Time Bias of Laser Ranging Observations

The Time Transfer by Laser Link (T2L2) experiment on-board Jason-2 allowed the comparison of remote clocks located at the Satellite Laser Ranging (SLR) stations of the International Laser Ranging Service (ILRS) to a level of a few nanoseconds (ns). The ILRS has a standard requirement that all sites maintain their local time within ±100 ns of Coordinated Universal Time UTC(GPS). To accomplish this comparison, we computed a dynamical on-board time scale using the many ground-to-space time transfers that were observed by the Grasse SLR station, together with a frequency model of the reference oscillator (clock) of the Jason-2 satellite. Calibrated ground links at the Grasse station achieved an accuracy of 3 ns relative to the "GPS time" scale. From many calculations of the time transfer in non-common view from Grasse to the other SLR stations, we established a history of time biases per station over 8 years (2008-2016). We observe time biases of up to several microseconds for some stations. Following the recommendation of the ILRS-Analysis Working Group, we converted these series into a common file of time bias and phase drift values using the Solution INdependent EXchange (SINEX) format in order to be considered for inclusion in the Precise Orbit Determination (POD) and analyses for estimation of geophysical products using SLR observations.