windows phone 8.1地图移动速度

Question

我正在努力获得运动的速度，但它不起作用，我不知道为什么。这是我试过的

async private void locator_PositionChanged(Geolocator sender, PositionChangedEventArgs e)
    {
        await dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
        {
            Geoposition geoPosition = e.Position;
            GeolocatorPositionChanged(geoPosition);

            var speed = e.Position.Coordinate.Speed; // en metres
            speed = speed * 3600 / 1000;
            speedTextBlock.Text = "speed : " + Convert.ToString(speed) ;
        });
    }

但是coordinate.speed总是返回5.8（米/秒）！