Elsevier

Acta Astronautica

Volume 211, October 2023, Pages 116-129
Acta Astronautica

Research paper
An analysis tool for collision avoidance manoeuvres using aerodynamic drag

https://doi.org/10.1016/j.actaastro.2023.05.038Get rights and content

Highlights

  • Analysis of effects of collision avoidance manoeuvres using aerodynamic drag.

  • Consideration of additional constraints on the satellite attitude.

  • Consideration of uncertainties in respective parameters.

  • Applicable to every satellite able to change its ballistic coefficient.

  • Proved feasibility and further analyses for the university satellite Flying Laptop.

Abstract

Aerodynamic collision avoidance manoeuvres provide an opportunity for satellites in Low Earth Orbits to reduce the risk during close encounters. With rising numbers of satellites and objects in orbit, satellites experience close encounters more frequently. Especially those satellites without thrusting capabilities face the problem of not being able to perform impulsive evasive manoeuvres. For satellites in Low Earth Orbits, though, perturbing forces due to aerodynamic drag may be used to influence their trajectories, thus offering a possibility to avoid collisions. This work introduces a tool for the analysis of aerodynamic collision avoidance manoeuvres for satellites in circular Low Earth Orbits. Current space-weather data are employed to estimate the density the satellite encounters. Achievable in-track separation distances following a variation of the ballistic coefficient through a change in attitude are then derived by evaluating an analytical equation from literature. Considering additional constraints for the attitude, e.g., charging phases, and uncertainties in the used parameters, the influence of a manoeuvre on the conjunction geometry and the collision probability is examined. The university satellite Flying Laptop of the University of Stuttgart is used as an exemplary satellite for analysis, which show the general effectiveness of evasive manoeuvres employing aerodynamic drag. First manoeuvring strategies can be deducted and the influence of parameter uncertainties is assessed.

Introduction

The regime of Low Earth Orbits (LEO), i.e., altitudes below 1500  km [1], offers great possibilities for Earth observation, thermospheric investigations, and the global connectivity market. This draws the attention of various actors and leads to a sharp rise in satellite numbers. Growing numbers of satellites (and debris) do not only pose a threat to functional satellites. Every collision produces a huge amount of debris objects, which in turn threaten other satellites. Analyses have shown that this could end in an avalanche-like process, referred to as the Kessler syndrome, rendering the LEO regime useless for future generations [2], [3], [4]. This highlights the need to minimize collision risk.

Besides active debris removal, an obvious option for functional satellites is the implementation of collision avoidance manoeuvres (CAMs) in case of a predicted close encounter with another object. Typically, such manoeuvres are performed with impulsive thrusters to deflect the satellite trajectory. Satellites without thrusting capabilities, on the other hand, need other strategies to evade potential collisions. In LEO, aerodynamic drag due to the remaining atmospheric particles represents significant natural perturbing forces, which can be used to control and manoeuvre asymmetrically-shaped satellites or satellites with dedicated control panels. This control method has, for example, been researched for satellite formation flight by applying differential aerodynamic forces to affect relative motion [5]. Furthermore, aerodynamic CAMs have been studied [6], [7], even in combination with solar pressure [8]. Other approaches include using drag sails [9], [10], [11], [12] or tethers [13] to alter satellite trajectories for manoeuvring as well as de-orbiting.

Although achievable forces through aerodynamic drag are magnitudes smaller than what is possible with chemical thrusters, satellite orbits can be measurably altered given enough time, which allows for the implementation of CAMs. Satellites without thrusters can, therefore, greatly benefit from CAMs using aerodynamic drag.

One such satellite is the Flying Laptop of the Institute of Space Systems, University of Stuttgart. It orbits the Earth in a nearly circular polar orbit at an altitude of 600km. From its launch into LEO in July 2017 until September 2022, the Flying Laptop’s operators received over 5000 warnings for over 150 close encounters from the Combined Space Operations Center (CSpOC), with the lowest predicted miss distance reported as 29  m. Since the Flying Laptop does not possess any thrusters, it has no possibility to perform impulsive collision avoidance manoeuvres. Thus, so far the operators had to remain without action upon reception of a collision warning. As previously pointed out, though, the orbital altitude and the asymmetric shape of the satellite, in principle, allow control via aerodynamic drag. The Flying Laptop will be used as an example satellite in this work.

The paper is structured as follows: At first, fundamental concepts are covered. The developed analysis tool is then introduced in Section 3. Lastly, results of exemplary analyses for the Flying Laptop are presented and discussed in Section 4. The research presented here is based on the corresponding author’s master thesis [14].

Section snippets

Satellite aerodynamics

Satellites in LEO are subject to a measurable perturbing force due to the remaining atmospheric particles, which impinge on the satellite surface. The aerodynamic forces acting on a satellite can be separated into lift and drag. Drag is defined as the component acting anti-parallel to the satellite velocity relative to the local atmosphere, whereas lift acts in perpendicular direction to that. The specific drag force fD depends on the satellite’s dimensionless aerodynamic drag coefficient CD,

Analysis tool

In the following section, the methodology behind the analysis tool for collision avoidance using aerodynamic drag is explained.

Results

This section presents the results of analyses performed with the tool on exemplary close encounters of the Flying Laptop. Achievable separation distances are investigated before the influence of parameter uncertainties on the collision probability is analysed. Solar and geomagnetic data for the different activity levels are defined as in Table A.2, the used atmosphere model is NRLMSISE-00. The aerodynamic data of the Flying Laptop are defined in Table B.3 and determined via interpolation

Discussion

In the following section the results of the previous analyses are discussed. At first, the feasibility of aerodynamic collision avoidance manoeuvres for the Flying Laptop is evaluated. After that, possible manoeuvre strategies are discussed before the aspect of uncertainties is dealt with. Lastly, limitations and the applicability of the tool are discussed.

Summary

In this work, a tool was developed to analyse aerodynamic manoeuvres for satellite collision avoidance. Different manoeuvres can be compared with respect to their resulting miss distance and collision probability, considering solar and geomagnetic activity conditions. Further constraints regarding charging phases during the manoeuvre can be considered and uncertainties in the various parameters that are used as input to the tool can be accounted for, as well. At this point, the tool relies

Outlook

The presented tool will be developed further. Alternatives for the analytical equation will be examined to estimate achievable separation distances. This is expected to lead to a broader applicability due to fewer assumptions, e.g., the use for satellites and or secondary objects in elliptic orbits.

More research is necessary for a realistic approximation of the uncertainties in the parameters, their correlation to the position uncertainty defined in a CDM to obtain a reliable and less

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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