Swarm. Document Change Record

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1 UK EXPORT CONTROL RATING : Not Listed Rated By: B Robinson This document is produced under ESA contract AO/1-4800/05/NL/CB, ESA export exemptions may therefore apply. These Technologies may require an export licence if exported from the EU UK EXPORT CONTROL RATING : DUAL USE 9E001(9A004), 9E002(9A004). NOT CONTROLLED BY ANNEXE IV UK EXPORT CONTROL RATING : Not Listed Rated By: B Robinson Rated By: B Robinson This document is produced under ESA contract AO/1-4800/05/NL/CB, ESA export exemptions may therefore This document is produced under ESA contract AO/1-4800/05/NL/CB, ESA export exemptions may therefore apply. These Technologies may require an export licence if exported from the EU apply. These Technologies may require an export licence if exported from the EU UK EXPORT CONTROL RATING : DUAL USE 9E001(9A004), 9E002(9A004). NOT CONTROLLED BY ANNEXE IV Rated By: B Robinson This document is produced under ESA contract AO/1-4800/05/NL/CB, ESA export exemptions may therefore apply. These Technologies may require an export licence if exported from the EU

2 Document Change Record Issue / Rev. Date Changes DR1 16/03/2006 First Draft Issue 1 29/03/2006 First Issue Astrium GmbH Page i

3 This page is left intentionally blank Astrium GmbH Page ii

4 Table of Contents 1 INTRODUCTION REFERENCE DOCUMENTS MISSION SUMMARY ENERGETIC PARTICLE RADIATION Quantification of Radiation Effects Trapped Electrons Trapped Protons Fluxes Solar Protons Cosmic rays SPACE RADIATION EFFECTS Ionizing Dose Single Event Phenomena Displacement Damage Solar cell degradation SHIELDING ELEMENTS OF SWARM SPACECRAFT Equipment located in main spacecraft structure Star Tracker Heads on Optical Bench ASM Sensor Head on Spacecraft Boom EFI on S/C RAM Side...25 List of Figures Figure 1. Parameter for the first 24h in orbit...4 Figure 2. Trapped Electrons Fluxes / NASA GSFC AE8...8 Figure 3. Trapped Protons Fluxes / NASA GSFC AP Figure 4. Solar Protons Fluence...12 Figure 5. Cosmic ray LET spectra (Creme96)...14 Figure 6. Dose Depth Curve for Swarm mission...16 Figure 7. Displacement Damage Equivalent Fluence Depth Curve (Si)...19 Figure 8. Displacement Damage Dose Curve (Si)...19 Figure 9. Solar cell equivalent 1MeV electron fluences...22 List of Tables Table 1. List of parameter used for qualification and types of tests of verify compatibility...7 Table 2. AE-8-MAX trapped electron spectrum (Integral)...9 Table 3. AE-8-MIN trapped proton spectrum (Integral)...11 Table 4. Solar proton fluences outside the magnetosphere...13 Table 5. Cosmic ray integral flux as a function of LET...15 Table 6. Dose depth as a function of the shielding thickness...17 Table 7. Displacement Damage Equivalent Fluence Depth Curve (Si)...20 Table 8. Displacement Damage Dose (Si)...21 Table 9. Solar cell equivalent electron fluences (GaAs and Si)...23 Astrium GmbH Page iii

5 1 INTRODUCTION The purpose of this document is to define the space radiation environment for the Swarm mission. All equipment/instruments shall be designed to survive the space radiation environment encountered during the mission lifetime as defined herein. The Swarm concept consists of a constellation of three satellites at three different near polar orbits between 530 km (BOL) and 300 km (EOL) altitudes. All calculations are done on basis of satellite SWARM C which will receive a higher radiation level because of the higher altitude and higher inclination compare to SWARM A and B ([RD1]). For a further assessment of the radiation impact on equipment/instruments an adequate design margin is required (not included in this analysis). Astrium GmbH Page 2

6 2 REFERENCE DOCUMENTS RD_1 System Requirements Document, SW-RS-ESA-SY-001 RD_2 ESA/BIRA Space Information System (Spenvis) Link: RD_3 "CREME96: A Revision of the Cosmic Ray Effects on Micro-Electronics Code" IEEE Trans. Nucl. Sci NS-44, (1997). A.J. Tylka et al. Astrium GmbH Page 3

