Magnetic Cleanliness Program on Cubesats and Nanosatellites For

JOURNAL OF AERONAUTICS AND SPACE TECHNOLOGIES (ISSN : 1304-0448) January 2020 Volume 13 Number 1 www.jast.hho.edu.tr Research Article Magnetic Cleanliness Program on CubeSats and Nanosatellites for Improved Attitude Stability Abdelmadjid LASSAKEUR 1 , Craig UNDERWOOD 2 , Benjamin TAYLOR 2 , Richard DUKE2 1 Satellite Development Center, Algerian Space Agency, BP 4065, Ibn Rochd USTO, 31130 Oran, Algeria, [email protected], https://orcid.org/0000-0002-4538-6985 2 Surrey Space Centre, University of Surrey, Guildford GU2 7XH, United Kingdom, [email protected], [email protected], [email protected], https://orcid.org/0000-0002-7001-5510, https://orcid.org/0000-0003-3635-003X, https://orcid.org/0000-0003-4450- 7981 Article Info Abstract CubeSats are being increasingly specified and utilized for demanding astronomical and Earth observation missions where precise pointing and stability are critical requirements. Such precision is difficult to achieve in the case of CubeSats, mainly because of their small moment of inertia, this means that even small disturbance torques, such as those due to a residual magnetic moment are an issue and have a significant effect on the attitude of nanosatellites, when a high degree of stability is required. Also, hardware limitations in terms of power, weight and size make the task more challenging. Recently, a PhD research program has been undertaken at the University of Surrey to investigate the Received: July 18, 2019 magnetic characteristics of CubeSats. It has been found that the disturbances may Accepted: November 22, 2019 be mitigated by good engineering practice, in terms of reducing the use of Online: January 23, 2020 permeable materials and minimizing current-loop area. This paper discusses the dominant source nanosatellites disturbances and presents a survey and a short Keywords: ADCS, CubeSats, description of magnetic cleanliness techniques to minimize the effect of the Nanosatellites, Magnetic Disturbances, residual magnetic field. It is mainly intended to supply a guide for CubeSat Magnetic Cleanliness, Magnetic Dipole community to design future CubeSats with improved attitude stability. We Moment determination, Attitude present then our findings to date of a new technique of the residual magnetic Stability. dipole determination for CubeSats and nanosatellites. This method is performed by implementing a network of eight miniature 3-axis magnetometers on the spacecraft. These are used to determine the strength, the direction and the center of the magnetic dipole of the spacecraft dynamically in-orbit and in real-time. This technique will contribute to reduce the effect of magnetic disturbances and improve the stability of CubeSats. A software model and a hardware prototype using eight magnetometers controlled via a Raspberry-Pi were developed and successfully tested with the boom payload of the Alsat-1N CubeSat and a magnetic air coil developed for validation purposes. To Cite This Article: A. Lassakeur, C. Underwood, B. Taylor, and R. Duke "Magnetic Cleanliness Program on CubeSats and Nanosatellites for Improved Attitude Stability," Journal of Aeronautics and Space Technologies, Vol. 13, No. 1, pp. 25-41, Jan. 2020. 1. INTRODUCTION have been launched up to 10th of June 2019 [5]. CubeSats are a demonstration that small size satellite technology CubeSats are nanosatellites built to standard dimensions, developed, mostly, using commercial off-the-shelf with a base unit volume (U) of 10 cm x 10 cm x 10 cm. (COTS) components. CubeSats have recently attracted They can be in different sizes, 1U, 2U, 3U, 6U or even the interest of many national space agencies, professional bigger scale, and the total mass budget is nominally less space-tech companies and universities as a new tool of than 1.33 kg of mass per unit [2]. This concept was space development and research due to their cost and ease proposed by Professor Robert Twiggs from Stanford of technology using COTS components [2]. They can University in November 1999 at the 2nd Space System perform practical space missions, often as university Symposium. It has then been adopted widely by the projects for space engineering students wanting to test out universities and the space industry [3, 4], 1088 CubeSats some new technology or techniques [6]. This class of 25 Magnetic Cleanliness Program on CubeSats and Nanosatellites for Improved Attitude Stability spacecraft is increasingly used for Earth observation and in the spacecraft – both in the wiring harness and in the astronomical applications where precise pointing and layout of the solar panels. Some CubeSats also carry high attitude stability are critical requirements [7, 8]. permanent magnets – e.g., for electric motors – often used in the ADCS systems for momentum wheels [7, 17]. By Knowing the power, volume and cost limitations of contrast, the other typical attitude disturbance sources for CubeSats, many challenges need to be addressed by the spacecraft decrease significantly when the satellites CubeSat engineers. One key