问为什么网络位置提供程序返回null？安卓

EN

public class MyGPS extends Service implements LocationListener
{
    protected LocationManager locationManager;
    private final Context mContext;

    public MyGPS (Context context) {
        this.mContext = context;

        getLocation();
    }

    public Location getLocation() {
        try {

            locationManager = (LocationManager) mContext.getSystemService(LOCATION_SERVICE);

            // Check network status
            if(locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER)) {
                Log.e("Location ", "Network enabled");

                locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER,1000,1, this);
                if (locationManager != null) 
                {
                    Log.e("Location ", "Location manager is not null");
                    Location networkLocation= locationManager.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
                    if (networkLocation != null) {
                        return networkLocation;
                    }
                    else
                    {
                        Log.e("Location ", "networkLocation object is null");
                    }
                }
            }
        }
        catch(Exception e) {
            Log.e("Location ", "Problum in getLocation()");
        }
        return null;

    }

    @Override
    public void onLocationChanged(Location arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onProviderDisabled(String arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onProviderEnabled(String arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onStatusChanged(String arg0, int arg1, Bundle arg2) {
        // TODO Auto-generated method stub

    }

    @Override
    public IBinder onBind(Intent arg0) {
        // TODO Auto-generated method stub
        return null;
    }

}