Update data sent by php form using ajax

i cant really figure out this problem , lets say that on the page there is a form with one input in it , if you submit the form then the text that you save will be sent to the database and stored now , i would like to get user location:

Longtitude Latitude

whenever he submits the form , now i found that its better to get the location using Javascript and then send it to the server using AJAX so basicaly when i submit the form the text of input is stored in the database and , i marked the submit button with id "submit" and i want to use ajax with jquery so:

$.ajax({ 
    url:'update_the_last_form_submit_data.php', 
    type:'POST', 
    data:{lat:lat,lat:lat}
    success:, 
    error: 
});

so what i want to do is , when ever the user submits the form , ajax will add the current longtitude and latitude to the mysqlquery which is being sent to database

and fially the table will look like this :

Time------------User Input--------Latitude---------Longitude
xx-xx-xx--------What you tiped----Detected longtitude-------detected latitude