TimeMon uses the signals provided by the LIGO timing board and monitoring software running on one of the DMT machines  to measure the delay in every second. We  measure delays modulo one second.

The LIGO timing boards are equipped with a 1PPS/RAMP output to allow the calibration of the DAQ timing from the inputs of filter boards to the DAQ time stamp. The 1PPS is a 1ms square pulse with a rising edge precisely aligned with the GPS second tick. The RAMP is a 1 ms (~16 points at 16384 Hz sampling rate) long, steep ramp, which starts exactly at the GPS tick (rising edge of the 1PPS). A snapshot of the two signals is shown on Figure 1.


Figure 1. An example of the RAMP and 1PPS (TRIG) signals recorded by the Diagnostic Test Tool.

The RAMP signal makes it possible to determine the exact time of the GPS second tick relative to the time stamp: the crossing of the base line and ramp relative to the full second time stamp is the offset. Both, mid-stations and the data collection units in the 1x5, 1x22 crates in the LVEA have timing cards and acquire RAMP and TRIG.