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Jan 6, 2009

How Quick GPS Works

quickgps

I recently started using iGo Navigator.

I've noticed that the times I use QuickGPS makes the Cold Start process work much faster, so I thought it would be nice to share what QuickGPS does.

I found a great article in HTCWiki:

Quick GPS does not "triangulate" using cellular networks, not even close. Let me start by telling you how a GPS gets a fix on its location, and then explain how Quick GPS offers a shortcut.


The GPS chip receives a synchronized timed signal from the satellites (birds) the US DoD launched into orbit. Each bird uses a different channel. With a good signal from three or more birds, GPS starts to get a fix. It does this by calculating the time difference between the reception of the different signals, then calculating the relative distances to each of the birds, then figuring out where it sits in the middle of the birds.

The GPS calculates four spheres, and its located somewhere on the surface of each of these spheres. There is only one point where these four spheres intersect. That's you.

If you're good at geometry, you might think that it needs 4 birds to get a fix, but in fact, if the GPS assumes that you are on earth, that provides another sphere, so it only requires 3 birds to get a decent fix. More adds altitude and accuracy.

But the GPS needs to know where each of the birds is before it can decide where IT is. That information is not calculated, but is available in a detailed download of orbital data provided by the DoD called ephemeris data. It is transmitted...from the bird to your GPS. For a cold start, your GPS needs to download the entire file, without interruptions in the signal - BEFORE it can even start getting a fix. In fact, the GPS may not even know which channels to tune into, since it doesn't yet know which birds are currently overhead. This causes a slow first fix.

Quick GPS, is simple. Ephemeris data is passed thru WiFi or cellular data connection ahead of time, and you never have to wait for it to download over the satellite. Awesome.

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6 comments:

Ivan Dimitrov said...

Great post!
However do you have any idea if similar software is available for Symbian based devices?

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