Evaluation of Electronics Shielding in Micro-satellites

Transcription

1 Evaluation of Electronics Shielding in Micro-satellites L. Varga and E. Horvath Defence R&D Canada - Ottawa TECHNICAL MEMORANDUM DRDC Ottawa TM February 2003

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3 Evaluation of Electronics Shielding in Micro-satellites L. Varga DRDC Ottawa E. Horvath JERA Consulting Defence R&D Canada - Ottawa Technical Memorandum DRDC Ottawa TM February 2003

4 Her Majesty the Queen as represented by the Minister of National Defence, 2003 Sa majesté la reine, représentée par le ministre de la Défense nationale, 2003

5 Abstract This report investigates radiation shielding capabilities of micro-satellite bus model structures, incorporating different designs and materials for protecting internal spacecraft electronics from the ionizing radiation of the space environment. The modelling calculations have been carried out with a 3D Monte Carlo radiation transport code. The results indicate that the greatest reduction of total ionizing dose (TID) is observed with traditional aluminum spacecraft structures, although structures made with lighter poly-carbon materials with added thin layer of high-z material can provide comparable radiation protection in addition to some spacecraft mass reduction. Résumé Ce rapport étudie les possibilités d'armature de rayonnement des structures de modèle de micro-satellite, de différentes conceptions d'incorporation et des matériaux pour protéger l'électronique interne de vaisseau spatial contre la radiation ionisante de l'environnement de l'espace. Les calculs modelants ont été effectués avec le code de transport derayonnement de 3D Monte Carlo. Les résultats indiquent que la plus grande réduction de la dose s'ionisante totale (TID) est observée avec les structures traditionnelles de vaisseau spatial d'aluminum, bien que, les structures faites avec des matériaux plus légers de poly-carbone avec la couche mince supplémantaire du haut-z matériel puissent assurer la radioprotection comparable en plus d'une certaine réduction de la masse de vaisseau spatial. DRDC Ottawa TM i

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7 Executive summary Electronic devices located inside orbiting satellites in the near-earth space environment are exposed during the mission to ionizing space radiation propagating through the micro-satellite structure and creating damage. Because of such damage, over a period of time electronic components can fail thus jeopardising the mission success. A trade-off exists between the amount of shielding the spacecraft needs for protection from the space radiation effects and the mass reduction effort to reduce the launch cost. The results of this study indicate that new lighter materials can be utilized to reduce the weight budget of the mission. Materials such as poly-carbon PEEK can be used to build the micro-satellite structure. Shielding effectiveness can be also improved by lining the interior of the spacecraft structural panels with a thin layer of high-z material such as tantalum. This design can provide protection that is comparable to traditional aluminum structures but also can lead to weight reduction and thus reduction in launch cost. Varga L., Horvath E Evaluation of Electronic Shielding in Micro-satellites. DRDC Ottawa TM Defence R&D Canada - Ottawa. DRDC Ottawa TM iii

8 Sommaire Des satellites orbitaux intérieurs localisés par dispositifs électroniques dans l'environnement de l'espace de la proche-terre sont exposés pendant la mission au rayonnement s'ionisant de l'espace propageant par la structuremicro-satelliteet créant des dommages. En raison d'un tel dommages, sur une certaine période de temps les composants électroniques peut échouer de ce fait compromettant le succès de mission. Une compensation existe entre la quantité de protéger les besoins de vaisseau spatial de protection contre les effets de rayonnement de l'espace et l'effort de masse de réduction de réduire le coût de lancement. Les résultats de cette étude indiquent que de nouveaux matériaux d'allumeur peuvent être utilisés pour réduire le budget de poids de la mission. Des matériaux tels que le PEEK de poly-carbone peuvent être employés pour établir la structure satellite. L'armature de l'efficacité peut être également améliorée en rayant l'intérieur des panneaux structuraux de vaisseau spatial avec une couche mince de haut-z matériel tel que le tantale. Cette conception peut assurer la protection qui est comparable aux structures traditionnelles d'aluminum mais également peut mener à la réduction de poids et ainsi à la réduction en coût de lancement. Varga L., Horvath E Evaluation of Electronic Shielding in Micro-satellites. DRDC Ottawa TM R & D pour la défense Canada Ottawa. iv DRDC Ottawa TM

9 Table of contents Abstract... i Executive summary...iii Sommaire... iv Table of contents... v List of figures... vi List of Tables...viii 1. INTRODUCTION MICRO-SATELLITE STRUCTURE SCHEMES SHIELDING EFFECTIVENESS SOLAR FLARE EFFECT ENERGY WINDOW CONTRIBUTION SHIELDING BY LOCATION DISCUSSION SUMMARY REFERENCES List of symbols/abbreviations/acronyms DRDC Ottawa TM v

