Monday, April 12, 2010

Is Assisted GPS better than Standalone?

Personally, I think GPS is one of the great marvels of science. The ability to precisely pinpoint your location with an accuracy of a few feet is incredible and is exceptionally useful. While GPS technology has been around for a long while, it's becoming more pervasive in the recent years after it found it's way into cell phones. Most medium to high-end cell phones today have inbuilt GPS hardware. In the context of cell phone GPS, I am sure everyone would have heard the term "A-GPS" which stands for "Assisted GPS". There's been poor understanding about the difference between the GPS hardware present in conventional navigation systems and these A-GPS hardware present in recent cell phones. So far, I've been under the impression that A-GPS  is something that cannot function without data about the initial co-ordinates (orbit/ephemeris) from a cell tower, until I talked about this to one of my friends (an MIT graduate) who is an expert in this field. There's been conflicting information about this on the web, and it also appears that some of the Wikipedia articles on this topic have incorrect information.

So apparently conventional navigation systems have a stand-alone version of GPS which obtains a location fix from scratch on their own, including the initial orbit/ephemeris data. This initial setup is pretty computation-intensive and takes a long time to obtain, thereby consuming significant battery power. Once the ephemeris information is obtained, the precise location fix is pretty fast. Stand-alone GPS systems can sometimes take even tens of minutes to obtain a fix because of this initial computation. They can also sometimes never get a fix when they are not in clear view of the sky.

A-GPS however, has an add on to the standalone GPS chip which is capable of obtaining precomputed orbit/ephemeris data from the cell tower instead of computing it on it's own from scratch, and hence is many times faster and energy efficient compared to standalone GPS. A-GPS chips' functionality is however a strict super-set of the functionality of a standalone GPS, as even if the cell tower fails to deliver the data, the A-GPS can fall back to normal standalone mode. According to my friend, there is no such GPS chip that cannot fall back to the standalone mode in the event of not being able to get the assistance of the cell tower, and if at all such a limitation exists, say, in a cell phone, it is artificially imposed by software. So according to my friend, if you have a cell phone whose GPS does not work without a cell signal, you should blame the phone software for it.

So yes, Assisted GPS is better than standalone, in all respects.

Note: Of course, obtaining approximate location using cell phones by cell tower "triangulation" is totally different from the above, and it can be done in pretty much any cell phone, as it does not require any GPS hardware.


  1. nice I got the clear idea about A-GPS.

  2. Thank you. I was looking for the last line especially.

  3. awesome and pretty much useful, keep up the good work !! :)