I noticed that when I activate my cell phone internet connection, I get my GPS position much faster.
What does the internet connection provide for the GPS receiver ?
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asked Feb 16, 2022 at 9:14
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2$\begingroup$ Mobile phones can also use cell towers to determine location. $\endgroup$– Bojan KogojFeb 16, 2022 at 9:31
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$\begingroup$ You mean with no need for the GNSS satellites ? $\endgroup$– Yahya Aouled AmerFeb 16, 2022 at 9:36
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$\begingroup$ Correct, but last I checked precision is not the best. Could also check for exact time, as answered in space.stackexchange.com/a/58287/40071 $\endgroup$– Bojan KogojFeb 16, 2022 at 10:10
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1$\begingroup$ I don't know if it does, but it certainly could be to verify the current date and time and general location which could really speed up satellite location and identification. Options on your phone to use the internet data and to use cell tower data might each have its own on/off switch in your system preferences; just thinking out loud here. $\endgroup$– uhohFeb 16, 2022 at 10:50
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$\begingroup$ I vote to close this as duplicate in favor of the older one, as that one is more accurate, but close the other one is also fine. $\endgroup$– LudoFeb 16, 2022 at 12:28
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2 Answers
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My GPS sports watch has the same feature.
A GPS receiver needs an actual set of the satellite orbit data to calculate the satellite positions. This data is transmitted from the satellites with 50 bits per second needing 12.5 minutes for the full set of 37.5 kilobit. This almanac data is transmitted in 25 frames of 1500 bit each.
Using the internet, this data is actualized in a few seconds or even less than a second.
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(copy from What is "Time to First Fix" in GNSS, as it is the same question
A "fix" is a solution to navigation problem, i.e. determining where you are. The Time To First Fix is the time it takes to arrive at that solution from the time you turn on your device
In order to determine where you are with GNSS you need:
- acquire signals from at least 4 satellites (to solve for four unknowns: 3 position variables and time, see also this question)
- know where those satellites are (recall that you use the satellites as beacon with a one-way distance measurement to determine where you are).
- compute the distances from you to each satellite
Especially the acquisition process takes a lot of time, because:
- you need to know which satellites are in view
- you need to know how fast they are traveling relative to you (to correct for Doppler shift)
This information is embedded in the satellite almanac (ephemerides for all satellites) plus information about where you are (you can't see satellites below the horizon) and how fast you are going and in which direction.
Typically, three cases are considered in TTFF performance:
- cold start: you know nothing about the satellites or your own position and velocity
- warm start: you have a valid almanac, but only a rough idea of where you are (e.g. you turned of your device for an while)
- hot start: you have a valid almanac and a (fairly) accurate estimate of your position and velocity
If you have a valid almanac and a good idea of where you are, you can speed up the acquisition process because you don't need to look for all the satellites, only the ones that are in view (recall that you need to correlate PRN codes against the incoming signals, so it helps if you don't need to correlate them all).
The almanac has limited validity, because the GNSS satellites orbits are not perfect due to various disturbances.
GNSS system distribute almanacs in their navigation messages that are modulated on top of the PRN codes. However, these are transmitted at low rate (only 50bps for GPS for example) and worst case it takes 12.5 minutes (!) to get the entire almanac.
If you have other means of access to the almanac (e.g. via an augmentation service via internet) you can speed up the cold-start scenario considerably. Also, internet access (via cellular or WiFi) can help in determining your location, further improving the acquisition process.
