申論 4Although the maritime community relies heavily on Global Position System (GPS) orGlobal Navigation Satellite System (GNSS), operational vulnerabilities of the GPS or theGNSS resulting from jamming noise and spoofing signals may be encountered. Foroceanic areas this means that astro navigation skills are essential for the majority ofmariners and there are some obstacles to conduct successful astro navigation. Please describethese obstacles and explain why these obstacles occur.（請以英文作答）（20 分）