It is possible to jam GPS nevigation system. The jamming techniques is to limit the effectiveness of an opponent’s communications and/or detection equipment. There are several case, intentional or unintentional, has been recorded including Air Force conducted GPS jamming exercises in 2003. There is also a detailed description of how to build a short range GPS L1 C/A jammer was published in Phrack issue 60 by an anonymous author.
If stronger signals were generated intentionally, they could potentially interfere with aviation GPS receivers within line of sight. Also GPS signals can also be interfered with by natural geomagnetic storms, predominantly at high latitudes. There is a feature called Receiver Autonomous Integrity Monitoring(RAIM) is designed to provide a warning to the user if jamming or another problem is detected. Some aviation and marine receivers usually use this kind of featured in their GPS system.
GPS jamming is specially important since many modern weapon such as GPS-guided bomb use GPS. The U.S. government believes that such jammers were also used occasionally during the 2001 war in Afghanistan.
But there are many people believe that GPS jammer is not effective. Regardless it is effective or not, it is an important uspect of GPS user and everyone should be aware of it.