Radio timing service , such as GPS , BPC , JJY , WWVB , DCF77 , WWVH have been widely used as a basic time source for industry or individual systems . These signals are used by millions of people to synchronize consumer electronic products like wall clocks , clock radios , and wristwatches . NTP sever also use these signals to get a precise time . In this presentation we show how these signals can be forg . . .