7 3 MISSION SUMMARY The orbit of the Swarm mission for the applicable radiation environment is described as follows: Altitude Orbit type 530 km (above the mean radius of the earth) general Inclination 88.0 Eccentricity 1e-6 R. asc. of asc. note 1.4 Argument of perigee 0 True anomaly 0 The launch date and lifetime is assumed to be: Launch date Lifetime 4.25 years Figure 1 shows the orbit data for the first day. Figure 1. Parameter for the first 24h in orbit Astrium GmbH Page 4

8 4 ENERGETIC PARTICLE RADIATION This chapter gives an overview of the energetic particle radiation environment. They always have to be considered early in the design cycle. Energetic charged particles, which can penetrate outer surfaces of spacecraft (for electrons, this is typically above 100keV, while for protons and other ions this is above 1MeV), are encountered throughout the Swarm orbit. The following several environments have to be discussed in more details: Radiation belts Energetic electrons and ions are magnetically trapped around the earth forming the radiation belts, also known as the Van Allen belts. The radiation belts are crossed by low altitude orbits as well as high altitude orbits (geostationary and beyond). The radiation belts consist principally of electrons of up to a few MeV energy and protons of up to several hundred MeV energy. The so-called south Atlantic anomaly is the inner edge of the inner radiation belt encountered in low altitude orbits. The offset, tilted geomagnetic field brings the inner belt to its lowest altitudes in the south Atlantic region. Solar energetic particles / Solar Wind Energetic solar eruptions (solar particle events - SPEs) produce large fluxes of solar energetic particles (SEPs) which are encountered in interplanetary space and close to the earth. The Earth s magnetic field provides a varying degree of geomagnetic shielding in near- Earth orbits from these particles. Galactic cosmic-rays There is a continuous flux of galactic cosmic-ray (GCR) ions. Although the flux is low (a few particles /(cm²*s)), GCRs include energetic heavy ions, which can deposit significant amounts of energy in sensitive volumes and so cause problems. Secondary radiation Secondary radiation is generated by the interaction of the above environmental components with materials of the spacecraft (e.g. Bremsstrahlung). A wide variety of secondary radiation is possible. These radiation environments represent important hazards to space missions. Energetic particles, particularly from the radiation belts and from solar particle events cause radiation damage to electronic components, solar cells and materials. They can easily penetrate typical spacecraft walls and deposit considerable doses during a mission. Energetic ions, primarily from cosmic rays and solar particle events, lose energy rapidly in materials, mainly through ionisation. This energy transfer can disrupt or damage targets such as a memory element, leading to single-event upset (SEU) of a component, or an element of a detector (radiation background). SEUs can also arise from nuclear interactions between very energetic trapped protons and materials (sensitive parts of components, detectors). Here, the proton breaks the nucleus apart and the fragments cause highly localised ionisation. Energetic electrons can penetrate thin shields and build up static charge in internal dielectric materials such as cable and other insulation, circuit boards, and on ungrounded metallic parts. These can subsequently discharge, generating electromagnetic interference. Apart from ionizing dose, particles can loose energy through non-ionizing interactions with materials, particularly through displacement damage, or bulk damage, where atoms are displaced from their original sites. This can alter the electrical, mechanical or optical properties of materials and is an Astrium GmbH Page 5

9 important damage mechanism for electro-optical components (solar cells, opto-couplers, etc.) and for detectors, such as CCDs. Astrium GmbH Page 6

10 4.1 Quantification of Radiation Effects The table below recalls the parameter used for quantification of various radiation effects, and for illustration purposes, lists the types of testing which must be done to verify compatibility with the effects. See ECSS-E-20 for further details. Radiation effect Parameter Test means Electronic component degradation Material degradation Material degradation (bulk damage) CCD and Sensor degradation Solar cell Single-event upset, Single-event transient Latch-up, etc. Sensor interface (background signals) internal electrostatic Charging Total ionizing dose Radioactive sources (Co 60 ) Total ionizing dose Non-ionizing dose (NIEL) Non-ionizing dose (NIEL) Non-ionizing dose (NIEL) & equivalent fluence LET spectra (ions)protons energy spectra explicit SEU/L rate Flux above energy threshold, flux threshold explicit background rate Electron flux and fluence dielectric E-field particle beams (e -, p + ) Radioactive sources (Co 60 ) particle beams (e -, p + ) Proton beams Proton beams Proton beams Heavy Ion particle beams Proton particle beams Radioactive sources particle beams test data feedback to calculation Electron beams test data feedback to calculation Discharge characterisation Table 1. List of parameter used for quantification and types of tests of verify compatibility Astrium GmbH Page 7

