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Internal Charging

Background

Internal charging can lead to an electrostatic discharge (ESD) which can cause damage to components, phantom commands, disruption to services, and in exceptional cases total satellite loss. It is one of the most important risk factors.

Internal charging takes place when the flux of high energy electrons in the radiation belts increases or remains at a high level for a substantial period of time. Typically energetic electrons with energies great than 1 MeV are important. Energetic electrons can penetrate the outer skin of a spacecraft and accumulate in insulators such as cables and circuit boards, and ungrounded conductors. If the charge builds up faster than it can leak a way an ESD can result. Usually shielding is used to protect sensitive components but if the radiation belts are enhanced during a major space weather event the shielding may not be sufficient. The risk indicators here are designed to help assess this risk.

Update frequency

The plots are updated hourly.

Internal charging risk indicator

The internal charging risk indicator has been selected as the charging current behind 0.5 mm of Al shielding. The charging current has been selected since it can be compared to the recommendations provided by the European Cooperation for Space Standardisation (ECSS, 2008) and the NASA Technical Handbook (NASA, 2011). Note, that the incident electron flux is converted into a charging current passing through a surface area. Although the units are A m-2, it is usually referred to as a charging current, or just current, and this terminology is used here.

Typically, the satellite skin, which may be in the form of a honeycomb panel, offers a shielding of 0.5 mm of Al in the form of a slab. Sensitive electronic components are usually placed inside a shielded box and may have an additional 1.5 mm of Al shielding. Thus connectors outside the box are likely to pose the greater risk. A shielding of 0.5 mm of Al has therefore been selected for calculating the internal charging risk indicator.

The ECSS recommends shielding for some components if the incident flux corresponds to a current greater than 2 x 10-10 A m-2 (0.02 pA cm-2) and 10-9 A m-2 (0.1 pA cm-2) for materials in excess of 25° C. The NASA Technical Handbook states that if the incident flux corresponding to a current greater than 0.1 pA cm-2 then the components should be shielded so as to reduce the current to that level, and if the incident flux corresponds to a current greater than 1 pA cm-2 then electrostatic discharge (ESD) problems may exist. Using the ECSS and NASA recommendations the following risk levels have been set:

  • Level 4 (Red) if the internal charging current is greater than 0.1 pA cm-2
  • Level 3 (Amber) if > 0.02 pA cm-2
  • Level 2 (Yellow) if > 0.01 pA cm-2
  • Level 1 (Green) otherwise

Method of Calculation

To calculate the risk indicator the electron flux measured along the orbit by a satellite is used wherever possible. The BAS radiation belt model (BAS-RBM) is used to provide any missing data, for example to ‘fill-in’ the electron spectrum at selected energies, or to provide the complete spectrum for orbits where no measurements are available. The full electron spectrum from 150 keV up to 10 MeV is then used as input to the DICTAT radiation effects code to calculate the current behind the level of shielding specified above.

A dielectric material similar to Kapton with a conductivity of 10-18 Ohm-1 m-1 is used to calculate the leakage currents. The calculations take into account the time history of electron flux variations since 1 June 2018, (provided the web site has run continuously without interruption), or since the last re-set of the web site.

Benefits

The benefits of our risk indicator are:

  • The radiation environment is integrated with radiation effects on materials to give a better risk assessment
  • The full electron spectrum is used to calculate the charging
  • The time history of the electron spectrum is used to calculate charging, and not the instantaneous or daily average flux

References

  • ECSS-E-ST-20-06C (2008), European Cooperation for Space Standardization, Space Engineering: Spacecraft charging, ESA Requirements and Standards Division, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands.
  • NASA Technical Handbook (2011), Mitigating in-space charging effects - A guideline, NASA Technical Handbook, NASA-HDBK-4002A, National Aeronautics and Space Administration, Washington, DC 20546-0001, USA.