Light time calculations in high precision deep space navigation

5 pages, submitted to proceedings of "International Symposium on Orbit Propagation and Determination, Lille 2011"During the last decade, the precision in the tracking of spacecraft has constantly improved. With the recent discovery of few astrometric anomalies, such as the Pioneer and Earth flyby anomalies, it becomes important to deeply analyze the operative modeling currently adopted in Deep Space Navigation (DSN). Our study shows that some traditional approximations can 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. Here we suggest a way to improve the light time calculation used for probe tracking