Timing

Timing and synchronisation of measurements at the different stations are critical for essentially all scientific measurements. The timing-system must be decentralised since the GLIPs are not necessarily online all the time during a measurement. Several possibilities were considered, such as frequency references at each station, the Omega navigation system, the Global Positioning System (GPS) etc. Since GPS provided superior accuracy and reliability at an affordable cost, this system was selected for the GLIPs (Figure A.8).

Figure A.8: The GPS receiver currently used in ALIS. Left: Receiver board with connectors for RS-232 and power (12 V DC). Right: GPS antenna.

The first GPS receiver used in ALIS was contained on an ISA-board that was plugged into the station computer. Later GPS units were external, communicating over RS-232. Both versions had a 1 Pulse Per Second (PPS) TTL-level output for precision timing. Early on the GPS devices had their own software, but they are now controlled by the Internet standard network time protocol (ntp) [Mills, 1992]. The ntp daemon (ntpd) running at each station interfaces directly to the output of the GPS receiver, as well as to other time-servers on the Internet, synchronising the PC clock to better than 1 ms precision against UTC. A software Phase-Locked Loop (PLL) implemented in the ntpd software allows timing to be preserved with good accuracy during periods when the timing sources are lost. While presently not needed, it is possible to further enhance timing accuracy to better than $1 \mu s$ by using the 1 PPS signal from the GPS-receiver, giving almost atomic-clock precision to the stations. The GPS-receiver also provides information about the geographical position of the station.