Ensemble Simulations of the Thermosphere to Quantify the Relationship between Uncertainties in the Space Environment Drivers and the Orbital Position of LEO Satellites.

Abstract:

The prediction of the position and the velocity of low Earth orbit (LEO) satellites and orbital debris has become particularly important as their number has considerably increased. Some space missions that include measurements of the Earth require the knowledge of the state (position and velocity) of the spacecraft that takes the measurements. Collision avoidance is also a major issue that can only be solved with a particular accurate orbit determination of the objects in orbit. The CYclone Global Navigation Satellite System (CYGNSS) that aims to improve the prediction of tropical cyclones by using a constellation of eight satellites is a good example of a mission that requires a high level of accuracy in the orbit propagation of each of the satellites.

After gravity, the main force acting on LEO satellites is atmospheric drag. The drag force acting on the satellite is directly proportional to the thermospheric density, which is complex to model and predict because of the strong coupling of the thermosphere with the lower atmosphere, the ionosphere (itself driven by the magnetosphere), and the Sun. Therefore, uncertainties in the density are not negligible at these altitudes and are an important cause of the uncertainties on the position of a satellite.

Many groups have worked on the prediction of inputs of the space environment that impact the density of the thermosphere (solar wind speed, Interplanetary Magnetic Field, F10.7, Ap/Kp). However, quantifying the uncertainty on these predictions is also essential. Ensemble forecasts can help quantify how uncertainties in drivers affect the uncertainty in the orbital position of satellites. We use ensemble simulations of the thermosphere, including both empirical and physics-based models, to quantify the relationship between uncertainties in the drivers and orbital position.