Tuesday, September 23, 2008

You have A-GPS on your Cellphone - so what?

a-gps systems and gps systemsQuite a number of cellphones nowadays are built with A-GPS in the handset. But what exactly is this A-GPS? And how does it differ from traditional GPS devices? For starters, A-GPS stands for Assisted Global Positioning System while GPS stands for Global Positioning System (which is not assisted). So what assists the A-GPS system? If you have not understood this technology, this article is for you.

Traditional GPS system

Traditional GPS systems typically find orbit and clock data from relevant satellites (typically 3 or more) and triangulate your position. This will enable the GPS device to locate your position. One important factor to note is the TTFF or otherwise known as Time To First Fix. This is the time it takes for the GPS device to locate your position during a cold start and it can range from anywhere between 30 seconds to a couple of minutes. It really depends on the GPS chip which is on board the GPS device and also depends on the environment - the performance will not be good if you are around tall buildings or indoors. This is the typical characteristic of a traditional GPS system.

Assisted GPS system

Assisted GPS systems or A-GPS systems work under a different model. Typically the A-GPS system leverages something called as an Assistant Server which provides information on cell ID and other information which helps the device to identify which satellite it needs to lock to. The assistance server is also known as a Mobile Location Server. The server uses your closest cell phone tower and uses this to calculate your position. Since the server has better processing power than your cellphone or your GPS device, the calculation is typically much faster. Hence we are able to reap benefits such as:

- Faster TTFF
- Do not need a high end processor on the device
- Better battery life for your A-GPS device (your cellphone)
- You could even get location acquisition indoors

Traditional GPS systems were designed basically for outdoor use (remember, GPS systems were primarily designed for military purposes). But as we start to use GPS in our cellphones for locating points of interest such as the nearest restaurant, hospital or bar the technology has evolved to include A-GPS which is now almost the de-facto standard in most GPS enabled cellphones. So the next time you use your A-GPS cellphone to locate that watering hole during your vacation, you will be able to appreciate it better!