Aerodynamic Orbit Control

We investigate how the Earth's residual atmosphere affects satellite orbits at low altitudes, how aerodynamic forces can be utilized for orbit control and, conversely, how orbital data can be used to draw conclusions about the properties of the atmosphere.

The figure shows a satellite with the body-fixed and aerodynamic coordinate axes originating from its centre of mass, together with the aerodynamic forces: drag (opposing the relative velocity to the atmosphere), lift (perpendicular to the relative velocity), and their sum, along with the angle of attack and sideslip angle describing the body's orientation relative to the flow. ©
Aerodynamic lift (index "L") and drag (index "D") acting on a satellite due to interaction with the atmosphere. The satellite's attitude with respect to the incoming flow is described by the angle of attack ("alpha") and the sideslip angle ("beta").

The Earth's atmosphere shows an approximately exponential decrease in density with altitude but its effect on satellites at low altitudes is not negligible: Due to interaction of the particles in the residual atmosphere, satellites experience aerodynamic forces. In Very Low Earth Orbits (VLEO, below 450 km), aerodynamics are the second-largest cause of orbital motion perturbation after gravitational effects, and thus need to be considered in the design and operation of spacecraft. At the same time, they offer the opportunity to deliberately adjust the trajectory. As shown in the figure on the right, the aerodynamic force is separated into two components: drag acts anti-parallel to the inflow velocity, while lift acts perpendicular to it. Both heavily depend on the attitude of the satellite, defined by the angle of attack and the angle of sideslip.

Our activities include:

  • Characterization of aerodynamic properties of satellites to determine drag and lift coefficients
  • Design of satellite geometries with improved aerodynamic properties to enhance lifetime and increase manoeuvrability
  • Estimation of atmospheric conditions, such as density and wind, based on observable effects on satellite orbits
  • Modelling the variable effects of aerodynamic forces on satellite orbits
  • Planning and optimization of aerodynamic manoeuvres considering uncertainty

Very Low Earth Orbits (VLEO, below 450 km) enable high-resolution Earth observation, rapid communication and lower launch costs than traditional, higher orbits. However, the increased aerodynamic drag reduces the useful lifetime of satellites and thus makes sustainable satellite operation at these altitudes more challenging. New technologies and strategies are therefore needed to make VLEO satellite operations sustainable.

We investigate, how interaction with the atmosphere affects satellite orbits and derive manoeuvre strategies. Applications range from orbit control for individual satellites, formation flying and even collision avoidance. By deliberately varying the aerodynamic force, e.g., via the satellite's attitude relative to the incoming flow, trajectory changes can be achieved without the need for propellant. While drag affects mainly the in-plane motion, lift allows corrections of the orientation of the orbital plane. However, all lift applications lead to increased drag. Accordingly, we also study how to optimize such manoeuvres with respect to the additional loss of orbital altitude. Manoeuvre profiles for a simple and decay-optimal inclination change can be seen in the figure below.

The aerodynamic force depends on various parameters, which all are subject to uncertainty. Density and chemical composition of the atmosphere show large fluctuations, among others they depend on solar and geomagnetic activity, and can only be predicted with a degree of uncertainty using models. The interaction of the atmospheric particles with a satellite's surface is also highly complex, and the relevant models have so far been validated experimentally only to a limited extent.

As the atmosphere has a measurable effect on a satellite’s orbit, this relationship can also be reversed: based on observations of the orbits of active satellites, conclusions can be drawn about atmospheric density and winds. This enables models of the upper atmosphere (thermosphere) to be improved and reduces the inaccuracy of density estimates. In the long term, improved modelling of the thermosphere based on real-time data will provide more precise predictions of re-entries, lifetimes, and collision risks.

Left: 3D illustration of the Earth's sphere with an inclined orbital plane, where the simple lift profile of a decay-optimal inclination-change manoeuvre is shown in green: the lift coefficient follows a piecewise-constant shape over one revolutions, changing signs over the poles. Right: 3D illustration of the Earth's sphere with an inclined orbital plane, where the optimal lift profile of a decay-optimal inclination-change manoeuvre is shown in green: the lift coefficient follows a cosine shape over one revolutions, changing signs over the poles. ©
Simple (left) and optimal (right) lift coefficient profile for an inclination change.

Contact

This image showsFabrizio Turco

Fabrizio Turco

M.Sc.

Research assistant

This image showsPaul J. Haufe

Paul J. Haufe

M.Sc.

Research Associate

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