private void validate_preferences() {
        // verify that X-Plane directory exists
        if (new File(this.preferences.getProperty(PREF_APTNAV_DIR)).exists() == false) {
            logger.warning("AptNav Resources directory not found. Will not read navigation data!");
            XHSIStatus.nav_db_status = XHSIStatus.STATUS_NAV_DB_NOT_FOUND;
        } else if (
                ! ( new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/Resources/default data/earth_nav.dat").exists() ||
                    new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/earth_nav.dat").exists() ) ||
                ! ( new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/Resources/default data/earth_fix.dat").exists() ||
                    new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/earth_fix.dat").exists() ) ||
                ! ( new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/Resources/default data/earth_awy.dat").exists() ||
                    new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/earth_awy.dat").exists() ) ||
                ! ( new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/Resources/default scenery/default apt dat/Earth nav data/apt.dat").exists() ||
                    new File(this.preferences.getProperty(PREF_APTNAV_DIR) + "/apt.dat").exists() )
                )
        {
            logger.warning("One or more of the navigation databases (NAV, APT, FIX, AWY) could not be found!");
            XHSIStatus.nav_db_status = XHSIStatus.STATUS_NAV_DB_NOT_FOUND;
        } else {
            logger.fine("Navigation databases found");
            XHSIStatus.nav_db_status = XHSIStatus.STATUS_NAV_DB_NOT_LOADED;
        }
    }