gps
I understand the basic concepts of GPS positioning system and the use of trilateration but I don't understand why it takes so much time to get a fix.
On commercial GPS like TomTom it usually takes about one minute and on my phone it can take more than 5 minutes to be located.
Is it because of the computation time or it needs to collect more data before having a good approximation?
Thank you
Just a graphic to add to M'vy's answer.
From Geocommons:
This is
a high-tech version of triangulation,called trilateration. The first satellite locates you somewhere on a sphere (top left of Figure). The second satellite narrows your location to a circle created by the intersection of the two satellite spheres (top right). The third satellite reduces the choice to two possible points (bottom left). Finally, the forth satellite helps calculate a timing and location correction and selects one of the remaining two points as your position (bottom right).
Update
As R.K. points out, this is not a form of triangulation. Even when GPS is leveraging more than 4 satellites, it is still doing trilateration, as opposed to multilateration, which GPS does not use.
Multilateration should not be confused with trilateration, which uses distances or absolute measurements of time-of-flight from three or more sites, or with triangulation, which uses the measurement of absolute angles. Both of these systems are also commonly used with radio navigation systems; trilateration is the basis of GPS.
Very interesting problem. There are many questions here so I'll give my shot at one. There is such a thing as real-time differential GPS. When we did NASA flight tests, we sent an engineer up to our landing site to setup up the GPS station ahead of the airplane. The station was placed at a known accurate location and broadcast corrections to a GPS receiver on the airplane correcting our position data during the flight. As the airplane approached the airfield (where the station was located), we could begin obtaining DGPS on the first approach. So, we could get good enough accuracy to auto-land an airplane.
If you really need meter/sub-meter accuracy, then DGPS is going to be (at least part) of your solution.
Best Answer
The time to first fix (TTFF) is dependent on many variables such as the state of the almanac and ephemeris data on the device, the device's dilution of precision (DOP) thresholds, the current positions of satellites in the sky and any obstructions, and whether the device is using normal GPS or assisted GPS (aGPS) (which theoretically should reduce the TTFF but perhaps there are circumstances where aGPS isn't enough to make up for the quality of the device's GPS functionality).