By: Yannis Braux and Stephen Murray
CST Computer Simulation Technology
Introduction
EMC and EMI analysis plays an important role in ensuring the correct functioning of electronic systems and guaranteeing reliability throughout their lifetimes. This task is all the more difficult and crucial when the system is complex or exists in a challenging EM environment. This is the case for any electronics intended to be sent into space, which have many operating constraints and need exceptional durability. Testing a prototype in the environment of space is prohibitively expensive, and once the system is launched it can’t be repaired. The success of a spacecraft mission, both in its launch phase and when installed in its orbit, requires careful study of electromagnetic compatibility. This article explores how EM simulation can help engineers in the space industry master environmental electromagnetic effects and susceptibility in these complex systems.
Today’s satellites and launchers contain many densely-packed and complex high-power systems. A single satellite will have many communication systems, sensors and high-end technologies. Making such a complicated platform EMC compliant can be extremely difficult. Confidence in the performance of the system before launch is essential.
There are now many players in the space market, including both government agencies and commercial companies. New competitors are very aggressive in proposing innovative solutions at the cheapest price. Because of the very competitive situation in the satellite and launcher markets, over-testing and over-engineering is no longer a viable solution and shortening the design stage is mandatory. This is where simulation can play a role.
At the design stage, EMC simulation can anticipate risks by predicting the electromagnetic behavior of the equipment and propose solutions even before the first prototype. Simulation does not replace testing but helps to predict potential failures, and allows the investigation of technical issues and novel concepts.
In the next sections, four EMC simulation workflows will be demonstrated. These cover both environmental electromagnetic effects (E3) and emissions within the satellite.
Lightning attachment analysis
As a tall metal object, a launcher is especially prone to lightning strikes. For this reason, launch pads are surrounded by several grounded metal towers linked by cables. These act as lightning rods and reduce the likelihood of the rocket being struck.
Lightning attachment analysis allows engineers to calculate the effectiveness of a lightning protection system. The odds of lightning striking a given point are related to the electric field around it during a thunderstorm. The build-up of charge means that a strong electrostatic potential develops between the earth and the clouds. The gradient of this potential is the electric field strength, and this is highest around sharp metal objects (Figure 1). The lightning leader is therefore most likely to strike here.
This can be modeled and analyzed using electrostatic simulation. A model of the system of interest is imported into the simulation environment and the potential is defined. From this, the gradient of the potential and the electric field around the structure can be quickly calculated with a static simulation. Figure 2 shows a lightning attachment simulation performed with the CST STUDIO SUITE® Electrostatic Solver. The lightning protection system reduced the electric field strength at the tip of the rocket fairing by 44%, significantly reducing the likelihood of lightning striking the launcher.
Lightning strike simulation
An electrostatic simulation is a good starting point for a full lightning simulation. Lightning is a transient current pulse typically modeled with a double-exponential waveform, and is effectively broadband (the frequency spectrum of lightning runs from DC up to around 10MHz). This means that it is best simulated in the time domain.
The lightning attachment simulation results suggest the best place to attach the lightning channel, which is then modeled as a wire that defines the contact position. The lightning stroke is modeled as a double exponential, per MIL-STD-464.[1] Currents can propagate through very fine structures, such as the rods that comprise the pylons, seams and vents in the structure, and cables within the launcher. These can be challenging to simulate with traditional simulation methods, since they are very small compared to the overall size of the structure that is simulated, and therefore require a very fine mesh and a short time-step.
Compact models, available in the Transmission-Line Matrix (TLM) Solver, are a more efficient way to simulate these fine structures. The compact model replaces the detailed model in the simulation, and can offer a significant speed-up while maintaining the same accuracy. A lightning strike simulation using the TLM Solver in CST STUDIO SUITE is shown in Figure 3 and Figure 4. This simulation made use of octree meshing and the PERFECT BOUNDARY APPROXIMATION (PBA)®, with cable harness and compact models. This approach also allows the incorporation of circuit elements into the model – for example transient voltage suppressors.
Radiated susceptibility
Once in orbit, there are other sources of interference. In this example, the excitation is a plane wave which mimics an incoming communication – for example, a telemetry signal. The same technique detailed in this section can be applied to other effects such as solar flare and electromagnetic pulse.
The immunity of a satellite across the entire frequency spectrum can be calculated in a single run by performing a time domain simulation. In this case a Gaussian pulse was used for a plane wave excitation with a circular polarization in order to excite equally all the frequencies up to 1 GHz.
Again, seams, vents and cables are crucial to the immunity of the satellite, and need to be modeled with cable harness and compact models. Probes are placed within the structure at relevant points to measure currents, voltages and field strength.
Once the simulation has been run, the spectrum of the interference can be investigated in order to find which frequencies correspond to high voltages. Problematic frequencies can then be analyzed by visualizing the fields in 3D to reveal the coupling paths and identify where shielding is most needed (Figure 5).
Oversize Cavity Theory
Frequencies used on a space-based communication system can reach into the tens of gigahertz. At these frequencies, the satellite may be hundreds or thousands of wavelengths in each dimension. This can be challenging to simulate with full wave simulation. Oversize cavity theory (OCT) offers a different approach which is well suited to satellites and launchers where leakage from one cavity to another is a major coupling path for interference.
OCT was developed by Lehmann in 1993 and is a statistical theory of electromagnetic field distribution in over-moded, large and complex cavities.[2]
It is based on the principle that the field in the cavity is statistically homogeneous and isotropic with a known statistical distribution: for the electric field, this known distribution is a chi2 distribution with 6 degrees of freedom. This theory is based on a power balance. For each single cavity, the total input must be equal to the total output. The inputs are the incident power and the power coming from other cavities, and the outputs are the dissipated power and the power coupling to other cavities. OCT can then calculate quantities such as field strength, power density and Q-factor.
Satellite systems can be modeled as a set of cavities, with sources, losses and connections, as shown in Figure 6, and the results of an OCT analysis with CST STUDIO SUITE is shown in Figure 7. With OCT analysis, complex structures such as these can be analyzed in a matter of seconds. OCT is a useful alternative to full wave in these scenarios, as it is very fast even on a basic computer, and is perfect for quick EM field assessments on satellite or launcher. However, it is only suitable for large, complex and over-moded cavities and cannot fully calculate 3D structures such as cables or complex vents; it is best seen as a complement to 3D simulation.
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Conclusion
This article has demonstrated several EMC simulation workflows for aerospace applications. 3D EM simulation can be used to analyze environmental electromagnetic effects, susceptibility and coupling and can be used to develop countermeasures against EMC issues that arise. 3D technologies can be complemented by cable and circuit co-simulation and by additional analysis tools like OCT. Analysis can be performed at both the system and sub-system level, and simulation is useful at design, pre-testing and investigation stage.
References
- “Electromagnetic Environmental Effects – Requirements for Systems”, MIL-STD-464, United States Department of Defense.
- T.H. Lehman, “A statistical theory of electromagnetic fields in omplex cavities”. Interaction Notes, note 494, May 1993.
