Long-Term Behavior of the DORIS Oscillator Under Radiation: The Jason-2 Case

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.