10 List of figures Figure 1. Schematic picture of the interior of the model micro-satellite, showing the locations of the electronic housings... 2 Figure 2. Traditional aluminum micro-satellite bus structures schemes. Structure B models spot shielding by increasing the electronic housing thickness Figure 3. The solid aluminum panel is replaced with honeycomb aluminum panel in C and with carbon composite PEEK honeycomb in D. Both honeycomb panels are covered with 0.1mm aluminum sheeting inside and outside. The electronic housings are 1mm thick solid aluminum...4 Figure 4. Carbon composite PEEK has replaced aluminum material in structural panels, aluminum electronic housings, however, remain. A thin coating (0.1mm) of high Z value material tantalum is added to the interior surface in F...4 Figure 5. Electronic housings are attached to the structural panels. The panels have the same composition as in structure F Figure 6. Model representation of the flex-board structure panel. Aluminum housings have been eliminated Figure 7. Calculated annual trapped electrons TID values into micro-satellite structures A to H and unprotected solid-state device, trace "I", for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 30 degrees Figure 8. Calculated annual trapped electrons TID values into micro-satellite structures A to H and unprotected solid-state device, trace "I", for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 60 degrees Figure 9. Calculated annual trapped electrons TID values into micro-satellite structures A to H and unprotected solid-state device, trace "I", for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 60 degrees Figure Name... 8 Figure 10. Calculated trapped protons annual TID values into micro-satellite structures A to H and unprotected solid-state device, trace "I", for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 30 degrees... 9 Figure 11. Calculated trapped protons annual TID values into micro-satellite structures A to H and unprotected solid-state device, trace "I", for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 60 degrees Figure 12. Calculated trapped protons annual TID values into micro-satellite structures A to H and unprotected solid-state device, trace "I", for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 85 degrees vi DRDC Ottawa TM

11 Figure 13. Relative shielding effectiveness of the micro-satellite model structures A to F for two specific orbit scenarios. The ratio is with respect to the fully exposed solid-state device..11 Figure 14. Calculated solar protons annual TID values into micro-satellite model structures A to H and exposed solid-state device (shown for reference as trace 'I"). No contribution to TID is observed at low altitude orbits and low angle of inclination Figure 15. Calculated solar protons annual TID values into micro-satellite structures A to H and unprotected solid-state device (shown for reference as trace 'I") at medium orbit inclination angle Figure 16. Calculated solar proton annual TID values into micro-satellite structures A to H and unprotected solid-state device (shown for reference as trace 'I") at high orbit inclination angle Figure 17. Energy window contribution to TID from protons for the micro-satellite structures A to H and unprotected solid-state device Figure 18. Energy window contribution to TID from electrons for the micro-satellite structures A to H and unprotected solid-state device Figure 19. Relative TID values inside the micro-satellite housings in electron dose dominated environment. The ratio is taken with respect to benchmarked housing Figure 20. Relative TID values inside the micro-satellite housings in proton dose dominated environment. The ratio is taken with respect to benchmarked housing Figure 21. Ratio of TID values of micro-sat structure E with respect to structure F for protons and electrons. Shielding effect of adding 4 mils of tantalum (structure F) is evident in case of electrons DRDC Ottawa TM vii

12 List of tables Table 1. List of model structures, shielding thickness and micro-satellite model bus mass... 6 Table 2. Proton to Electron Dose ratio for micro-satellite structures A to H and exposed solidstate device at 30 degrees inclination orbits Table 3. Proton to Electron Dose ratio for micro-satellite structures A to H and exposed solidstate device at 60 degrees inclination orbits Table 4. Proton to Electron Dose ratio for micro-satellite structures A to H and exposed solidstate device at 85 degrees inclination orbits Table 5. Total ionizing dose (TID) in Rad(Si) Y -1 from trapped radiation for micro-satellite structures A to H and exposed solid-state device ( I ) in orbits having 30 degrees inclination. Mass of the model structures is compared relative to structure "A Table 6. Total ionizing dose (TID) in Rad(Si) Y -1 from trapped radiation for micro-satellite structures A to H and exposed solid-state device ( I ) in orbits having 60 degrees inclination. Mass of the model structures is compared relative to structure "A Table 7. Total ionizing dose (TID) in Rad(Si) Y -1 from trapped radiation for micro-satellite structures A to H and exposed solid-state device ( I ) in orbits having 85 degrees inclination. Mass of the model structures is compared relative to structure "A viii DRDC Ottawa TM

