  During the last decade, the precision in the tracking of spacecraft has
constantly improved. The discovery of few astrometric anomalies, such as the
Pioneer and Earth flyby anomalies, stimulated further analysis of the operative
modeling currently adopted in Deep Space Navigation (DSN). Our study shows that
some traditional approximations lead to neglect tiny terms that could have
consequences in the orbit determination of a probe in specific configurations
such as during an Earth flyby. Therefore, we suggest here a way to improve the
light time calculation used for probe tracking.
