Speakers
Dr
Josep Virgili
(Naval Postgraduate School)Dr
Peter Roberts
(University of Manchester)
Description
The residual atmospheric density present at orbital altitudes produces an aerodynamic disturbance, both to satellite orbits and attitude, that at Very Low Earth Orbits (<450 km) can be significant, usually being the strongest disturbance. Certain spacecraft shapes are considered to be aerostable as a restoring aerodynamic torque appears when the spacecraft aerodynamic equilibrium attitude - where no aerodynamic torque is produced - is lost. The equilibrium attitude is defined with respect to the relative flow which is composed of the spacecraft's inertial velocity, the atmospheric co-rotation and the atmospheric wind. A cone is a simple example of an aerostable shape, with its aerodynamic equilibrium attitude achieved when the cone's axis of symmetry is aligned with the relative flow.
The proposed method is able to measure the atmospheric wind by observing the spacecraft's attitude motion when it is allowed to freely react to the aerodynamic torques caused by the relative flow. Estimates of the other attitude disturbance sources are required to isolate the response cause by the aerodynamic torque and knowledge of the spacecraft's aerodynamic properties and atmospheric density is needed to determine the contribution from the wind magnitude and direction in the observed aerodynamic torque.
Aerostable spacecraft behave as an undamped oscillators with their natural frequency depending on the dynamic pressure and their aerostable properties (i.e. the aerodynamic stiffness). If the spacecraft aerostability is strong enough the attitude motion will remain a bounded oscillation around the aerodynamic equilibrium point and thus no additional control input is required. The natural frequency of the system along with the velocity of the spacecraft, determines the achievable spatial resolution of the wind measurements. As the atmospheric density increases exponentially with decreasing altitudes, improved spatial resolutions at lower altitudes are achieved by using the same aerostable properties. A high spacecraft inertia to aerodynamic stiffness ratio also increases the natural frequency and implies that small spacecraft (with high area per unit of mass) are better suited to be used in the proposed method.
Aerostable spacecraft only provide an aerodynamic torque that is normal to the relative flow direction (i.e. pitch and yaw). So the cross-track wind components have a stronger effect on the spacecraft's attitude than its in-track wind counterpart. The dynamic pressure is a function of the in-track wind and thus the in-track wind component is also observable although it has a much weaker effect and thus it imposes more stringent requirements.
The method described in this paper can provide global cross-track and in-track wind measurements. The measurements accuracy and spatial resolution with respect to the system parameters (i.e. altitude, aerostable properties and its uncertainty, inertia and uncertainty on the knowledge of atmospheric density) are also analyzed.
| Applicant type | Co-author |
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Primary author
Dr
Peter Roberts
(University of Manchester)
Co-author
Dr
Josep Virgili
(Naval Postgraduate School)
