta D of the distances to points A and B a hyperboloid? (I get the of revolution part) | |
| Feb 14, 2022 at 22:21 | history | answered | Mark | CC BY-SA 4.0 |