13 1. INTRODUCTION Parts of the Earth s magnetosphere that are capable of trapping ionizing radiation form the socalled Earth s Radiation Belts. Occupying predominantly the inner portion of the magnetosphere, the trapped radiation display spatial and temporal variation in their spectra. The inner part of the radiation belts (L< 2.5), and the outer part (3 < L < 12) contain trapped electrons and protons, the proton belt being predominantly confined to smaller L values, L being the McIlwain magnetic shell parameter. In the inner part of the radiation belts, the particle population density is more stable, while the outer part is variable and the particle density responds readily to solar wind activity. The void region, located between the two parts of the belts, gets filled during large magnetospheric activity. Satellites located in Earth s orbit will be required to operate in these radiation belts and in the process will be exposed to trapped ionizing radiation, galactic cosmic rays and solar flare radiation. Since the radiation is dependent on altitude and latitude, satellite orbits such as for example LEO to GEO transfer orbit will pass through various regimes of radiation belts involving different electron and proton spectra. For some of these orbits, and at different orbital points, the radiation could be either proton or electron dose dominated. The integrated space radiation environment per orbit will depend on a number of parameters such as the spectrum of the radiation and the time of exposure to that spectrum and the local temporal modulation by the magnetospheric activity. This is specifically true for elliptic orbits where the velocity of the spacecraft will change at various points of the trajectory. In addition, geomagnetic shielding and Earth shadowing will modulate exposure of the satellite to radiation originating outside the Earth s magnetosphere, specifically solar flare radiation and galactic cosmic rays. The total ionizing dose (TID) environment of the mission, to a very large extent, will be affected by the design of the micro-satellite bus. Material composition of exterior walls, relative location inside the satellite, location of solar cells with respect to the main microsatellite body, presence/absence of equipment housing, cable harness locations and many other structural features will affect TID value. In order that the solid-state devices operating on board a micro-satellite can meet the TID requirement of the mission, as a precursory step at the design stage of the mission it is necessary to carry out mission dose estimates. In this work, we will examine how the structure of the micro-satellite bus, relative location inside the bus, material selection and orbit parameters affect the TID environment inside a satellite. A schematic picture of the model micro-satellite is shown in Figure 1[1]. The electronic housings have been numbered, as shown in Figure 1; the numbers also reflect the ID number that the specific housing unit has inside the input file of the Monte Carlo radiation transport simulation code used in this work. The objective of this work is to compare shielding effectiveness of several micro-satellite shielding configurations for selected orbits against trapped radiation and solar flare protons. DRDC Ottawa TM

14 Figure 1. Schematic picture of the interior of the model micro-satellite, showing the locations of the electronic housings. 2. MICRO-SATELLITE STRUCTURE SCHEMES Several micro-satellite bus structures have been modeled to ascertain their shielding capability against TID at selected orbits. The TID mitigation effort is often compared against the weight budget of spacecraft, these two being opposing factors. Various shielding schemes are employable for sensitive components protection, each placing different amounts of shielding between the radiation environment and the radiation sensitive electronic solid-state device. The solid-state device model is made of a 10mil thick silicon layer enclosed into a 100mil thick molded epoxy package. Eight micro-satellite structures have been studied; the structures 2 DRDC Ottawa TM

15 and materials are shown in Figures 2, 3, 4, 5 and 6. The dose deposition is calculated in the model solid-state device, one in each of the 5 electronic housings. Throughout most of this work, the results will refer to TID in the solid-state device located in the electronic equipment housing numbered 407. The shielding effectiveness of other electronic housings is explored later. Structures A and B (Figure 2) can be termed as conventional buses with aluminum electronic housings and aluminum structural panels. Electronic housings in structure B are 2mm thick; all other structures (A, C, D, E and F) have the electronic housings 1mm thick. The supporting shelves are also 1mm thick, made of aluminum. The structures C to H have the body panels made of honeycomb mesh, covered on the outside and inside with thin layers of material, such as aluminum or PEEK. The body panels of structure C are made of 8mm thick aluminum honeycomb mesh covered on both sides with 0.1mm aluminum sheeting. Structure D has the honeycomb portion of the body panels made of PEEK, a carbon composite material, which is covered with thin, 0.1mm layer of aluminum sheeting on both sides (see Figure 3). Satellite structure E is like structure D, except the honeycomb sheeting is made also from PEEK material. Structure F is like structure E, however, on the inside of the body panels, there is a 0.1mm thick layer of tantalum, a high Z material (Figure 4). Structure G has no support shelves because the electronic housings are attached directly to the spacecraft structural body panels (Figure 5). Structure H has no electronic housings to house sensitive devices (Figure 6). Instead, the rigid electronic housing and electronic boards are replaced with flex-boards and flex-cables that are attached directly to the structural body panels of the micro-satellite[2]. An electronic solid-state device would in this configuration be attached directly to the flexboard as shown in Figure 6. Table 1 provides a summary of shielding thickness and the mass of the micro-satellite structures. The mass reflects only the mass of the supporting structure and excludes all subsystems. Structure "A" Structure "B" Structural Structral panels pannels 1mm Al 1mm Al Electronic housings 1mm Al 2mm Al Figure 2. Traditional aluminum micro-satellite bus structures schemes. Structure B models spot shielding by increasing the electronic housing thickness. DRDC Ottawa TM

