ABSTRACT
This paper discusses the issues of radiation exposure that especially occur on the Doppler Orbitography and Radiopositioning Integrated on Satellite (DORIS) ultrastable oscillator (USO) that has been placed on the Jason-2 oceanography mission as the reference clock. We have studied the frequency response of the Jason-2 USO (launched in June 2008, at 66° of inclination and 1336 km of altitude) over 8.5 years. This is the only mission that provides different sources of data, internal and external, which permit a precise description of the oscillator behavior using several assumptions. While the ICARE-NG instrument provided continuous measurement of the electron and the proton fluxes, the Time Transfer by Laser Link (T2L2) experiment demonstrated its ability to monitor the "reading" of the oscillator at a level of a few parts in 10-13 with respect to a ground network of laser stations equipped with atomic clocks. From these inputs, and also environmental parameters such as the orbit, the attitude law of the platform, and the temperature of the oscillator, we have developed a complete physical model of the DORIS USO that fits the data at less than slightly below the threshold of 10-12 that limits the use of the DORIS data in current space geodesy analyses. In this way, the model also makes it possible to provide an assessment of the new Jason-3 USO to radiation effects after the satellite launch in January 2016.
ABSTRACT
The Time Transfer by Laser Link (T2L2) project allows for the synchronization of remote ultrastable clocks over intercontinental distances. The principle is derived from the satellite laser ranging technology with a dedicated space equipment designed to record arrival times of laser pulses at the satellite. The space segment was launched in 2008 as a passenger instrument on the ocean altimetry satellite Jason 2. For the first time, we have conducted by the end of 2016 a dedicated time transfer comparison campaign between Global Positioning System and T2L2 over intercontinental distances. The campaign was carried out between two laboratories in Europe and two in China. The campaign has demonstrated a consistency of the time transfer techniques at the 1-ns level, together with the confirmation of a subnanosecond level for continental distances.
ABSTRACT
The Time Transfer by Laser Link (T2L2) experiment has been developed in close collaboration between Centre National d'Etudes Spatiales and Observatoire de la Côte d'Azur. The aim is to synchronize remote ultra-stable clocks over large-scale distances using two laser ranging stations. This ground to space time transfer has been derived from laser telemetry technology with dedicated space equipment designed to record arrival time of laser pulses on board the satellite. For 3 years, specific campaigns have been organized to prove T2L2 performance. In April 2012, we performed a 2-week campaign with our two laser ranging stations, Métrologie Optique and French Transportable Laser Ranging Station, to demonstrate the T2L2 time transfer accuracy in co-location. We have compared three independent time transfer techniques: T2L2, GPS, and direct measurement, with both an event timer and an interval counter. The most important result obtained in this campaign was a mean agreement between T2L2 and a direct comparison better than 200 ps. This is the first major step to validate the uncertainty budget of the entire T2L2 experiment. This paper focuses on this campaign setup and the obtained results.
