To discover the location of the device, LBS must use real-time positioning methods. Accuracy depends on the method used. Locations can be expressed in spatial terms or as text descriptions. A spatial location can
be expressed in the widely used latitude-longitude-altitude coordinate system. Latitude is expressed as 0-90 degrees north or south of the equator, and longitude as 0-180 degrees east or west of the prime meridian, which passes through Greenwich, England. Altitude is expressed in meters above sea level. A text description is usually expressed as a street address, including city, postal code, and so on.

Applications can call on any of several types of positioning methods. Using the mobile phone network: The current cell ID can be used to identify the Base Transceiver Station (BTS) that the device is communicating with and the location of that BTS. Clearly, the accuracy of this method depends on the size of the cell, and can be quite inaccurate. A GSM cell may be anywhere from 2 to 20 kilometers in diameter. Other techniques used along with cell ID can achieve accuracy within 150 meters.

Using satellites: The Global Positioning System (GPS), controlled by the US Department of defense, uses a constellation of 24 satellites orbiting the earth. GPS determines the device's position by calculating differences in the times signals from different satellites take to reach the receiver. GPS signals are encoded, so the mobile device must be equipped with a GPS receiver. GPS is potentially the most accurate method (between 4 and 40 meters if the GPS receiver has a clear view of the sky), but it has some drawbacks: The extra hardware can be costly, consumes battery while in use, and requires some warm-up after a cold start to get an initial fix on visible satellites. It also suffers from "canyon effects" in cities, where satellite visibility is intermittent. Using short-range positioning beacons: In relatively small areas, such as a single building, a local area network can provide locations along with other services. For example, appropriately equipped devices can use Bluetooth for short-range positioning.

In addition, location methods can connect to a mobile position center that provides an interface to query for the position of the mobile subscriber. The API to the mobile position center is XML-based. While applications can be fully self-contained on the device, it's clear that a wider array of services is possible when a server-side application is part of the overall service.

Some applications don't need high accuracy, but others will be useless if the location isn't accurate enough. It's okay for the location of a tourist walking around town to be off by 30 meters, but other applications and services may demand higher accuracy.

The Location API for J2ME specification defines an optional package, javax.microedition.location, that enables developers to write wireless location-based applications and services for resource-limited devices like mobile phones, and can be implemented with any common location method. The compact and generic J2ME location APIs provide mobile applications with information about the device's present physical location and orientation (compass direction), and support the creation and use of databases of known landmarks, stored in the device.

JSR 179 (location api)requires the Connected Device Configuration (CDC) or version 1.1 of the Connected Limited Device Configuration (CLDC). CLDC 1.0 isn't adequate because it doesn't support floating-point numbers, which the API uses to represent coordinates and other measurements. The Location API doesn't depend on any particular profile -- it can be used with MIDP or the Personal Profile.

The hardware platform determines which location methods are supported. If it doesn't support at least one location provider, LBS won't be possible. Applications can request providers with particular characteristics, such as a minimum degree of accuracy. Some location methods may be free; others may entail service fees. The application should warn the user before any charges are incurred.

It is up to the application to determine the criteria for selecting the location method. Criteria fields include: accuracy, response time, need for altitude, and speed. Once the application obtains a LocationProvider instance that meets the criteria, it can use that object to obtain the location, in either of two ways: Invoke a method synchronously to get a single location.
Register a listener and get periodic updates at application-defined intervals.
The Location class abstracts the location results. Its object contains coordinates, speed if available, textual address if available, and a time stamp that indicates when the location measurements were made.

Coordinates are represented by either of two classes:

A Coordinates object represents a point's latitude and longitude in degrees, and altitude in meters.
A QualifiedCoordinates object contains latitude, longitude, and altitude, and also an indication of their accuracy, represented as the radius of an area.


The following segment of code demonstrates how to obtain the present location of the device synchronously:

// Set criteria for selecting a location provider:
// accurate to 500 meters horizontally
Criteria cr= new Criteria();
cr.setHorizontalAccuracy(500);

// Get an instance of the provider
LocationProvider lp= LocationProvider.getInstance(cr);

// Request the location, setting a one-minute timeout
Location l = lp.getLocation(60);
Coordinates c = l.getQualifiedCoordinates();

if(c != null )
{
// Use coordinate information
double lat = c.getLatitude();
double lon = c.getLongitude();
}