16 Structure "C" 8mm Al Honeycomb 0.1mm Al 0.1mm Al Structure "D" 8mm PEEK Honeycomb 0.1mm Al 0.1mm Al Structural Structral panels pannels Electronic housings 1mm Al 1mm Al Figure 3. The solid aluminum panel is replaced with a honeycomb aluminum panel in C and with a carbon composite PEEK honeycomb in D. Both honeycomb panels are covered with 0.1mm aluminum sheeting inside and outside. The electronic housings are 1mm thick solid aluminum. Structure "E" 8mm PEEK Honeycomb 0.5mm PEEK 0.5mm PEEK Structure "F" 8mm PEEK Honeycomb 0.5mm PEEK 0.5mm PEEK Structural Structral panels pannels 0.1mm Tantalum exterior Electronic housings 1mm Al 1mm Al Figure 4. Carbon composite PEEK has replaced aluminum material in structural panels; aluminum electronic housings, however, remain. A thin coating (0.1mm) of high Z value material tantalum is added to the interior surface in F. 4 DRDC Ottawa TM

17 Structure "G" 8mm PEEK Honeycomb 0.5mm PEEK 0.5mm PEEK Structural panels Structral pannels exterior 0.1mm Tantalum Electronic Electronic equipment equipment housing attached housing to the structural attaced panels to the structural pannels Electronic housings 1mm Al Figure 5. Electronic housings are attached to the structural panels. The panels have the same composition as in structure F. Structure H H Satellite Body Structural Panel Satellite body pannel 0.1mm Tantalum / 0.5mm PEEK / 8mm PEEK Honeycomb / 0.5mm PEEK 0.1mm Tantalum / 0.5mm PEEK / 8mm PEEK Honeycomb / 0.5mm PEEK Kapton flex-board exterior Silicon detector encapsulated in Epoxy Figure 6. Model representation of the flex-board structure panel. Aluminum housings have been eliminated. DRDC Ottawa TM

18 Table 1. List of model structures, shielding thickness and micro-satellite model bus mass Model Structure Effective Shielding Thickness Mass of the Model Supporting Structure A g cm g B g cm g C g cm -2 + Al Honeycomb 4530g D g cm -2 + PEEK Honeycomb 5090g E g cm -2 + PEEK Honeycomb 5500g F g cm -2 + PEEK Honeycomb 7500g G g cm -2 + PEEK Honeycomb 5990g H g cm -2 + PEEK Honeycomb 4050g 3. SHIELDING EFFECTIVENESS Shielding effectiveness of bus structures is examined at 5 elliptical orbits with a common perigee of 600km and with apogees of 1100km, 1500km, 3000km, 20000km and 36000km (LEO to GEO transfer orbit) respectively. Three inclination angles, low (30 degrees), medium (60 degrees) and high (85 degrees) have been used. The particle data were obtained from the SPENVIS system, ESA s space environment software package, available to run on the World Wide Web. The radiation transport simulation code used in this study was the 3D Monte Carlo code MCNPX/LAHET. The code is a general-purpose time-dependent transport code for neutrons, photons, and electrons in combination with the LAHET module; it also calculates transport and interaction of nucleons, pions, muons, light ions, and antinucleons. The micro-satellite bus structures provide radiation protection of varying degree for the sensitive electronic devices located on board, as shown in Figures 7 to 9 for trapped electron radiation and in Figures 10 to 12 for the trapped protons. The contributions to TID are shown separately for the purpose of ascertaining the relative shielding effectiveness of these structures in both the electron and proton environments. This is useful as, for example, many military missions require fission electron dose analysis for electronics on board. For the sake of completeness, the TID for the exposed solid-state device is also shown. It is evident that in the trapped electron environment, the least protection is provided by the bus structure type H; however even this bus reduces the total annual electron dose by about an order of magnitude from what a fully exposed device would receive. Bus structure B, with 1mm thick body panels and 2mm thick aluminum electronic circuit housings, and structure F, with light polycarbon honeycomb body panels and a 0.1mm (4 mils) tantalum layer, provide the best protection. A thin, 4 mils tantalum layer is virtually as much effective shield as an extra 1mm of aluminum added to the aluminum housing for electrons. The other bus structures (A, C, D, 6 DRDC Ottawa TM

