Minimisation of the Solar Array Generated Electrical Interference on the GEOS Satellite

The international satellite GEOS will carry very sensitive electromagnetic wave-field experiments operating in the frequency range 0 - 54 KHz. It is essential that the satellite does not interfere electromagnetically with the function of the experiments. One of the most severe problems is posed by a pair of electric sensors having a sensitivity of 100 μV/m at zero frequency and 1pV/m for frequencies up to 70 KHz. These sensors are located at the ends of two diametrically opposed 10 metre radial booms. A major source of on-board electric interference is the body mounted solar array, due to the sensor passing through the spatially stationary field of the array as the satellite rotates and by scintillation of the finite module distribution. This interference can be minimised by special design of the solar generator, A second source of interference not discussed in great detail here is due to the potential acquired by the spacecraft by the various charging mechanisms. Solar array generated interference has been investigated by the development of an accurate computer model, and the results have been verified by two independent experiment techniques. The electric field computation is based on the derivation of the potential at a point knowing the potential distribution over a sphere. A cylinder was approximated by a series of coaxial equal radii spheres where the potential distribution over the cylindrical solar array is represented by mid-bands of potential on each of the spheres. The field was then computed by superposition for different solar array configurations. The results show that a compensating centre-tapped lay-out referenced to zero potential and with limited parallel redundancy is sufficient to keep this interfering field below the sensitivity of the experiment. Model measurements were made in an electrolytic tank giving an order of magnitude agreement with the computed field values. A second experimental verification has been made using a life-size model of the spacecraft and a sensor of the GEOS experiment type. Considerations of the charging mechanisms of the spacecraft body imply a careful choice of the materials making up the exterior surfaces based on their secondary emissive and photo-emissive properties.