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Geolocation is short for Geographic location and defines the location of your computer or cell phone. Ideally this is computed using a GPS. A GPS is generally inside cell phones these days and Web Tablets that include cellular capabilities. Otherwise they can be hooked up to a serial port. Note that Geolocation is an HTML5 feature.



[edit] How does GPS work

This is an abbreviated description of how a GPS works. Other geolocation systems around the world are similar. The GPS receiver depends on a sky full of GPS satellite transmitters that are up to 26 or more transmitting their location to the your receiver. Each satellite also has several atomic clocks that have the time. The GPS receiver uses this information to compute your location in Latitude and Longitude very much like a surveyor would do. It can also provide your elevation in feet above or below sea level (Altitude). The calculation is done by a technique called triangulation.

Each of the satellites send a signal that includes their position and the current time. The GPS receiver also knows the current time. The time it takes for the signal, moving at the speed of light, to reach the GPS receiver is calculated and converted to the distance to the satellite. The GPS then computes the size of a sphere would be based on the center of the satellite location and a radius of the distance to the satellite. The user is somewhere on the surface where it intersects the earth, which is also shown as a sphere with a known size. The two spheres form a circle where they intersect. Repeating this for a second satellite in another part of the sky forms a second circle. The intersection of those two circles is two points. The user is standing on one of those two points. A third satellite calculation can tell you which one because only one point will be available. In actual practice the point of intersection will be close but not exact.

Another satellite calculation will also be close but not exact forming a relative small triangle of locations. You are inside that triangle. Additional corrections to the GPS clock are made by the GPS until the triangle gets really small. The altitude is computed at the same time since it is a 3D fix but it needs to be corrected since the earth isn't a perfect sphere. This is done by a table inside the GPS itself that corrects the earth sphere to match the real earth altitude at that location. In addition you will know the exact time of day.

While this sounds difficult it is made even more difficult since there are a lot signals on the same frequencies the satellites are using so they must be singled out by the receiver. This is done by knowing the approximate locations of the satellites ahead of time. The GPS does that by searching the entire sky and downloading an almanac which takes several minutes. It then saves this almanac of locations locally for use in the future. If you use it often it can use this data to speed up the fix time but if you let it expire it will have to search the sky again.

[edit] Cell phones

Location systems include Assisted GPS and GLONASS, (Galileo and QZSS).

GPS (Global Positioning System) is US based but reception is available all over the world. It was initially designed for the Military, but has always been available for consumers since 1993 although the accuracy has improved. The Assistance portion is a stationary Satellite or a location on the ground that improves the accuracy of civilian GPS. Civilian GPS with assistance is used for aircraft and water navigation and improved landing accuracy.

GLONASS is owned by Russia and has been operational from 2011. They were worried that the US would pull its system in case of war.

Galileo is a global navigation satellite system that went live in 2016, created by the European Union through the European Space Agency. It is more accurate that the civilian GPS system with location within a meter.

QZSS is the Quasi-Zenith Satellite System, also known as Michibiki. It is a four-satellite regional time transfer system and a satellite-based augmentation system development by the Japanese government to enhance the United States-operated Global Positioning.

[edit] How is a cell phone so fast?

If you have seen someone with a regular GPS receiver you might ask: "How a cell-phone so much quicker at calculating than the stand alone GPS receiver?" Your friend, on a walk, will have to stop and get a fix before starting while your cell phone is always ready it seems. Well, a cell phone is in contact with a cell tower that is always active. The cell tower has its own specialized GPS that can collect some critical data and keep it up to date always. When your phone contacts the tower, it can supply this data to the phone. Thus the phone does not ever have to do a search the sky and download satellite data. Instead the ephemeris data can be downloaded from the cell tower directly to the unit itself. The current time can also be accurately downloaded. This drastically speeds up the fix calculation. This is called assisted GPS or A-GPS.

The downside of A-GPS is that these GPS receivers cannot collect the data for themselves so if you don't have cell phone reception the builtin GPS will not work. If you leave the city it is always a good idea to have a regular GPS as a backup.

[edit] Desktops

GPS is not in a desktop computer so often it will place you at the location of your Internet provider. It is possible to create a script that will cause some of the web browsers to report any particular location that you wish. This is very useful to provide your real location. The page Findlocation can be used to test the Geolocation information.

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