Galleon Systems provides the NTS-6000-Dual reference NTP time server with both a MSF, WWVB and DCF radio antenn and a GPS antenna. The more accurate GPS antenna is used by the ntp time server by default, due to it being more accurate than radio. However, in the event of the GPS antenna failing, the radio antenna can be used by the ntp time server as backup.
Both references are continually monitored, however the radio reference is only used a backup reference due to it being slightly less accurate than the GPS timing reference.
Many customers install 2 seperate NTP time servers, a GPS device (NTS-6000) and a radio device (NTS-4000) this has the added advantage of prividing a redundancy solution to overcome complete failure of one device.