J2ME and Location-Based Services

Location-based services (LBS) provide users of mobile devices personalized services tailored to their current location. They open a new market for developers, cellular network operators, and service providers to develop and deploy value-added services: advising users of current traffic conditions, supplying routing information, helping them find nearby restaurants, and many more.

This article introduces you to the field of LBS and to the Location API for J2ME (JSR 179), a set of generic APIs that can be used for developing location-based services. In addition, the article offers guidelines for designing location-based services.

What Location-Based Services Do

Location-based services answer three questions: Where am I? What's around me? How do I get there? They determine the location of the user by using one of several technologies for determining position, then use the location and other information to provide personalized applications and services. As an example, consider a wireless 911 emergency service that determines the caller's location automatically. Such a service would be extremely useful, especially to users who are far from home and don't know local landmarks. Traffic advisories, navigation help including maps and directions, and roadside assistance are natural location-based services. Other services can combine present location with information about personal preferences to help users find food, lodging, and entertainment to fit their tastes and pocketbooks.

There are two basic approaches to implementing location-based services:

1. Process location data in a server and deliver results to the device.
2. Obtain location data for a device-based application that uses it directly.

This article focuses on device-based location services.

Determining the Device's Location

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.

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