The Pioneer 10 and 11 spacecraft are subject to an unexplained acceleration which has a constant value of (8.74±1.33)10-10 ms-2 and seems to be directed toward the Sun. In 2006, five french laboratories formed the Groupe Anomalie Pioneer (GAP) in order to analyse a set of the Doppler data received from the Pioneer spacecraft. To perform this analysis, a specific orbit determination software has been developped which lead to the independant confirmation of the existence of an anomaly. This software has also been developped to study some aspects of the anomaly : nature, direction... This Pioneer anomaly which may be seen as challenging General Relativity has motivated an interest in flying new probes to the distances where the anomaly was first discovered, that is beyond the Saturn orbit, and studying gravity with modern techniques. A mission named Odyssey has been proposed for the ESA Cosmic Vision 2015-2025. The mission has the objective to verify General Relativity up to 50 AU. The main tracking observable is the range measured between Earth Deep Space Network stations. The principle of the test is to compare this measured value to the prediction of General Relativity. The motion of the probe is calculated by a navigation software with the non geodesic acceleration provided by an accelerometer.