Orbit Determination from Two Line Element Sets of ISS-Deployed CubeSats

Deploying nanosatellites from the International Space Station (ISS) has become prevalent since the addition of the NanoRacks CubeSat Deployer in early 2014. Since then, 61 CubeSats have been deployed from the ISS, with the majority coming from the Planet Labs Flock 1 and Flock 1B constellations. CubeSats often rely on two-line elements (TLEs) made publicly available by the Joint Space Operations Center (JSpOC) for orbit determination and conjunction assessments, so the accuracy of JSpOC TLEs for ISS-deployed CubeSats is important to examine. In this work, the accuracy of TLEs of Flock 1B satellites are analyzed by comparison to orbits as derived from twoway ranging. Ten Flock 1B satellites are examined for the month of September 2014, using 634 TLEs from start date to end date across the flock. Prior TLE assessments for CubeSats in LEO have estimated error to be within 1 km. We found that error for ISS-deployed CubeSats is substantially higher than prior estimates. Using only forward propagation with the most recent TLE, as is the case for operational TLE use, median error in position is found to be 4.52 km with a first quartile of 2.01 km and a third quartile of 10.6 km. The 1-σ in-track propagation error after one day ranges from 10-30 km among the 10 satellites, and the two-day 1-σ error ranges from 20-70 km. To improve TLE accuracy for on-orbit operations, a batch least squares estimation technique is used to estimate some or all elements of the current TLE based on prior TLEs. It is shown that this method can improve the propagation of a TLE significantly, particularly in cases of sparse updates, with up to 95% error reduction. This can potentially enable operations in cases where they would otherwise be lost due to inaccurate orbital knowledge.