19 E and G) are comparable to each other in shielding effectiveness. Utilization of honeycomb body panels provides just as effective shielding against electrons as solid aluminum panels if one compares the results of structure A against structures C, D, E and G. Electronic housings can be used as an effective means of spot shielding by comparing results between structure G and H, where structure H is effectively like G but without the electronic housing or structure B and A, where structure B has thicker electronic housing. In the proton environment, the shielding effectiveness differences of the structures are less pronounced; only structure H and the unprotected device show distinct TID values and even this difference decreases with increasing orbit of inclination. This is shown in Figures 10, 11 and 12, showing results of TID from trapped protons into the micro-satellite structures A to H. Trace I is again the dose that a totally unprotected device would receive in these orbits. The electron dose peaks in all cases at the orbit with the apogee of 20000km and becomes lowest for LEO (1100km apogee). This is what one would expect from the distribution of the trapped electron population inside the inner magnetosphere, where the trapped electron population is predominantly located at higher magnetic L shell values. The proton dose peaks at lower orbits than the electron dose, again based on similar arguments that the trapped proton population is located predominantly in the inner portion of the radiation belts. Specifically, for the considered orbit examples, the orbit with apogee of 3000km has the proton dose peak for all three inclination angles. The dominance of either proton or electron TIDs inside the spacecraft is summarized in Tables 2, 3 and 4 as the ratio of proton/electron dose. The data show both orbit dependency (vertical columns) and spacecraft structure dependency, shown horizontally.. Figure 7. Calculated annual trapped electrons TID values into micro-satellite structures A to H and unprotected solid-state device, trace I, for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 30 degrees DRDC Ottawa TM

20 Figure 8. Calculated annual trapped electrons TID values into micro-satellite structures A to H and unprotected solid-state device, trace I, for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 60 degrees. Figure 9. Calculated annual trapped electrons TID values into micro-satellite structures A to H and unprotected solid-state device, trace I, for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 85 degrees. 8 DRDC Ottawa TM

21 Figure 10. Calculated trapped protons annual TID values into micro-satellite structures A to H and unprotected solid-state device, trace I, for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 30 degrees. Figure 11. Calculated trapped protons annual TID values into micro-satellite structures A to H and unprotected solid-state device, trace I, for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 60 degrees. DRDC Ottawa TM

22 Figure 12. Calculated trapped protons annual TID values into micro-satellite structures A to H and unprotected solid-state device, trace I, for LEO, MEO and LEO-GEO transfer orbits with inclination angle of 85 degrees. Table 2. Proton to Electron Dose ratio for micro-satellite structures A to H and exposed solid-state device at 30 degrees inclination orbits ORBIT APOGEE A B C D E F G H I 1100km 1500km 3000km 20000km 36000km Table 3. Proton to Electron Dose ratio for micro-satellite structures A to H and exposed solid-state device at 60 degrees inclination orbits ORBIT APOGEE A B C D E F G H I 1100km km km km km DRDC Ottawa TM

23 Table 4. Proton to Electron Dose ratio for micro-satellite structures A to H and exposed solid-state device at 85 degrees inclination orbits ORBIT APOGEE A B C D E F G H I 1100km km km km km The amount of relative shielding individual structures can provide is presented in Figure 13, showing TID ratios at five locations inside the spacecraft; the locations are numbered after the individual electronic housings as shown in Figure 1 (Further discussion to relative shielding by individual housings is given later). The TID ratio is taken with respect to a fully exposed solid-state device to the space environment at the specific orbit. Two orbit examples are given; one is a high inclination angle, high apogee orbit (lower cluster of curves) and the other is a low inclination angle, low apogee orbit (upper cluster of curves). As shown, the microsatellite bus structures can cut the TID values (in comparison to exposed solid-state device) down to between 7% and 15% in low inclination, low altitude orbit environment and down to between less then 2% and 6% in the high inclination angle, high altitude orbit environment. Figure 13. Relative shielding effectiveness of the micro-satellite model structures A to F for two specific orbit scenarios. The ratio is with respect to the fully exposed solid-state device. DRDC Ottawa TM

24 4. SOLAR FLARE EFFECT The JPL-91 solar flare model, available from SPENVIS, ESA s space environment model, was used to determine the solar proton fluence into the micro-satellite structures for the five elliptic orbits used in this work. The mission was assumed to be five years long, geomagnetic shielding was taken into account and the magnetosphere was considered to be stormy. The stormy magnetospheric condition will provide greater geomagnetic shielding at the low energy end of the proton spectrum up to about 50 MeV. Figures 14, 15, and 16 show the annual TID contribution into the micro-satellite test structures, specifically into the polymer encapsulated solid-state device located in the electronic housing numbered 407 (see Figure 1). Figure 14. Calculated solar proton annual TID values into micro-satellite model structures A to H and exposed solid-state device (shown for reference as trace I ). No contribution to TID is observed at low altitude orbits and low angle of inclination. Again, for comparison purposes, the fully exposed solid-state device is also shown. For the externally (external to magnetosphere) originating radiation, such as solar flare protons, geomagnetic shielding is very effective at low inclination angles and low apogee orbits. At the 30 degrees orbital inclination angle, effective screening for the selected orbits is well beyond the 3000km apogee. At the 20000km apogee orbit, only solar protons with energy greater then 60MeV deposit some dose into the satellite. The proton fluence is, however, low at these energies and therefore contribution to TID is also low. The LEO-to GEO orbit receives contributions to TID from all the energies, although this is smaller than it would be for GEO because the micro-satellite becomes geomagnetically shielded in the vicinity of the orbit perigee. At higher inclination angles, geomagnetic shielding is less effective and more low energy solar protons contribute to TID. Shielding against solar flare protons, in these examples, the traditional aluminum bus structure B provides marginally the best protection, other structures are similar as shown in Figures 14, 15 and 16. The multifunction PEEK 12 DRDC Ottawa TM

