Aspects of Extremely Low Frequency Electric and Magnetic Cleanliness on Space Platforms

Introduction

The necessity as well as the difficulties of achieving electromagnetic cleanliness in space missions are well known issues that constantly concern EMC engineers ((Junge & Marliani, 2011), (Polirpo & Cucca, 2012), (Weikert, Mehlem, & Wiegand, 2012) (Boschetti, Gervasio, & Marziali, 2012) (Lassakeur & Underwood, 2019) (Michelena, Rivero, Frutos, Ordóñez-Cencerrado, & Mesa, 2019) and other). In fact, every electronic device situated inside a spacecraft emits electromagnetic radiation that in conjunction with the properties of the materials selected for the design, can pose a serious threat to the whole system’s performance. Therefore, at the early phases of the design, a lot of effort is made in order to achieve acceptable levels of EM radiation at certain regions and prevent system failures, such as permanent damage of power supplies, false commanding, noisy measurements etc. A typical example of equipment pieces that require strict electromagnetic cleanliness constraints are field and particle sensors. Those, aiming to widen humanity’s understanding regarding the laws governing the universe, from planet formation to particle interactions, require precise and accurate measurements that cannot be obtained if the measuring environment is not free from spacecraft-induced interference.

The techniques that can be utilized in order to achieve this challenging objective of electromagnetic cleanliness vary from the implementation of adequate shielding on the onboard equipment, to the use of low-emissions interconnecting wiring methods between the mission’s units (ECSS Secretariat, 2019). But except for the unit-level work, certain design choices can also contribute to the attainment of this goal. In particular, based on the electromagnetic theory and the modeling approximation of the problem, the authors of (Kountantos, Nikolopoulos, Baklezos, & Capsalis, 2019), (Nikolopoulos, Baklezos, & Capsalis, 2020) and (Baklezos A., Nikolopoulos, Vardiambasis, Kapetanakis, & Capsalis, in press) have developed a methodology that aims to the formation of an environment inside or outside the boundaries of the spacecraft, where the system’s interference is minimized, if not eradicated. This methodology deploys an optimization algorithm, namely the Differential Evolution (DE) algorithm (Storm & Price, 1995), which calculates the proper ordinance of the equipment in order to achieve EM cleanliness either at a single observation point or on a set of points (boundaries of the area of interest). More specifically, the optimal spatial arrangement (positions) as well as the proper orientation angles of the devices (DUTs) are calculated. In order for this process to be realized, the electric and magnetic signatures of the units comprising the spacecraft’s equipment should be known. Usually, measuring and modeling campaigns are performed at the early stages of the design, therefore such information is available.

The algorithmic process also accounts for the dimensions of the equipment, thus avoiding the overlap of the units in the proposed solution, as well as for the Spacecraft Hull effect, i.e. the contribution of the conductive surfaces of the spacecraft to the distribution of the system’s field. In addition, if the strict cleanliness requirements cannot be achieved with the proper placement of the onboard equipment, additional auxiliary sources can be included in the design, the ordinances of which are calculated similarly to that of the DUTs. It should be noted that in a wide range of missions, with the arrangement of the units, a trade-off aims to be achieved, between not only EM cleanliness restrictions but also thermal, mechanical, accessibility etc. Hence, those may take precedence over the currently discussed cleanliness requirements, such that the spatial arrangement of the equipment cannot be addressed as proposed. But even in those cases, this methodology can be implemented for determining the position and orientation of the potentially used compensation sources.

Overall, this stochastic methodology, given a set of previously characterized devices, can calculate the optimal arrangement of the equipment in a predefined search space (spacecraft boundaries), aiming at the minimization of the total field in a given area. In the framework of this chapter, DC and extremely low frequency (ELF) sources are examined, operating in the frequency range up to 250 KHz. Simulation results show that, in these cases, due to the strong dependence of the emissions on the relative distance between the source and the observation point(s), even small rearrangements can have noteworthy impact to the distribution of the system’s field.