Effects of Increasing Drag on Conjunction Assessment

Conjunction Assessment Risk Analysis relies heavily on the computation of the Probability of Collision (Pc) and the understanding of the sensitivity of this calculation to the position errors as defined by the covariance. In Low Earth Orbit (LEO), covariance is predominantly driven by perturbations due to atmospheric drag. This paper describes the effects of increasing atmospheric drag through Solar Cycle 24 on Pc calculations. The process of determining these effects is found through analyzing solar flux predictions on Energy Dissipation Rate (EDR), historical relationship between EDR and covariance, and the sensitivity of Pc to covariance. It is discovered that while all LEO satellites will be affected by the increase in solar activity, the relative effect is more significant in the LEO regime around 700 kilometers in altitude compared to 400 kilometers. Furthermore, it is shown that higher Pc values can be expected at larger close approach miss distances. Understanding these counter-intuitive results is important to setting Owner/Operator expectations concerning conjunctions as solar maximum approaches. I. Introduction ONJUNCTION assessment risk analysis (CARA) is the process of quantifying the risk associated with any identified close approaches between two orbiting objects. NASA Goddard Space Flight Center (GSFC) has implemented a routine CARA process that currently supports approximately 55 robotic spacecraft [References 1,2]. Often the collision risk associated with a conjunction is quantified in the form of the Probability of Collision, Pc. The computation of Pc requires both state and uncertainty information of the two conjuncting objects. The uncertainty information is provided in the form of a covariance. In Low Earth Orbiting (LEO), satellite covariance is predominantly driven by perturbations due to atmospheric drag. It is also well-known that atmospheric drag is a dynamic effect dictated by solar activity. This analysis seeks to understand the effects of increasing atmospheric drag on CARA through analysis of the Pc calculation. C