Locating a Device Using Global Positioning System and Network/WiFi Technology

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The Android platform uses both GPS and network/WiFi to locate a device. There is no direct way to specify a unique location source, but a workaround is to only add the relevant permission, fine or coarse as defined earlier, in the manifest file.

Using GPS

GPS is a manmade navigation system. Satellites in space constantly broadcast messages with their ephemeris, or position in the sky, and the time. Your mobile device, as a GPS receiver, uses the messages received to determine the distance to each satellite by measuring the transit time of the message. Using triangulation, the device is able to determine its own position as latitude and longitude or on a map. Other information derived from this is direction and speed, calculated from changes in position. Visit the TomTom site for a simple explanation and good graphics on how GPS works (http://www.tomtom
.com/howdoesitwork/page.php?ID=8&CID=2&Language=1).

GPS is the most accurate positioning technology, and it provides the most frequent updates. However, it is not always available, particularly indoors. It also consumes the most battery power.

Position acquisition is often not very accurate initially, but it improves over time. The GPS sensor can take several minutes to get a proper position from satellites. Do not make your application full screen so that the status bar is hidden from the user so that they cannot see network activity and signal strength. If a GPS connection is established, a GPS icon is visible on the top right. If it is trying to establish or fix a lost GPS connection, the icon will blink.

Signal-to-noise ratio

SNR is an algorithm that compensates for inaccurate or ambiguous results due to interference and multipath errors. Interference could be due to weather conditions or signals bouncing off mountains or large buildings. A multipath error is a problem with signals from multiple satellites arriving out of sync if one of them experiences interference.

Assisted GPS

Assisted GPS, also called A-GPS, is used to improve the startup performance or Time To First Fix (TTFF). In this mode, your device uses cell tower positions and triangulates from there to get an initial location quickly. It then switches to GPS when it is available and accurate enough. Figure 10-2 shows GPS Test, an application developed by Chartcross Ltd. It displays the position and signal strength of satellites within view as obtained by the device’s GPS receiver.

Using the Cellular Network and WiFi

If GPS is not available, your application will switch over to cell/WiFi. The network system is a combination of cell towers, WiFi hotspots, and IP-based geolocation. Cell tower signals, although not affected by architecture or bad weather, are less accurate and are dependent on the infrastructure. WiFi hotspots are very precise with enough data points but are only available in urban areas.

GPS Test application developed by Chartcross Ltd.

How to Know if GPS or WiFi Is Active

The static flash.net.NetworkInfo class provides information about the network interfaces on computers and on some mobile devices. Your application needs permission to access network information. Add it to your application manifest as follows:

<uses-permission android:name=”android.permission.ACCESS_NETWORK_STATE” />
<uses-permission android:name=”android.permission.ACCESS_WIFI_STATE” />

Check that it is supported on your device:

if (NetworkInfo.isSupported) {
// network information supported
}

NetworkInfo stores a list of possible network interfaces that you can get by calling:

import flash.net.NetworkInfo;
var network:NetworkInfo = NetworkInfo.networkInfo;
for each (var object:NetworkInterface in network.findInterfaces()) {
trace(object.name);
}

For example, the list of interfaces for the Nexus One, Droid 2, and Samsung Galaxy Tab is:

mobile, WIFI, mobile_mms, mobile_supl, mobile_dun, mobile_hipri

You can find out which method is active, and its name, using the following code:

import flash.net.NetworkInterface;
var network:NetworkInfo = NetworkInfo.networkInfo;
for each (var object:NetworkInterface in network.findInterfaces()) {
trace(object.name);
if (object.active) {
if (object.name == “WIFI”) {
// you are using wifi
} else {
// you are using GPS
}
}
}