This function can make navbar become fixed-top navbar when user do scrolling and top of the navbar meets the top border of browser.

magellan for Bootstrap

       jQuery(function() {
            window.magellan = function() {
                obj = $('.magellan');
                pos = obj.offset().top;
                pos2 = obj.next().offset().top-obj.innerHeight();
                $(window).scroll(function() {
                    if (pos < $(this).scrollTop() && !obj.hasClass('navbar-fixed-top')) return obj.addClass('navbar-fixed-top');
                    if (pos2 > $(this).scrollTop()) return obj.removeClass('navbar-fixed-top');
                });
            }    
        })

// Run with call : window.magellan() and don't forget to add class "magellan" into navbar that you want to work with.