a method and apparatus of initial synchronization , or acquisition , of time modulated ultra - wideband communications uses a transmitted - reference preamble . the method and apparatus require that the transmitter first send a time - reference , delay - hopped burst ; such a burst is easily detected and can be processed to provide a time mark accurate to within a few nanoseconds . following the transmission of the tr / dh burst , the transmitter waits a fixed period of time , the duration of which is known to the receiver , and then the transmitter sends a burst of pulse position modulation , time hopped or other time modulated uwb . after the reception of the first burst , the receiver can estimate the time of reception of the second burst to the accuracy of the time mark .