申論題

四、Although the maritime community relies heavily on Global Position System (GPS) or Global Navigation Satellite System (GNSS), operational vulnerabilities of the GPS or the GNSS resulting from jamming noise and spoofing signals may be encountered. For oceanic areas this means that astro navigation skills are essential for the majority of mariners and there are some obstacles to conduct successful astro navigation. Please describe these obstacles and explain why these obstacles occur.(請以英文作答）（20 分）