9.1. Location-Based Services

Mobile phones use several related methods, alone and in combination, to determine where they are:

Cell ID

Regardless of whether you're actually talking on the phone, as long as it's powered up, your mobile phone carries on a constant conversation with nearby cell towers. It has to do this in order to be able to respond when someone calls you, so every few seconds it "pings" the cell tower it was using last to tell it that it's still in range and to note network parameters such as the current time, the current signal strength (uplink and downlink), etc.

If you happen to be moving, your phone may do a handover to another cell tower, all in the background without a need for you to intervene. Each cell tower worldwide has a unique identifier, called (appropriately enough) its Cell ID, and each tower knows its latitude and longitude, so it's easy enough for a mobile phone to know "approximately" where you are located by taking note of the current Cell ID's geographic location. Cell sizes vary depending on the expected traffic in an area, but in the U.S., their radius ranges from a half mile (cities) to five miles or more (wide-open spaces).

Triangulation

Most of the time your mobile phone is in range of more than one cell tower. In 2G and later mobile technologies, the cell tower has the ability to tell what direction your signal is coming from. If there are two or three towers that can see your phone, together they can triangulate on your phone's location. With some operators, your phone then has the ability to query the network to find out where it's been located. This sounds a little backward, but it can be very accurate, and doesn't depend on any extra hardware on the mobile phone.

GPS

The satellite-based Global Positioning System (GPS) is ubiquitous these days, found in car navigation units, handheld navigators, and mobile phones. The good news is that, using GPS, your mobile phone can determine its location very accurately, including its altitude if that's important for some particular application. There are several downsides to GPS, but it is gaining popularity nonetheless. The downsides:

Increased cost

GPS radios and processors are fairly inexpensive, but still, an increase of even $10 in the bill-of-materials cost of a mobile phone is considerable.

Reduced battery life

There have been great strides in reducing the power required by GPS radios and processors, but they still suck battery power. Most phones that include GPS also have a feature that lets the user turn it on and off. If your application depends on GPS accuracy, it's good to remember that your application might have to check whether the GPS device is turned on and notify the user if it isn't.

Unreliable availability

Nothing "always works," but GPS in particular depends on your mobile device being able to see the satellites currently overhead. If you're in the basement of a high-rise building, surrounded by steel-reinforced concrete, you probably aren't going to be able to use GPS.

It's reasonable to expect that all Android phones will include one or all of these location-finding methods. The T-Mobile G1 in particular can use them all. So now we'll proceed to techniques for using the location capabilities.