Background

Total ionising dose causes degradation to electronic components on a spacecraft. It can result in device failure (and biological damage to humans), increased power consumption, and timing changes. It is one of the main factors that limits the lifetime of a spacecraft.

The total ionising dose is the amount of radiation absorbed in matter due to all sources. In space this is mainly due to energetic electrons and protons arising from the radiation belts, high energy ions from a Solar Energetic Particle event and cosmic rays. As these particles are absorbed they emit Bremsstrahlung radiation, usually in the form of X rays, which is also later absorbed contributing to the total dose.

Update frequency

The plots are updated hourly.

Ionising dose risk indicator

The ionising dose rate has been selected as the risk indicator as it can be compared against the design standard. Although the total ionising dose is more important, the total dose depends on the launch date of the satellite and hence the total radiation exposure to date. This is specific to each satellite and not possible to calculate without user input. However, both the ionising dose rate and the total ionising dose since 1 June 2018 are provided.

To test if a satellite is aging faster or slower than expected the total dose for a spacecraft launched after 1 June 2018 can be calculated by subtracting the total dose before launch, provided the orbit type is the same. This will provide an approximation to the total dose.

The total dose indicator is more appropriate for electronic components inside a shielded box than connectors outside. Therefore, a total spherical shielding of 0.5 + 1.5 = 2.0 mm of Al has been selected for calculating the total ionising dose risk indicator

The risk levels for the ionising dose rate have been set as:

• Level 4 (Red) if the dose rate is greater than 5,000 rads hr^-1
• Level 3 (Amber) if > 1,000.0 rads hr^-1
• Level 2 (Yellow) if > 100.0 rads hr^-1
• Level 1 (Green) otherwise

Method of Calculation

To calculate the dose rate and total ionising dose we use the electron and proton energy spectrum measured along the orbit of a satellite where possible. We then use results from the BAS radiation belt model to ‘fill-in’ the electron energy spectrum, or to provide the complete energy spectrum for orbits where no data are available. For orbits where there are no proton observations we use the AP8 Max model for the proton radiation belt and the proton energy spectrum measured by GOES at geostationary orbit, transformed to lower altitudes using the Tsyganenko 89 magnetic field model and the appropriate rigidity cut-off, in order to capture Solar Energetic Particle events. We use the SHIELDOSE-2 code to calculate the radiation dose due to electrons, protons and Bremsstrahlung radiation behind 2 mm of Al shielding (spherical shielding). Heavy ions are not included. Cosmic rays are not included, but the dose from cosmic rays is small compared to other sources.

Benefits

The benefits of our risk indicator are:

• The radiation environment is integrated with radiation effects to give a better risk assessment
• The full electron and ion spectra are used
• The risk indicator is specific for each orbit type
• The ionising dose rate can be compared against design standards for any satellite
• The total ionising dose can be calculated from the launch date of the satellite