Abstract
Nowadays, a wide range of space missions accommodate ever-stricter electromagnetic cleanliness requirements arising either from the need for more precise measurements or from the implementation of highly sensitive equipment. Therefore, the establishment of a methodology that ensures the minimization of the electric and/or magnetic field in specific areas inside or outside the spacecraft structure is crucial. Towards this goal, the current chapter proposes that utilizing the results of a process completed during the early design stages of a mission, that is, the measurement and characterization of each implemented device, the desired elimination of the field can be achieved. In particular, the emerged electromagnetic signatures of the units are proven essential for the proposed methodology, which, using a heuristic approach, defines the optimal ordinance of the equipment that leads to system-level electromagnetic field minimization in the volume of interest. The dimensions of the devices and the effect of the conductive surfaces of the spacecraft's hull are also taken into account.
TopIntroduction
Since the very beginning of the first space missions in 1957, the scope of the majority of these missions is to measure extraterrestrial electromagnetic fields and particle population in plasma (Benkhoff et al., 2010; Drinkwater et al., 2003; Scheeres et al., 1998; Escoubet & Schmidt, 2000; Antonucci et al., 2011; Müller et al., 2013; Bayle et al., 2016). GOCE, EXOMARS, Rosetta, Cluster, BepiColombo, LISA and Solar Orbiter, are some examples that aim to facilitate these measurements, carrying particle detectors and sensitive sensors (e.g. fluxgate or search-coil magnetometers) in their satellites or space probes. A lot of effort is carried out in the early design phases of a mission in order to place these sensors in the optimum electromagnetically clean environment. Moreover, and regarding the payload of these missions that is necessarily sensitive to electric and magnetic fields, requires stringent electromagnetic cleanliness with emphasis to random and periodic AC electric and magnetic field variations in frequency and time domain. From the perspective of extremely low frequency (ELF) electric cleanliness and AC & DC Magnetic cleanliness, these instruments are meant to measure slow time- variant fields (both electric and magnetic), equivalent to frequencies not higher than 250 KHz, based on recent study of (Vaivads et al., 2016). The targeted frequency range is not usually studied in standard ElectroMagnetic Compatibility (EMC) space engineering techniques (ECSS-E-ST-20-07C rev-1 Space engineering Electromagnetic compatibility, 2012) and thus the developed test measurement methods of this activity, that are presented in this chapter, are rather unique.
The acceptable mission threshold both for radiated AC electric (E-field) and magnetic (B-field) emissions in terms of spectral density in case of THOR mission are presented in Figures 1 and 2 respectively. Table 1 & 2 tabulated the mission requirements. It should be noted that in order to compare them with the measured radiated emission, these values shall be rescaled to the measured distance according to the equation 1. In the same Figures, radiated emissions of a Command and Data Handling Management unit (CDMU) from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission as measured in Thales Alenia Space (TAS-I) premises on the Pre-verification of THOR mission Electromagnetic Cleanliness Study (ESA contract 4000118770/16/NL/BW) are also present and compared with the mission requirements both for steady state and transient behavior. It is apparent that the unwanted radiated electromagnetic fields from spacecraft’s equipment cannot be higher than the accepted limits (predefined mission requirements).
where
Rreq the distance between the magnetic (or electric) sensor and the satellite’s (or spacecraft’s) closest edge, referenced in the spacecraft’s coordinate system (in this case, 6.3 m is assumed),
Freq defines the magnetic (or electric) cleanliness requirements at the location of the sensitive or/and measuring instruments (sensors) and
Rmeas is the distance that the sensitive or/and measuring instruments are located during the measurement of the device under test (DUT). It should be noted, that according to the Equation 1 and under the quasi-static regime, the electric field is falling with the
distance law. Therefore, the rescaled mission requirements shall be calculated at the measurement distance where, in our case, for the electric field the capturing antenna monopole was at 1 m while in the magnetic test campaign the magnetometers were placed in the distance of 50 cm.
Key Terms in this Chapter
Extremely Low Frequency (ELF): Is the ITU designation for electromagnetic radiation (radio waves) with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively.
Magnetic Cleanliness: The generated magnetic field from the spacecraft is required to not exceed specific thresholds at the location of the magnetic sensors and therefore, disturb their measurements.
Magnetic Signature: The magnetic field generated by an DUT typically depends on the distribution of magnetic material inside the unit, as well as the electronics and the current paths that are incorporated in its design. Therefore, each unit is expected to emit an individual distinctive magnetic field.
Equipment or Device Under Test (EUT or DUT): Any instrument or unit related to the space mission that produces an electric field. These units can be inside (e.g., Power Conditioning and Distribution Unit, RF Switch, Battery, etc.) or outside the main body of the spacecraft (e.g., Solar Arrays, Boom, etc.) and need to be tested and electrically characterized.
Magnetic Sensors: Instruments that are designed to measure magnetic fields (fluxgate magnetometers, search coil magnetometers, etc.). For instance, the fluxgate magnetometers have a magnetically susceptible core wrapped by two coils and measure the induced electric currents from a background magnetic field.
Differential Evolution Algorithm (DEA): Is a method that optimizes a problem by iteratively trying to improve a candidate solution with regard to a given measure of quality.
Electric Dipole Model (EDM): A method that optimizes a problem by iteratively trying to improve a candidate solution with regard to a given measure of quality. Such methods are commonly known as metaheuristics as they make few or no assumptions about the problem being optimized and can search very large spaces of candidate solutions.
Induced Magnetization: Magnetic properties of a material that purely depend on the presence of an applied external magnetic field and directly vanish in its absence.
Electromagnetic Interference (EMI): Also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction.
Remnant Magnetization: Magnetic properties of a material that are always present, even in the absence of external fields (e.g., ferromagnetic materials).
Multiple Magnetic Dipole Modeling: The representation of a unit’s generated magnetic field with a dipole model able to recreate and reproduce its magnetic signature.
European Space Agency (ESA): Is an intergovernmental organization of 22 member states dedicated to the exploration of space. Established in 1975 and headquartered in Paris.
Turbulence Heating Observer (THOR): A plasma mission, aiming at understanding the physical mechanisms underlying the energy dissipation of turbulent fluctuations in plasmas.
Multi-Frequency Electric Dipole Model (MFEDM): A method using electric dipoles to predict the radiated emission of an DUT.
Electromagnetic Compatibility (EMC): The branch of electrical engineering concerned with the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment.
Magnetic Source: Any source that generates a magnetic field. These sources typically include magnetic materials (e.g., iron, steel, etc.) and flowing electric currents.
Electrical Ground Support Equipment (EGSE): Is an integrated suite of electrical satellite testing solutions to make sure that the satellites you are launching work perfectly.
Electric Cleanliness: The generated electric field from the spacecraft is required to not exceed specific thresholds at the location of the electric sensors and therefore, disturb their measurements.