Javascript navigator.geolocation.watchPosition以速度返回NaN

我在iOS上的phonegap/cordova中从GPS获取速度时遇到问题。不管我是开车还是站着不动，它都会回来

    function speed_start() {
        speed_timer_id = navigator.geolocation.watchPosition(speed_update_data,
                                                  speed_update_error,
                                                  { enableHighAccuracy: true });
}

var speed_update_data = function(position) {
        $("#speed-debug").html('Latitude: '          + position.coords.latitude          + '\n' +
              'Longitude: '         + position.coords.longitude         + '\n' +
              'Altitude: '          + position.coords.altitude          + '\n' +
              'Accuracy: '          + position.coords.accuracy          + '\n' +
              'Altitude Accuracy: ' + position.coords.altitudeAccuracy  + '\n' +
              'Heading: '           + position.coords.heading           + '\n' +
              'Speed: '             + position.coords.speed             + '\n' +
              'Timestamp: '         + position.timestamp                + '\n');
};

var speed_update_error = function(error) {
        $("#speed-debug").html(error.message);
}

这是一个硬件“问题”。似乎GPS没有足够的信号