Do Open Source Tools Rival Heritage Systems? A comparison of tide models in OCEAN and Orekit

Open source software tools have been gaining acceptance in the astrodynamics community for some applications, though heritage tools still dominate precision orbit determination and propagation. This paper examines recent tide modeling improvements in the open source Orbit Extrapolation Toolkit (Orekit) and compares it with the US Naval Research Laboratory’s (NRL) heritage Orbit Covariance Estimation And ANalysis (OCEAN) system. First, the two tools are compared directly against each other by propagating a given state vector for Stella, a geodetic satellite sensitive to tidal variations in the geopotential. Second, orbits were fit to International Laser Ranging Service (ILRS) laser ranging data using OCEAN and orbit determination software built around Orekit so that a more useful comparison could be made. Five days of data were used to solve for orbital parameters using OCEAN and Orekit. This solution orbit is then propagated forward 25 days and compared to subsequent five day orbit solutions. This comparison between predicted and fitted orbit solutions is used as a metric to compare the quality of each piece of software’s dynamic modeling capability. Results from the direct orbit propagation comparison indicate the RSS of postion difference between the OCEAN and Orekit propagated orbit grow to only 7 meters over 25 days. It is also seen that the difference between OCEAN’s and Orekit’s implementation of Earth tides are less than 3% of the total tidal effect. The results of the orbit determination analysis show that the Orekit orbit solution comparison is at worst on the same order of magnitude in accuracy as the OCEAN orbit solution comparison, and at best more accuate than the OCEAN orbit solution comparison. While OCEAN produces a more accurate orbit prediction than Orekit in the majority of the cases studied, more testing is need to understand the origin of the difference.