11 4.2 Trapped Electrons Trapped Electrons Fluxes on Swarm Orbit are calculated using the AE8 NASA GSFC model, during the Solar Maximum period. This model is included in the Space Information System (Spenvis) [RD3]. Trapped Electrons Fluxes are given in figure 2 and table 2. Figure 2. Trapped Electrons Fluxes / NASA GSFC AE8 Astrium GmbH Page 8

12 Energy (MeV) Total mission average flux (/cm2/s) Integral electron spectra Total mission fluence (/cm2) Average flux (/cm2/s) Segment fluence (/cm2) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+00 Table 2. AE-8-MAX trapped electron spectrum (Integral) Astrium GmbH Page 9

13 4.3 Trapped Protons Fluxes Trapped Protons Fluxes on Swarm Orbit are calculated using the AP8 NASA GSFC model, during the Solar Minimum period. This model is included in the Space Information System (Spenvis) [RD3]. Trapped Protons Fluxes are given in figure 3 and table 3. Figure 3. Trapped Protons Fluxes / NASA GSFC AP8 Astrium GmbH Page 10

14 Energy (MeV) Total mission average flux (/cm2/s) Integral proton spectra Total mission fluence (/cm2) Average flux (/cm2/s) Segment fluence (/cm2) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+07 Table 3. AE-8-MIN trapped proton spectrum (Integral) Astrium GmbH Page 11

15 4.4 Solar Protons During energetic events on the sun, large fluxes of energetic protons are produced which can reach the Earth. Solar particle events, because of their unpredictability and large variability in magnitude, duration and spectral characteristics, have to be treated statistically. However, large events are confined to a 7-year period defined as solar maximum. The reference model, which was used for engineering consideration of time-integrated effects, is the JPL-1991 model, include within the SPENVIS environment with 95% confidence level and the geomagnetic shielding is included. Figure 4 shows the predicted spectrum for the Swarn orbit of solar protons based on this model. Figure 4. Solar Protons Fluence Astrium GmbH Page 12

16 Energy (MeV) Fluence at spacecraft Model fluence at 1.0 AU Total mission fluence Total prediction period Integral (cm -2 ) Differential (cm -2 MeV -1 ) Integral (cm -2 ) Differential (cm -2 MeV -1 ) Cycle 24: 4.25 yr in max Integral (cm -2 ) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+08 Table 4. Solar proton fluences outside the magnetosphere Astrium GmbH Page 13

17 4.5 Cosmic rays To estimate the Cosmic-Ray environment and effects the CREME96 [RD4] model is used. The model provides a comprehensive set of cosmic ray and flare ion LET and energy spectra, including treatment of geomagnetic shielding and material shielding. CREME96 also includes upset rate computation based on the pathlength distribution in a sensitive volume and also treated in a simple manner trapped proton-induced SEUs. Cosmic ray fluxes are anti-correlated with solar activity so the highest cosmic ray fluxes occur at solar minimum. CREME96 is the standard model for cosmic ray environment assessment. It is the standard for evaluation of single event effects from cosmic rays, from solar energetic particles and from energetic protons. Figure 5 shows composite LET for the Swarm orbit for three CREME96 environments Quiet - the nominal solar minimum cosmic ray flux Worst week - the average flux for a "worst week" of a large SEPE Peak 5 minutes - the peak flux from a large SEPE. Ions from Z=1 to 92 are included and, in the absence of a reason to use another value, shielding of 1g/cm 2 Aluminium is assumed. 1.E+09 1.E+07 Cosmic Ray Let Spectra Integral Flux [ /m2/s/ster] 1.E+05 1.E+03 1.E+01 1.E-01 1.E-03 1.E-05 1.E-07 1.E-09 Quiet Worst Week Peak 5 minutes 1.E-11 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 LET[MeV cm2/g] Figure 5. Cosmic ray LET spectra (Creme96) Astrium GmbH Page 14

18 LET [MeV cm 2 /g] Integral Flux [/cm 2 /s/ster] Quiet Integral Flux [/cm 2 /s/ster] Worst Week Integral Flux [/cm 2 /s/ster] Peak 5 minutes 1.00E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-09 Table 5. Cosmic ray integral flux as a function of LET Astrium GmbH Page 15

