The IRIG and 1PPS signals are generated in the timing center by a Trak Systems clock driven by the maser. The IRIG time code is synchronized to the 1PPS by the Trak clock. The clock is synchronized to GPS when the system is first powered up, and then it is left to free-run from the maser signal. These signals are distributed to the user instruments via fiber optics or coaxial cable, depending on the location of the user instruments.
The 1PPS signal generated is buffered and transmitted to various locations around the site. Because the 1PPS signal is used by the system backends and the tracking LO system for high precision timing, it is necessary to know the delays introduced into the signal by the delays in the transmission equipment and the transmission medium itself. This is accomplished by using a return cable to loop the signal from each location back to a multiplexer at the site timing center. The originating signal is also supplied to the multiplexer. Using this multiplexer, each signal can be applied to the gates of a counter, and the delay between the pulses may be measured and logged by the M&C system.
Each VME chassis in the system is fitted with a BC635 (See Sect. 2.4) card that processes the IRIG signal and extracts the timing information. The card uses interrupts to signal the computers in the VME chassis each 20 ms. The system controller in the chassis latches the current time into the memory. This time is then read out by the other processors on the bus. This timing has 100ns resolution. Accuracy is better than 1 microsecond.
The software driving this card is slated for upgrades; with the current software, if the IRIG signal goes berserk (which happens if the signal from the timing center jumps, for whatever reason), the system time happily follows along. A more graceful degradation of synchronization is desired. This is important for many reasons, not the least of which is that the conversion to Azimuth-Elevation servo coordinates from commonly used astronomical coordinate systems depends on knowing the correct time.
The current plan is to integrate NTP into the software driven by this card. This will allow a certain amount of filtering and validation to be performed on the time returned from the card, and will prevent the system time from taking a large step.