25 polycarbon/flex-board structure (H) offers the lowest protection against solar flare protons. The unprotected, fully exposed solid-state device is also shown. Figure 15. Calculated solar proton annual TID values into micro-satellite structures A to H and unprotected solid-state device (shown for reference as trace I ) at medium orbit inclination angle. Figure 16. Calculated solar proton annual TID values into micro-satellite structures A to H and unprotected solid-state device (shown for reference as trace I ) at high orbit inclination angle. DRDC Ottawa TM

26 5. ENERGY WINDOW CONTRIBUTION TO TID All structures display a threshold energy for protons and electrons at which there is a large jump in dose deposition. Below this energy, very little dose deposition occurs into any of the solid-state devices located inside the micro-satellite bus structure. This threshold energy is bus structure dependent; for the bus structures under consideration, the protons threshold is between 15MeV and 25MeV while for electrons the threshold energy lies between 1MeV and 1.5MeV. The results are shown in Figures 17 and 18, showing the contribution to TID as a function of proton and electron energy. It is also evident that for the high-energy protons and electrons the structure configuration becomes less important as for all the micro-satellite bus structures considered, the TID as a function of bus structure design and material converges into a single value. This indicates that shielding against high-energy particles, especially protons, which contribute to Single Event Effects (SEE), is very difficult and is not very feasible. However, the good news is that the population of trapped high energy protons is about 3 to 4 orders of magnitude less than the population at the low-energy end of the spectrum. For both electrons and protons, again configuration H requires the least energetic particles, the threshold energy being about 1 MeV for electrons and 15 MeV for protons. For bus type B, the threshold energy was found to be highest at 1.5MeV and 25MeV for electrons and protons respectively. Figure 17. Energy window contribution to TID from protons for the micro-satellite structures A to H and unprotected solid-state device. 14 DRDC Ottawa TM

27 Figure 18. Energy window contribution to TID from electrons for the micro-satellite structures A to H and unprotected solid-state device. 6. LOCATION SHIELDING EFFECTVENESS The shielding capability of the bus structure in other electronic housings (other than 407, see Figure 1) inside the micro-satellite structure type A was examined. Analyses were carried out for two types of orbits, namely, one with low angle of inclination and low altitude, and the other with high angle of inclination and high altitude. The results are shown in Figures 19 and 20, presented as the ratio of TIDs taken with respect to the TID in the electronic housing number 407. This electronic housing (407), as mentioned, has been used throughout this work as the reference location. Larger variation in TID values from location to location inside the micro-satellite is observed in the high inclination and high altitude case, i.e. in the electron-dose-dominated environment than in the proton dose environment case (low inclination, low altitude orbit) as evident if one compares results in Figure 19 and 20. This indicates that shielding by location would be more feasible in the electron dose dominated environment then in the proton environment. For the orbits, without adding any additional shielding, the TID values can vary by up to 100% as seen in Figure 19. The smaller variation at orbits with low altitude and low inclination angles points to larger difficulty of the bus structures to shield against protons. DRDC Ottawa TM

28 Figure 19. Relative TID values inside the micro-satellite housings in high altitude and high inclination angle orbit. The ratio is taken with respect to benchmarked housing 407. Figure 20. Relative TID values inside the micro-satellite housings in low altitude and low inclination angle orbit. The ratio is taken with respect to benchmarked housing DRDC Ottawa TM