19 5 SPACE RADIATION EFFECTS 5.1 Ionizing Dose The ionizing dose environment is represented by the dose-depth curve. This may provide dose as a function of shield thickness in planar geometry or as a function of spherical shielding about a point. The planar model is appropriate for surface materials or for locations near to a planar surface. In general, electronic components are not in such locations and a spherical model is recommended for general specification. The SHIELDOSE-2 model as part of SPENVIS has been used for ionizing dose. This method uses a pre-computed data-set of doses from electrons, electron-induced bremsstrahlung and protons, as derived from Monte-Carlo analysis. The doses are provided as functions of material shielding thickness. The reference geometrical configuration for this dose depth curve is a solid Aluminium sphere. Figure 6 shows the expected accumulated doses for the Swarm orbit as function of the thickness of the Aluminium shielding. It is seen that the dose is dominated by the contribution from electrons. For shielding higher than 4 mm Aluminium solar protons become the most important contributors.. Figure 6. Dose Depth Curve for Swarm mission In cases where more careful analysis of the shielding of a component or of other sensitive locations is necessary, a sectoring calculation is to be performed on the geometry of the system. This might be necessary if the doses computed from simple spherical shielding are incompatible with the specification of the allowable radiation dose. The sectoring method traces rays from the point of interest through the shielding in a large number of directions. Astrium GmbH Page 16

20 Along each direction the derived shielding, together with the data on dose as a function of shielding depth, d, is used to find the omnidirectional 4π dose contribution from each direction. The contributions, weighted by the solid angle increment around the rays, are then summed to give the total dose. In some cases, it is efficient to derive a shielding distribution. This is also the result of the ray-tracing described above and provides the distribution of encountered shielding. Total mission dose (rad) Al absorber thickness (mm) (mils) (g cm -2 ) Total Trapped electrons Bremsstrahlung Trapped protons Solar protons Tr. electrons+ Bremsstrahlung Tr. el.+bremss. +Tr. protons E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+02 Table 6. Dose depth as a function of the shielding thickness Astrium GmbH Page 17

21 5.2 Single Event Phenomena For Single Event Phenomena rates calculation, it is necessary to consider the following sources: Galactic Cosmic Rays, described in figure 5 and table 5 Solar Flare Protons described in figure 4 and table 4 Upset rate calculation models are included in CREME96 [RD4]. It is possible to make upset rate predictions only when details of the device under consideration are known, particularly the critical charge and the sensitive volume dimensions. If a device is uncharacterized, tests have to be performed. The test data has to show the normalized upset rate as a function of Ion LET in the range 1 to 100 MeV cm²/mg and proton energy in the range MeV. To compute an upset rate for an electronic device or a detector from the predicted fluxes, device characteristics must be specified, particularly the size of the sensitive volume and the critical charge, or equivalently, critical energy E c, in the volume which results in upset or registers as a "count". For SEUs resulting from direct ionization the rate is found by integrating over the composite differential ion LET spectrum and the distribution of pathlengths for the sensitive volume. An estimate of the upset rate from nuclear interactions of energetic protons can be obtained by integration of the product of the measured proton induced upset cross section σ(e) and the differential proton flux (E) over all energies. σ(e) can be derived directly from the test data, or the 2-parameter Bendel fit can be used. 5.3 Displacement Damage Both protons and electrons can induce displacement damage in semiconductor devices. The part of deposited energy involved in displacement defects creation is called Non-Ionizing Energy Loss (NIEL). The particles spectra are converted into a fluence of monoenergetic particles producing the same amount of defects (10 MeV). The attenuation of 10 MeV incident protons by an aluminium solid sphere shielding is evaluated using a routine from the CREME programme suite and is included in SPENVIS ([RD2]). The mission Displacement Damage Equivalent Fluence Depth Curve takes into account 4.25 years. It is given for a Silicon detector (figure 7, table 7). The displacement damage dose is the nonionizing equivalent of the ionizing dose, it is calculated by integrating the product of the differential fluence and the nonioizing energy loss values over energy (figure 8, table 8). Astrium GmbH Page 18

22 Figure 7. Displacement Damage Equivalent Fluence Depth Curve (Si) Figure 8. Displacement Damage Dose Curve (Si) Astrium GmbH Page 19

23 Damage equivalent 10.0 MeV proton fluence (cm -2 ) as a function of spherical Al shield radius Radius (mm) Mission Total Solar protons Total Trapped protons E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+09 Table 7. Displacement Damage Equivalent Fluence Depth Curve (Si) Astrium GmbH Page 20

