$ .ajax发布空数组

I'm trying to post browser's geoposition via jQuery Ajax

if (window.navigator.geolocation) {
    var failure, success;
    success = function(position) {
        var stringData = JSON.stringify(position, null);
        $.ajax({
            type: 'POST',
            url: 'file_2.php',
            data: { data: stringData }
        });
    };
    failure = function(message) {
        alert('Cannot retrieve location!');
    };
    navigator.geolocation.getCurrentPosition(success, failure, {
        maximumAge: Infinity,
        timeout: 5000
    });
}

But when its received in file_2 the array is empty and throw this alert "Notice: Undefined index: coords in..."

file_2.php

if(isset($_POST['data'])){               
    $dataString = $_POST['data'];      
    $Geoposition = json_decode($dataString, true);
    $lat = $Geoposition['coords']['latitude'];
    $lng = $Geoposition['coords']['longitude'];
}

Based on the log, stringData's value is {}. Its empty