challenge is the development become small. of a precise attitude determination and control system (ADCS). As CubeSats are small satellites ,they have less Table 1 represents a quantitative figure on a typical mass, less volume, and less power for sensors, actuators magnitude order of the different disturbance torques in and algorithms processing [9]. Besides, several CubeSats real life mission scenario in low Earth orbit (660km), have been observed to suffer from unwanted magnetic estimated on PRISM nanosatellite [10]. dipole moment (e.g. SNAP1, PRISM, Alsat1-N, etc.) which becomes the dominant source of attitude Table 1. Attitude disturbance in the PRISM mission [10]. disturbances for such a small moment of inertia platforms Disturbance type Value [Nm] [6, 10, 11]. Therefore, to minimize the effect of the magnetic disturbances and achieve the required level of Magnetic disturbance 3.0 ∙ 10−6 attitude control, the source of the magnetic disturbances must be reduced on the ground (e.g., by good design Gravity gradient 8.0 ∙ 10−7 practices) and, ideally, any residual magnetic dipole −8 moment should be cancelled in-orbit using Aerodynamic 3.0 ∙ 10 magnetorquers [7]. Solar pressure 1.0 ∙ 10−8 Surveys of COTS solar arrays and CubeSats subsystems indicate that they are often not designed with magnetic cleanliness in mind [8, 12]. This paper aims to describe Materials which retain their magnetism and are difficult the sources of the residual magnetic moment and evaluate to magnetize or demagnetize are called hard magnetic their effect on the attitude of CubeSats; we then introduce materials. They have a large hysteresis and low methods of magnetic cleanliness on CubeSats to permeability. In contrast, soft magnetic materials have a minimize the sources of the residual magnetic moment low hysteresis and a large magnetic permeability and thus (RMM) and reduce the effect of the magnetic respond very sensitively to the presence of an external disturbances on CubeSats in orbit. We finally discuss the magnetic field – they are easy to magnetize and characterization and estimation methods of the residual demagnetize. Therefore, the use of hard ferromagnetic or magnetic moment. (preferably) non-ferromagnetic materials, such as some forms of stainless steel (e.g. 304 or 316 alloys), aluminum, copper or titanium are recommended for 2. MAGNETIC DISTURBANCE CHALLENGES spacecraft magnetic cleanliness, whereas soft magnetic TO ATTITUDE DETERMINATION AND materials such as iron, nickel and mild steel are not [7, CONTROL SYSTEMS 18]. As CubeSats have tiny moments of inertia (e.g. MIO of Some CubeSats missions use the permanent magnet CSSWE 3U CubeSats 퐽 = 퐷푖푎푔[0.00551, 0.02552, control as the main attitude control, it is also known as a 0.02565] kg ∙ m2) and are usually operating in the low compass mode. It is used when only two-axis stabilization Earth orbits (160km to 2000km), it is found that the is required. The main drawback of this type of internal magnetic moment dominates over other stabilization is that it is not possible to remove the kinetic environmental disturbances (e.g., aerodynamic, Solar energy from the satellite, and the fact that the Earth radiation pressure and gravity-gradient torques) in terms magnetic field varies along the orbit [19, 20]. An example of producing unwanted attitude disturbances [11, 13]. of permanent magnet control applications on 3U This is due to the Geomagnetic field that interacts with CubeSats is the RAX (Radio Aurora Explorer) [21-23] any residual magnetic field of the spacecraft and results and TURKSAT-3USAT [24]. And on 1U CubeSat in a net magnetic dipole moment [14-16]. missions, OUFTI-1 (Orbital Utility For Telecommunications - Technology Innovations) [25], The effect of magnetic disturbances has shown itself by and ITUpSat-1 (Istanbul Technical University the problem of high tumbling rates observed on several PicoSatellite-1) which uses Alnico permanent magnet for CubeSat missions [7, 17]. Post-flight analysis indicates its attitude control [26]. that this is due to the un-modelled dynamically changing magnetic moments mainly caused by the current flowing LASSAKEUR et. al. 26 Magnetic Cleanliness Program on CubeSats and Nanosatellites for Improved Attitude Stability 2.1. Residual Magnetic Moment length dl, the generated magnetic flux density dB at a point (x, y, z) in a Cartesian coordinate system is given The primary sources of current loops in CubeSats that by [1]: generate a dynamic magnetic moment in the satellite come from the layout of the solar panels and the harness µ 퐼 푑푙⃗⃗⃗ × 푟 of the spacecraft. The current flowing in the solar panels ⃗⃗⃗⃗ 0 (2) 푑퐵 = 2 generates a residual magnetic field due to the resulting 4휋 푟 current loops (Figure 1). Many methods are available in the literature, which can reduce these current loops [27], Where: including placing solar cells tracks of opposite current µퟎ is the permeability of free space. flow next to one another or laying the tracks on top of one another in a multilayer PCB (printed circuit board).

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