24 Displacement damage dose (MeV g(si) -1 ) as a function of spherical Al shield radius Radius (mm) Mission Total Solar protons Total Trapped protons E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+06 Table 8. Displacement Damage Dose (Si) Astrium GmbH Page 21

25 5.4 Solar cell degradation The version of the EQUFRUX model as part of SPENVIS is used for the solar cell degradation calculations, EQFRUX-Si for silicon solar cells and EQFRUX-Ga for gallium arsenide solar cells 1.4E+14 Equiv. 1MeV electron fluence [ /cm2] 1.2E E E E E E E+00 Si cells - Pmax&Voc Si cells - Isc GaAs cells - Voc GaAs cells - Pmax GaAs cells - Isc Cover glass Thickness [µm] Figure 9. Solar cell equivalent 1MeV electron fluences In absence of other test data, it shall be assumed that 10MeV protons cause equivalent damage to 3000 times a 1 MeV electrons in silicon cells. Similarly it shall be assumed for gallium arsenide cells that the damage equivalence of a 10MeV proton is 400, 1000 and 1400 times a 1 MeV electron for short-circuit current, maximum power and open-circuit voltage degradation respectively. Since the default in these models is the assumption of infinite rear-side shielding of cells, this shall be the standard way of reporting results. However, account shall then be explicitly taken of radiation penetration through the rear-side of solar arrays. Figure 7-13 shows the predicted equivalent 1MeV electron fluence for solar cell degradation for the Swarm orbit as functions of the cover glass thickness. Astrium GmbH Page 22

26 Summary of 1 MeV equivalent electron fluences (cm -2 ) Cover glass thickness Total Trapped electrons Trapped protons Solar protons g cm -2 mils micron P max V oc I sc P max, V oc, I sc P max V oc I sc P max V oc I sc E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+11 Summary of 1 MeV equivalent electron fluences (cm -2 ) Cover glass thickness Total Trapped electrons Trapped protons Solar protons g cm -2 mils micron P max V oc I sc P max, V oc, I sc P max V oc I sc P max V oc I sc E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+12 Table 9. Solar cell equivalent electron fluences (GaAs and Si) Astrium GmbH Page 23

27 6 SHIELDING ELEMENTS OF SWARM SPACECRAFT To determine the radiation level received on all sensitive parts and materials, the shielding of the spacecraft, equipment and parts has to be taken into account. Note: The below specified total dose figures do not contain design margins. For Swarm a safety factor of two shall be maintained. 6.1 Equipment located in main spacecraft structure The outer structural elements of Swarm spacecraft (main body) consists out of 0.5 mm CFRP/ 40 mm Al-Honeycomb/ 0.5 mm CFRP sandwich (CFRP Carbon fibre reinforced plastics): CFRP density : 1.6 g/cm 3 Aluminium : 2.7 g/cm 3 Therefore a shielding of 0.59 mm Al equivalent is provided by the Swarm spacecraft. An additional shielding of 2 mm Al equivalent is taken into account from the electronic boxes. According to table 6, 7 and 8 the resulting accumulated radiation dose for all radiation sensitive components inside the electronic boxes is about: TID: 8 krad (Si) DD: MeV/g(Si) (10MeV proton fluence: /cm 2 ). In case the equipment/instrument box shielding is different from 2 mm Al equivalent the individual radiation environment has to be derived from and tables 6, 7 and Star Tracker Heads on Optical Bench The structure elements containing the Startracker heads consists out of 4 mm SiC Material. SiC density : 3.2 g/cm 3 Therefore a shielding of 4.76 mm Al equivalent is provided by the Swarm spacecraft. An additional shielding of 2 mm Al equivalent is taken into account from the electronic boxes. According to table 6, 7 and 8 the resulting accumulated radiation dose for all radiation sensitive components inside the electronic boxes is about: TID: 1.5 krad (Si) DD: MeV/g(Si) (10MeV proton fluence: /cm 2 ). In case the equipment/instrument box shielding is different from 2 mm Al equivalent the individual radiation environment has to be derived from and tables 6, 7 and 8. Astrium GmbH Page 24

28 6.3 ASM Sensor Head on Spacecraft Boom For the ASM Sensor head no spacecraft shielding is provided. Thus, only the shielding effects from the sensor cover can be taken into account by the ASM supplier. 6.4 EFI on S/C RAM Side TBD final accommodation Astrium GmbH Page 25