I would definitely suggest using Google Cloud Messaging here. This has the benefit that once a device runs your app, you can make it register against GCM and you can send them some message that would make them register their coordinates into your database.
This approach would need that you have to implement some server (for instance, a web server with PHP) and make the users communicate with it, so the flow would be:
Your server sends a broadcast message to all registered devices telling them they have to register against the GCM service.
The devices get the message, and you implement the BroadcastReceiver to get both latitude and longitude and send it to your remote server, say http://www.mysuperserver.com/getlonglat.php, via a POST request.
Your server takes both parameters and you save them or do whatever you need.
If you want to follow this approach, this are the steps you have to follow:
Go to https://console.developers.google.com. With your Google account, register a new project. This will provide you an API key and a Sender ID.
You'll need to make the device register against your project in GCM. You can achieve this by importing Google Play Services within your project, and once done, do something alike to this:
GoogleCloudMessaging gcm = GoogleCloudMessaging.getInstance(context);
final String regid = gcm.register("YOUR_SENDER_ID");
At this time, the GCM server already knows that this user has registered whithin your project and has given him a regid.
The next step is informing your own remote server that the user has done so, and save somewhere the regid that it has been given, so you can later send them messages. So in the client side, make a HTTP POST request against your server sending that regid and process it with somethat like this:
$regid = $_POST['regid'];
if (($regId) && ($ipAddr))
sql_query("INSERT INTO gcm_subscribers(regid) VALUES($regid')");
Once done, you know who have already registered against your database. You can SELECT all the users and send them a signal to send you back their coordinates. In the following example, the parameters would be, firstly, an array of registration IDs and secondly the message to send (for instance, $messageData = "sendMeYourCoords!").
function sendNotification($registrationIdsArray, $messageData) {
$apiKey = "YOUR_API_KEY";
$headers = array("Content-Type:" . "application/json", "Authorization:" . "key=" . $apiKey);
$data = array(
'data' => $messageData,
'registration_ids' => $registrationIdsArray
);
$ch = curl_init();
curl_setopt( $ch, CURLOPT_HTTPHEADER, $headers );
curl_setopt( $ch, CURLOPT_URL, "https://android.googleapis.com/gcm/send" );
curl_setopt( $ch, CURLOPT_SSL_VERIFYHOST, 0 );
curl_setopt( $ch, CURLOPT_SSL_VERIFYPEER, 0 );
curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt( $ch, CURLOPT_POSTFIELDS, json_encode($data) );
$response = curl_exec($ch);
curl_close($ch);
return $response;
}
Once in your client, just process that GCM message and make another POST to another URL on your remote server and pass it the longitude and latitude. To process the messages I'm including a few links below.
And for getting the coordinates without GPS seems there aren't too much solutions, but there are some. This would be an example: How to get Latitute Longitude value without using GPS in Android?
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