wp question – WP_Query – order with usort by customized meta

I’ve a customized submit sort location which has completely different classes (e.g. museums, theatres, purchasing) and I need to output every of those classes and order inside these classes by distance which is a customized meta discipline. Proper now, I’ve following code, however can’t make the ordering work by distance.

<?php
        $custom_terms = get_terms('location_category');

    foreach($custom_terms as $custom_term) {
        wp_reset_query();
        $args = array('post_type' => 'location',
            'tax_query' => array(
                array(
                    'taxonomy' => 'location_category',
                    'discipline' => 'slug',
                    'phrases' => $custom_term->slug,
                ),
            ),
         );

         $loop = new WP_Query($args);
         if($loop->have_posts()) {;?>

In the meanwhile, I am utilizing usort to order alphabetically (see under), however this must be modified to ordering by distance.

    <?php usort($loop->posts, perform($a, $b) {
   return strcasecmp( 
                $a->post_title, 
                $b->post_title 
            );
});?>

Can anybody please assist me with this?