Android : Android - Showing Alert Dialog leads to BadTokenException in Service class

I am reading GPS info and sending it to the network every 10 seconds.

What I am doing is using an handler which runs my service class GPSTracker

public void startGpsService() {
        handler = new Handler();
        task = new Runnable() {
            @Override
            public void run() {
                gps = new GPSTracker(getApplicationContext());

                if (gps.canGetLocation()) {
                    ... do something
                } else {
                    // GPS or Network is not enabled
                    gps.showSettingsAlert();
                }

                handler.postDelayed(task, 10000);
            }
        };
        task.run();
    }

if my GPS setting are not enabled gps.showSettingsAlert(); is executed

public class GPSTracker extends Service implements LocationListener {

    private final Context mContext;

    // Declaring a Location Manager
    protected LocationManager locationManager;

    public GPSTracker(Context context) {
        this.mContext = context;
        getLocation();
    }

   ...  

    /**
     * Function to show settings alert dialog
     * On pressing Settings button will lauch Settings Options
     * */
    public void showSettingsAlert(){
        AlertDialog.Builder alertDialog = new AlertDialog.Builder(mContext);

        // Setting Dialog Title
        alertDialog.setTitle("GPS is settings");

        // Setting Dialog Message
        alertDialog.setMessage("GPS is not enabled. Do you want to go to settings menu?");

        // On pressing Settings button
        alertDialog.setPositiveButton("Settings", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog,int which) {
                Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
                mContext.startActivity(intent);
            }
        });

        // on pressing cancel button
        alertDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
            }
        });

        // Showing Alert Message
        alertDialog.show();
    }
 ...

My problem is that alertDialog.show(); raises this exception:

08-21 09:49:23.906: E/AndroidRuntime(4088): java.lang.RuntimeException: Unable to start activity ComponentInfo{...}: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application

How to solve it? Similar solutions found on here and here dont fix my problem as in that case it happens inside activity classes.