These units obtain accurate time from the atomic clocks carried aboard the GPS navigation system's satellite vehicles. GPS works by measuring the time differences between the signals received from multiple satellites (four satellites' data provides enough information to solve four simultaneous equations for three dimensions plus the current time) - so their internal clocks need to be very accurate as the differences in signals' timing are very small. Hence GPS derived time is very accurate, and easy and affordable to obtain for most of the populated world and is accepted as an authenticated standard for legal purposes. These devices receive and/or display the time but do not have the ability to act as an NTP (Network Time Protocol) server - those units are listed separately. Often these units provide time output in the form of IRIG-B, NASA, ASCII etc timecode for use by downstream devices. GPS devices work best when paired with a professional grade antenna which is mounted outdoors with a full view of the sky. An indoor or window mounted antenna usually will not provide results commensurate with the investment you are making in the receiver. Most receivers provide a 5V feed for the antenna, but note that some Symmetricom/TrueTime units send 12V so require a suitably matched antenna to avoid damaging the receiver or antenna.