Modelization and Extrapolation of Time Deviation of USO and Atomic Clocks in GNSS-2 Context

In Global Navigation Satellite Systems (GNSS), the on-board time has to be modeled and predicted in order to broadcast the time parameters to final users. As a consequence, the time prediction performance of the on-board clocks has to be characterized. In order to estimate the time uncertainty of the on-board oscillator a linear or parabolic fit is performed over the sequence of observed time difference and extrapolated during the prediction period. In 1998 the Centre National d'Etudes Spatiales (CNES) proposed specifications of orbit determination and time synchronization for GNSS-2. The needs of synchronization were specified as the maximum error of the time difference prediction from the extrapolated fit. Using our work about the estimation of uncertainties in time error extrapolation, we have translated these time domain specifications into a noise level limit or an Allan deviation limit. Of course, these limits depend on the main type of noise for integration time of about 1 day and on the type of adjustment which is performed (linear for cesium clocks and quadratic for other oscillators). A table summarizing these limits is presented. These values are compared to experimental results obtained with different types of oscillators (quartz, rubidium, and cesium).