29 7. DISCUSSION Traditional micro-satellite bus structures having structural panels, shelves and equipment housings made of aluminum and bus structures made of lighter materials such as polycarbon have been compared for their space radiation shielding effectiveness. Structure B, with effective 3mm minimum aluminum shielding was the best in protection against the space environment ionizing radiation, due to the additional 1mm of aluminum shielding added to the equipment housing. Other structures with identical interiors provided good comparison in terms of the micro-satellite envelope performance. These results are summarized in Tables 5, 6 and 7. Multi-layered structure F with poly-carbon honeycomb structural panels and a 4 mils tantalum layer provided the second-best protection against the space environment ionizing radiation. By removing the 4 mils thick tantalum layer, structure F becomes structure E, and the shielding protection at high altitude orbits becomes reduced by a factor of between 2 and 3 (Figure 21 and Tables 2, 3 and 4), while shielding protection show only a small deterioration at the low altitude orbits. The high Z material (tantalum) is located in the particle path between low Z materials of the outside panel, i.e. poly-carbon PEEK structural panel, and the aluminum housing. Tantalum provides selfattenuation for bremsstrahlung X-rays while the poly-carbon structural panels reduce electron fluence via the inelastic scattering process [3]. Micro-satellite structures C and D are identical with the exception that the outer body panels of structure D, specifically the honeycomb portion, is made of composite carbon material PEEK (density 1.2g cm -3 ) compared to the honeycomb made of traditional aluminum (density = 2.7g cm -3 ) in structure C. In the high altitude orbit environment, the D structure performs slightly better; in the low altitude orbit environment, the two are about the same. In terms of the weight budget, about 0.5kg reduction can be realized with structure C; even though PEEK is only about as half as dense aluminum, thicker PEEK material making the honeycomb panel (0.1mm versus 0.5mm) accounts for the difference. The presence of other bus structures such as shelves, reduce the TID values by up to 60%, as was observed comparing results between structure F and structure G, however the weight budget increases by about 1.5kg. Although the presence of electronic housings increased the weight budget of the model micro-satellite by about 2 kg, the total TID was reduced by up to a factor of 5 in high altitude, high inclination orbits. In the low altitude orbit environment, a reduction was also observed, but was less than a factor of 2. The most dramatic change in TID occurs when both shelves and electronic housings are removed; the effect is shown in Tables 5, 6, and 7 by comparing results between structure F and structure H. Again, the change is most visible in the high altitude orbit environment. Protecting a sensitive electronic component by selecting a more shielded location inside a micro-satellite can be done relatively well in the high altitude orbit environment but it becomes a much less effective technique in the low altitude and low angle of inclination orbit environments. For example, up to a 100% change in TID can occur from location to location in structure A, by selecting a different location to house sensitive electronic device as evident for example between TID results inside housings #317 and #427. Again, much less change in protection by location can be accomplished when the micro-satellite is located in an orbit with low angle of inclination and low altitude. DRDC Ottawa TM

30 Orbit parameters determine either electron or proton dose domination, but micro-satellite structure also plays a role as to whether TID into the electronic components of the microsatellite is electron or proton dominated. There is a rather large (in some cases a factor of 7) variation from structure to structure for the same orbit, as can be seen, which can be tied to the previously made point that layered structures can be designed for electron shielding. The orbital dependence is a function of spectrum change from orbit to orbit. Every microsatellite structure has also a different capability for shielding out particles up to a certain particle energy. Threshold energies were determined at which a large, 4 to 5 orders of magnitude, jump in contribution to TID occurs. These values were determined for all of the micro-satellite structures and for both proton and electron radiation. Above this energy, the contribution to TID becomes much less energy and micro-satellite design dependent for both types of particles. Table 5. Total ionizing dose (TID) in Rad(Si) Y -1 from trapped radiation for micro-satellite structures A to H and exposed solid-state device ( I ) in orbits having 30 degrees inclination. Mass of the model structures is compared relative to structure A. Structure Mass Ratio ORBIT (Ref. A ) 1100km 1500km 3000km 20000km 36000km A 1.00 B 1.28 C 0.66 D 0.74 E 0.80 F 1.10 G 0.87 H 0.59 I Table 6. Total ionizing dose (TID) in Rad(Si) Y -1 from trapped radiation for micro-satellite structures A to H and exposed solid-state device ( I ) in orbits having 60 degrees inclination. Mass of the model structures is compared relative to structure A. Structure Mass Ratio ORBIT (Ref. A ) 1100km 1500km 3000km 20000km 36000km A 1.00 B 1.28 C 0.66 D 0.74 E 0.80 F 1.10 G 0.87 H 0.59 I DRDC Ottawa TM

31 Table 7. Total ionizing dose (TID) in Rad(Si) Y -1 from trapped radiation for micro-satellite structures A to H and exposed solid-state device ( I ) in orbits having 85 degrees inclination. Mass of the model structures is compared relative to structure A. Structure Mass ratio ORBIT (Ref. A ) 1100km 1500km 3000km 20000km 36000km A 1.00 B 1.28 C 0.66 D 0.74 E 0.80 F 1.10 G 0.87 H 0.59 I Figure 21. Ratio of TID values of micro-satellite structure E with respect to structure F for protons and electrons. Shielding effect of adding 0.16g cm -2 of Tantalum (structure F) is well evident in case of electrons. DRDC Ottawa TM

