Galileo Giove-A Radiation Monitors Review And Results
Taylor, B.1; Underwood, C.1; Evans, H.D.R.2; Daly, E.2; Ryden, K.A.3
1University of Surrey; 2ESTEC; 3QinetiQ
A review of the radiation monitors on board the Galileo Giove-A satellite, CEDEX and Merlin, and their first 600 days of data are presented. A comparison of the data with existing monitors and models is presented. In preparation for the Galileo global navigation satellite system, the European Space Agency commissioned two test satellites, the first of which is Giove-A, built by Surrey Satellite Technology Ltd (SSTL), UK. These satellites will test the technology required for the Galileo System and characterise the space radition environment that the final constellation will encounter. Giove-A was launched on the 28th December 2005 into a 23,260 km circular orbit with an inclination of 56°. To characterise the environment, Giove-A’s payload includes two radiation monitors: CEDEX (University of Surrey, UK) and MERLIN (QinetiQ Ltd., UK.) Both the instruments and the satellite have been operating successfully for over a year and have been measuring the radiation environment with few data gaps. In December 2006, a series of CMEs (Coronal Mass Ejections) were projected into the solar system, some influencing the Earth. These events and their effects on the MEO environment were measured. The Cosmic-Ray Energy Deposition Experiment (CEDEX) monitors high-energy proton fluxes and provides a detailed LET spectrum for the cosmic-ray ion environment with a pair of large-area diode detectors arranged to form a telescope. In addition, CEDEX carries four experimental dose-rate sensors, shielded by domes of varying shielding thickness. Merlin includes a PIN diode detector to measure proton fluxes, a LET telescope for measuring cosmic rays, two RADFETs for dosimetry and the SURF instrument for measuring energetic electrons and monitoring electrostatic charging. Results The Merlin and CEDEX monitors were both switched on soon after launch, and a near continuous dataset has been provided commencing at the beginning of January 2006. Both instruments are working well and are detecting the magnitude and variability of the outer radiation belts. Both the CEDEX photodiodes and the Merlin SURF plates are registering variability in the MEO environment due to solar activity in the form of both (Co-rotating Interaction Region) CIR and CME associated events. Analysis GRAS (GEANT-4 Radiation Analysis for Space) modelling of the two instruments allows comparison to the response expected to the environment as specified in the AE-8 model. A series of Monte Carlo simulations were run for mono-energetic electrons covering all energies contributing to the response of the instruments, thus determining their energy response. This was then combined with a differential flux spectrum from AE-8 to produce the mean measurement expected from the CEDEX photodiodes and Merlin SURF plates. These results suggest that AE-8 may under predict at higher energies, demonstrated in the heavier shielded channels. The Copper-shielded diodes are dominated by their thermal response under quiet conditions, but show a good dose-rate response to large flux events. A sectoring analysis using SSAT (Sector Shielding Analysis Tool) was carried out on the Merlin RADFETs. This combines the shielding distribution with SHIELDOSE-2 AE-8 predictions for a spherical shell, giving a predicted dose. From this it is clear that whilst the 3mm Al shielded RADFET is a fair match to the model predictions, the 6mm Al shielded RADFET greatly exceeds model predictions by a factor of ~3. Again, this suggests AE-8 under predicts higher energy electrons in the Galileo orbit. Data from both instruments has also been compared to further environmental models, such as CRRESELE, POLE, FLUMIC, etc, in addition to data from the IREM instrument on the INTEGRAL observatory.