var d = document;
onload = function() {
    var viewport_height = (self.innerHeight) ? self.innerHeight : (document.documentElement && document.documentElement.clientHeight) ? document.documentElement.clientHeight : (document.body) ? document.body.clientHeight : 0;
    var page_height = d.getElementsByTagName('body')[0].offsetHeight;
    var ttl = d.getElementById('to_top_link');
    if (page_height < viewport_height) ttl.style.visibility = 'hidden';
};