32 8. SUMMARY Radiation transport analysis into several micro-satellite bus structures was presented. The results indicate that it is possible to design a bus structure, optimal to operate in a specific orbit space environment. In the high-altitude orbit environment, multi-layered structure made of layers of low Z material and high Z material provides very effective protection against the total ionizing dose. In the low-altitude orbit environment, the presence of such layering is not necessary; structures made only of low Z materials are just as effective in shielding. 9. REFERENCES 1. QuickSat Space Technologies Micro-satellite Platform Development Project, CSA/DND Working Group Presentation, November 5, 1999 Meeting 2. B.D. Spieth, K.S. Quasim, R.N. Pottman and D.A. Russell, Shielding Electronics Behind Composite Structures, IEEE Transactions on Nuclear Science, Vol. 45, No. 6, December W.C. Fan, C.R. Drumm, S.B. Roeske and G.J. Scrivner, Shielding Consideration for Satellite Microelectronics, IEEE Transactions on Nuclear Science, Vol. 43, No. 6, December DRDC Ottawa TM

33 List of symbols/abbreviations/acronyms/initialisms DND TID LEO MEO GEO Department of National Defence Total Ionizing Dose Low Earth Orbit Mid-altitude Earth Orbit Geostatinary Earth Orbit DRDC Ottawa TM

34 22 DRDC Ottawa TM

35 UNCLASSIFIED SECURITY CLASSIFICATION OF FORM (highest classification of Title, Abstract, Keywords) DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified) 1. ORIGINATOR (the name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Establishment sponsoring a contractor s report, or tasking agency, are entered in section 8.) Defence R&D Canada - Ottawa Ottawa, Ontario K1A 0Z4 2. SECURITY CLASSIFICATION (overall security classification of the document, including special warning terms if applicable) UNCLASSIFIED 3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S,C or U) in parentheses after the title.) Evaluation of Electronic Shielding in Micro-satellites(U) 4. AUTHORS (Last name, first name, middle initial) Varga, L and Horvath, E 5. DATE OF PUBLICATION (month and year of publication of document) February a. NO. OF PAGES (total containing information. Include Annexes, Appendices, etc.) 30 6b. NO. OF REFS (total cited in document) 7. DESCRIPTIVE NOTES (the category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered.) 3 Technical Memorandum 8. SPONSORING ACTIVITY (the name of the department project office or laboratory sponsoring the research and development. Include the address.) DRDC-Ottawa 9a. PROJECT OR GRANT NO. (if appropriate, the applicable research and development project or grant number under which the document was written. Please specify whether project or grant) 9b. CONTRACT NO. (if appropriate, the applicable number under which the document was written) 15EW21 10a. ORIGINATOR S DOCUMENT NUMBER (the official document number by which the document is identified by the originating activity. This number must be unique to this document.) 10b. OTHER DOCUMENT NOS. (Any other numbers which may be assigned this document either by the originator or by the sponsor) DRDC Ottawa TM DOCUMENT AVAILABILITY (any limitations on further dissemination of the document, other than those imposed by security classification) ( x ) Unlimited distribution ( ) Distribution limited to defence departments and defence contractors; further distribution only as approved ( ) Distribution limited to defence departments and Canadian defence contractors; further distribution only as approved ( ) Distribution limited to government departments and agencies; further distribution only as approved ( ) Distribution limited to defence departments; further distribution only as approved ( ) Other (please specify): 12. DOCUMENT ANNOUNCEMENT (any limitation to the bibliographic announcement of this document. This will normally correspond to the Document Availability (11). However, where further distribution (beyond the audience specified in 11) is possible, a wider announcement audience may be selected.) UNCLASSIFIED SECURITY CLASSIFICATION OF FORM DCD03 2/06/87

36 UNCLASSIFIED SECURITY CLASSIFICATION OF FORM 13. ABSTRACT ( a brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual). Abstract This report investigates the shielding capabilities of several micro-satellite model structures and shielding materials in protecting internal spacecraft electronics from the ionizing radiation of the space environment. The calculations have been carried out with 3D Monte Carlo radiation transport code. The results indicate that the largest reduction of total ionizing dose (TID) is observed with traditional aluminium spacecraft structures, although, structures made with lighter poly-carbon materials with added thin layer of high-z material can provide comparable radiation protection in addition to much desired spacecraft mass reduction. 14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.) LEO, SEU, PEEK, Space Environment, Tantalum, Trapped Protons, Trapped Electrons, Solar Protons, Total Ionizing Dose UNCLASSIFIED SECURITY CLASSIFICATION OF FORM

37 Defence R&D Canada Canada s leader in defence and national security R&D R & D pour la défense Canada Chef de file au Canada en R&D pour la défense et la sécurité nationale